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Molecular and Cellular Biology, December 2004, p. 10126-10144, Vol. 24, No. 23
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.23.10126-10144.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

A Tel1/MRX-Dependent Checkpoint Inhibits the Metaphase-to-Anaphase Transition after UV Irradiation in the Absence of Mec1

Michela Clerici,{dagger} Veronica Baldo,{dagger} Davide Mantiero, Francisca Lottersberger, Giovanna Lucchini, and Maria Pia Longhese*

Dipartimento di Biotecnologie e Bioscienze, Università di Milano-Bicocca, Milan, Italy

Received 26 June 2004/ Returned for modification 10 August 2004/ Accepted 7 September 2004


   ABSTRACT
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
In Saccharomyces cerevisiae, Mec1/ATR plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while the role of the Tel1/ATM kinase in DNA damage checkpoints is not as well defined. We found that UV irradiation in G1 in the absence of Mec1 activates a Tel1/MRX-dependent checkpoint, which specifically inhibits the metaphase-to-anaphase transition. Activation of this checkpoint leads to phosphorylation of the downstream checkpoint kinases Rad53 and Chk1, which are required for Tel1-dependent cell cycle arrest, and their adaptor Rad9. The spindle assembly checkpoint protein Mad2 also partially contributes to the G2/M arrest of UV-irradiated mec1{Delta} cells independently of Rad53 phosphorylation and activation. The inability of UV-irradiated mec1{Delta} cells to undergo anaphase can be relieved by eliminating the anaphase inhibitor Pds1, whose phosphorylation and stabilization in these cells depend on Tel1, suggesting that Pds1 persistence may be responsible for the inability to undergo anaphase. Moreover, while UV irradiation can trigger Mec1-dependent Rad53 phosphorylation and activation in G1- and G2-arrested cells, Tel1-dependent checkpoint activation requires entry into S phase independently of the cell cycle phase at which cells are UV irradiated, and it is decreased when single-stranded DNA signaling is affected by the rfa1-t11 allele. This indicates that UV-damaged DNA molecules need to undergo structural changes in order to activate the Tel1-dependent checkpoint. Active Clb-cyclin-dependent kinase 1 (CDK1) complexes also participate in triggering this checkpoint and are required to maintain both Mec1- and Tel1-dependent Rad53 phosphorylation, suggesting that they may provide critical phosphorylation events in the DNA damage checkpoint cascade.


   INTRODUCTION
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Eukaryotic cells have developed sophisticated surveillance mechanisms called checkpoints to ensure proper response to the presence of damaged or incompletely replicated DNA molecules (reviewed in references 49 and 72) and to alterations in the mitotic apparatus (reviewed in references 46 and 64). The failure of checkpoints causes accumulation of genetic changes and chromosome instability that may lead to cancer in multicellular eukaryotes (reviewed in reference 87).

DNA damage checkpoints are specialized in detecting abnormal DNA structures, serving at least two primary purposes: (i) to arrest the cell cycle in response to DNA damage, thereby coordinating cell cycle progression with DNA repair capacity (109); and (ii) to regulate transcription of DNA damage response genes, as well as to regulate activation and recruitment of various repair and recombination proteins that help cells survive genotoxic stress to sites of damage (reviewed in references 72 and 90). Therefore, DNA damage checkpoints are considered one of the main lines of defense against genomic instability. In fact, similar to mutations in recombination, replication, and repair genes, defective S-phase checkpoint genes increase the rate of gross chromosome rearrangements in Saccharomyces cerevisiae in the absence of exogenous DNA damage (40, 65, 66).

The DNA damage checkpoints are envisaged as signal transduction cascades, where upstream sensors monitor and detect altered DNA molecules, while central transducers act in a protein kinase cascade to regulate a myriad of downstream effectors (reviewed in references 72 and 90). Keystones of DNA damage checkpoints are protein kinases of a phosphoinositide 3-kinase-related family, which include S. cerevisiae Mec1 (55, 75, 110) and Tel1 (33, 62), Schizosaccharomyces pombe Rad3 and Tel1 (7), Drosophila melanogaster Mei-41 (35), and mammalian ATM (86) and ATR (7). These kinases respond to various DNA stresses by phosphorylating key proteins, thus regulating numerous processes, depending on the spectrum of their substrates (reviewed in reference 90).

Several lines of evidence indicate a functional distinction between ATM- and ATR-dependent pathways in both yeasts and mammals. In fact, in humans, ATR responds to UV-induced DNA damage, double-strand breaks (DSBs), and stalled replication forks, while ATM seems to respond primarily to DSBs (reviewed in reference 90). Similarly, S. cerevisiae Mec1 and S. pombe Rad3, more closely related to human ATR, are the prototype transducers of the DNA damage and replication stress signals (reviewed in references 49 and 72), while evidence regarding activation by DSBs of the S. cerevisiae and S. pombe ATM homologs, Tel1, is less extensive.

Budding yeast (S. cerevisiae) Mec1 plays a critical role in both sensing and transducing the checkpoint signals and is required to delay G1/S and G2/M transitions as well as to slow down DNA replication in the presence of damaged DNA molecules, depending on the cell cycle stage at which the damage occurs (reviewed in references 49 and 72). Mec1 functions in a complex with Ddc2/Lcd1/Pie1 (74, 81, 108), functionally related to S. pombe Rad26 and human ATRIP, which interact with Rad3 and ATR, respectively (15, 20). These complexes seem to respond directly to DNA insults. In particular, Mec1 and Ddc2 are recruited to sites of DNA damage independently of other checkpoint proteins (41, 58, 82), and in vivo Ddc2 and Rad26 phosphorylation does not require other known checkpoint factors but Mec1 and Rad3, respectively, suggesting a pivotal role for these kinases in sensing DNA alterations (15, 20, 74). Similarly, ATR colocalizes with ATRIP in nuclear foci after DNA damage, indicating that the ATR-ATRIP complex may also be recruited to damaged DNA (116). ATR does not localize at damaged sites in the absence of the single-stranded DNA (ssDNA) binding complex replication protein A (RPA), which also stimulates in vitro ATRIP binding to ssDNA in human cells, indicating that RPA-coated ssDNA is critical for recruiting the ATR-ATRIP complex (115). Similarly, Ddc2 is recruited to DSBs in an RPA-dependent manner (115).

In budding yeast, once DNA perturbations are sensed, checkpoint signals are propagated through the evolutionarily conserved protein kinases Rad53 and Chk1, which undergo Mec1-dependent phosphorylation after DNA damage (84, 85). While Rad53 is required for proper response to DNA damage in all cell cycle phases, Chk1 contributes only to the activation of the G2/M checkpoint in a Rad53-independent manner (85). The DNA damage-sensing functions are linked with the downstream effectors by Mec1-dependent phosphorylation of the Rad9 adaptor (21, 95, 106). In particular, this phosphorylation triggers Rad9 interaction with Rad53 and the subsequent release of active Rad53 kinase, thus indicating that Mec1 can regulate both sensing and transducing of checkpoint signals (31, 88).

The response to DSBs in humans occurs primarily through ATM and leads to phosphorylation of many targets critical for checkpoint activation, apoptosis, and DNA repair (reviewed in reference 90). ATM/Tel1 functions depend on a highly conserved complex, called MRN (Mre11-Rad50-Nbs1) in mammals and MRX (Mre11-Rad50-Xrs2) in S. cerevisiae, which is required for recombinational DNA repair, DSB end resection, telomere metabolism, and meiotic recombination (reviewed in reference 105). The phenotypes of cells defective for Mre11 or Nbs1 show significant overlap with those of ATM-deficient cells (112), raising the possibility that MRN deficiency may somehow affect ATM activation. Indeed, the MRN complex stimulates ATM kinase activity in vitro by facilitating stable substrate binding (44). Moreover, ATM phosphorylates Nbs1 in response to ionizing radiation (IR), and this phosphorylation is important for IR-induced cell cycle arrest (28, 48, 111, 112). This response is evolutionarily conserved, as certain types of DNA damage also stimulate Tel1-dependent phosphorylation of the S. cerevisiae Mre11 and Xrs2 proteins (17, 34). Finally, the complex is involved in checkpoint activation in response to DSB-inducing agents in both mammals and yeasts (10, 17, 34).

The hallmark of ATM's response to DSBs is a rapid increase in its kinase activity immediately after the formation of DSBs (5, 8). Although a portion of nuclear ATM is recruited to broken DNA soon after DNA damage (3), even a limited number of DSBs was shown to be sufficient to activate the majority of intracellular ATM molecules, suggesting that the signals for ATM activation might be chromatin structure perturbations, rather than direct ATM contact with the broken DNA (4).

Less clear is the role in DNA damage response of the S. cerevisiae ATM counterpart, Tel1. Although TEL1 deletion by itself does not confer sensitivity to DNA-damaging agents, Tel1 functions in the DNA damage response are inferred from the finding that its absence increases the sensitivity of mec1 mutants to genotoxic agents (62, 84), and it impairs the checkpoint response to phleomycin treatment during S phase (67). Tel1 is also responsible for the G2 arrest after DNA damage in a dominant-negative MEC3 mutant (29). Moreover, TEL1 overexpression can suppress both cell lethality and hypersensitivity to DNA-damaging agents of mec1{Delta} and ddc2{Delta} mutants, indicating that excess Tel1 can bypass the requirements for the Mec1-Ddc2 complex (11, 84). Finally, altering DSB processing by the absence of Sae2 or non-null mutations affecting Rad50 or Mre11 functions in mec1{Delta} cells triggers Tel1/MRX-dependent Rad53 phosphorylation and interaction with Rad9 after DNA damage, suggesting that these mutations may allow accumulation of DNA lesions that specifically activate Tel1 (104). Although it was shown that Tel1 association to DSBs does not necessarily result in activation of the Rad53 kinase, the finding that this association is mediated by an Xrs2-dependent mechanism supports the hypothesis that Tel1 may be activated by DSBs (68).

Altogether, the data obtained in both yeasts and humans highlight the functional distinction between ATM- and ATR-dependent pathways and indicate that their interrelationships may be less straightforward than merely serving the same purpose at different times. In particular, although mec1{Delta} cells seem to retain the ability to trigger a Tel1-dependent DNA damage checkpoint, further studies are required to clarify the molecular mechanisms and signals involved in this response, as well as the in vivo consequences of its activation.

In this context, we have found and analyzed in detail a Tel1/MRX-dependent G2/M checkpoint that is triggered by UV irradiation in the absence of Mec1. Unlike Mec1-dependent Rad53 phosphorylation, which occurs after UV treatment independently of cell cycle progression in wild-type cells, Tel1-dependent Rad53 phosphorylation occurs in mec1{Delta} cells only concomitantly with completion of UV-damaged DNA replication. Moreover, it requires both entry into S phase and active Clb-CDK complexes, and the resulting metaphase arrest can be relieved by eliminating the anaphase inhibitor Pds1, whose UV-induced phosphorylation and stabilization in mec1{Delta} cells depend on Tel1.


   MATERIALS AND METHODS
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
S. cerevisiae strains and media. The relevant genotypes of all S. cerevisiae strains used in this study are listed in Table 1. All strains were constructed during this study, and all were derivatives of W303 (K699) (MATa or MAT{alpha} ade2-1 can1-100 his3-11,15 leu2-3,112 trp1-1 ura3 rad5-535). The rad5-535 mutation confers very slight sensitivity to methyl methanesulfonate (MMS) (23).


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  		TABLE 1. S. cerevisiae strains used in this study

 
Standard yeast genetic techniques and media (80) were used. Cells were grown in YEP medium (1% yeast extract, 2% Bacto Peptone, 50 mg of adenine per liter) supplemented with 2% glucose (YEPD) or 2% raffinose (YEP+Raf) or 2% raffinose and 1% galactose (YEP+Raf+Gal).

Deletions of the MEC1, RAD53, CHK1, SML1, TEL1, and RAD14 genes have been obtained as previously described (50, 51, 69, 74, 75). Deletion of MRE11 was generated from strain K699, where 2,111 bp of the MRE11 coding region was replaced by the HIS3 gene by one-step PCR disruption method (107). The rfa1-t11 mutant was kindly provided by R. D. Kolodner (University of California, La Jolla).

Strains carrying fully functional MRE11-HA3, CHK1-HA3, and PDS1-HA3 alleles at their corresponding chromosomal loci were constructed as previously described (6, 73, 75). W303-derived strains carrying either deletions of the PDS1 and MAD2 genes, or the GAL-CLB2 allele integrated in one copy at the TRP1 locus or in seven copies at the CLB2 locus (7XGAL-CLB2) (96), or a galactose-induced GAL-ubiCDC6 fusion at the CDC6 locus, were kindly provided by S. Piatti (University of Milano-Bicocca, Milan, Italy), while a W303-derived strain expressing the Scc1 version cleavable by the tobacco etch virus (TEV) protease (scc1{Delta}::HIS3 SCC1-TEV GAL-TEV) was kindly provided by F. Uhlmann (Cancer Research UK, London, United Kingdom). All these strains were used to disrupt MEC1 or TEL1 or both, as reported above. Strain YLL1437, carrying three copies of the GAL-SIC1{Delta}NT-MYC-HIS fusion integrated at the URA3 locus, was obtained by transforming strain K699 with ApaI-digested plasmid pLD1, kindly provided by J. Diffley (Clare Hall Laboratories, South Mimms, United Kingdom). The accuracy of integration was verified by Southern blot analysis.

The mec1{Delta} tel1{Delta} sml1{Delta} triple mutants were constructed by crossing mec1{Delta} sml1{Delta} and tel1{Delta} sml1{Delta} strains. The resulting diploid strain mec1{Delta}/MEC1 tel1{Delta}/TEL1 sml1{Delta}/sml1{Delta} was allowed to sporulate, and tetrads were dissected on YEPD plates. Forty-eight hours after tetrad dissection, segregant clones were streaked on YEPD plates and were used 24 h later for the experiments described in the text and concomitantly analyzed for the presence of the mec1{Delta} and tel1{Delta} markers.

Other techniques. Synchronization experiments were performed as described previously (74). Mechloretamine (nitrogen mustard or HN2) and mononitrogen mustard (2-dimethylaminoethylchloride hydrochloride or HN1) were purchased from Sigma-Aldrich. Flow cytometric DNA analysis was performed on a Becton-Dickinson FACScan. Nuclear division on cells stained with propidium iodide was scored with a fluorescence microscope. tet operators integrated at the URA3 locus of chromosome V (35 kb from the centromere) using the GFP-tetR fusion were visualized as described previously (60). For Western blot analysis, protein extracts were prepared by trichloroacetic acid precipitation as previously described (51). Rad53 was detected using anti-Rad53 polyclonal antibodies kindly provided by J. Diffley (Clare Hall Laboratories), and Rad9 was detected using anti-Rad9 polyclonal antibodies kindly provided by N. Lowndes (University of Ireland, Galway). Secondary antibodies were purchased from Amersham, and proteins were visualized by an enhanced chemiluminescence system according to the manufacturer.


   RESULTS
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Cellular responses to different genotoxic agents in the absence of Mec1. In order to gain new knowledge of the interrelationships between Mec1 and Tel1 in the DNA damage checkpoint response, we analyzed the abilities of mec1{Delta} cells to progress through the cell cycle and to activate Rad53 after treatment with different genotoxic agents. Rad53 activation was examined by monitoring its phosphorylation, which is known to be required for its activation as a kinase, and can be detected as an electrophoretic mobility shift (31, 84, 85, 95). It is worth pointing out that all strains carrying the mec1{Delta} allele also carried the sml1{Delta} allele (Table 1), which suppresses the lethality of MEC1 deletion, but not its effect on the DNA damage response (113).

In the experiments shown in Fig. 1A, B, and C, G1-arrested wild-type and mec1{Delta} cells were either released into the cell cycle under unperturbed conditions or UV irradiated before release or released in the presence of the alkylating agent MMS or the radiomimetic drug bleomycin. When 95% of cells had budded after release, {alpha}-factor was added back in order to prevent cells from entering a second cell cycle. As expected, due to activation of the DNA damage checkpoint by the genotoxic treatments, damaged wild-type cells slowed down cell cycle progression and started cell division much later than the untreated cell cultures (Fig. 1A). Accordingly, phosphorylated Rad53 appeared in wild-type cells immediately after UV irradiation or concomitantly with S-phase entry in the presence of MMS and bleomycin (Fig. 1B). Interestingly, although both untreated and UV-, MMS-, and bleomycin-treated mec1{Delta} cells completed DNA replication with similar kinetics, the damaged mec1{Delta} cells consistently went through the subsequent cell cycle phases slower than the untreated cells did. Moreover, they accumulated phosphorylated Rad53, which was detected concomitantly with completion of damaged DNA replication about 60 min after genotoxic treatment (Fig. 1A and B).



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  		FIG. 1. Rad53 and Mre11 phosphorylation in response to different genotoxic treatments in mec1{Delta} cells. (A to C) Logarithmically growing wild-type (wt) (YLL1072) and mec1{Delta} (DMP4225/4C) cells were arrested in G1 with {alpha}-factor and released from the pheromone block in YEPD alone or YEPD supplemented with 10 mU of bleomycin (bleo) per ml or 0.02% MMS or were UV irradiated (30 J/m2) prior to release in YEPD. When 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures to prevent them from entering a second cell cycle. Cell samples were collected at the indicated times after {alpha}-factor release to analyze the DNA content by FACS (A) and to detect the Rad53 (B) and Mre11 (C) proteins by Western blot analysis with anti-Rad53 and antihemagglutinin antibodies, respectively. (D) Logarithmically growing wild-type (YLL1072) and mec1{Delta} (DMP4225/4C) cells were arrested in G1 with {alpha}-factor and released from the pheromone block at time zero in YEPD containing HN1 (2 µM) or HN2 (0.5 µM). Extracts from cell samples collected at the indicated times after {alpha}-factor release were subjected to Western blot analysis as described above for panels B and C. Cell samples taken immediately before UV treatment were taken at time zero. exp, exponentially growing cells.

 
As shown in Fig. 1C, not only Rad53 but also Mre11 was phosphorylated in mec1{Delta} cells about 30 min after UV irradiation or bleomycin or MMS addition, whereas its phosphorylation in wild-type cells was detected only in the presence of bleomycin. Since Mre11 phosphorylation is known to be Tel1 dependent (17, 34) (see Fig. 3B), these data indicate that Tel1, which can substitute for the absence of Mec1 under specific circumstances, is activated in response to DNA damage in the absence of Mec1 and therefore might be responsible for the observed Mec1-independent phosphorylation events.



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  		FIG. 3. Tel1 and Mre11 are required for the response to UV irradiation in G1 in the absence of Mec1. (A) Cell cultures were arrested in G1 with {alpha}-factor and released from the G1 block either unirradiated (–UV) or after UV irradiation (30 J/m2) (+UV). As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Samples of untreated (left) and UV-treated (right) wild-type (wt) (DMP4290/27A), tel1{Delta} (DMP4062/19A), mec1{Delta} (DMP4290/13C), mec1{Delta} tel1{Delta} (DMP4062/5C), mre11{Delta} (DMP4290/3A), and mec1{Delta} mre11{Delta} (DMP4290/10B) cell cultures collected at the indicated times after {alpha}-factor release were analyzed by fluorescence microscopy to determine the percentage of separated sister chromatids (top) and the percentage of binucleate cells (bottom). (B) Samples of wild-type (YLL839), tel1{Delta} (DMP3641/2C), mec1{Delta} (DMP3642/8C), mec1{Delta} tel1{Delta} (DMP3817/9A), mre11{Delta} (DMP3815/3C), and mec1{Delta} mre11{Delta} (DMP3774/28A) cells were treated as described above for panel A and analyzed to evaluate Rad53, Chk1, and Rad9 phosphorylation in the UV-irradiated cultures by Western blot analysis of protein extracts with anti-Rad53, antihemagglutinin, and anti-Rad9 antibodies, respectively. The DNA content by FACS and the percentage of binucleate cells were also analyzed (not shown). Samples of wild-type (YLL1072), tel1{Delta} (DMP4226/1B), mec1{Delta} (DMP4225/4C), and mec1{Delta} tel1{Delta} (DMP4239/8A) cells were treated as described above for panel A and analyzed to evaluate Mre11 phosphorylation in the UV-irradiated cultures by Western blot analysis with antihemagglutinin antibodies. Cell samples taken immediately before UV treatment were taken at time zero. exp, exponentially growing cells.

 
Although UV irradiation can cause replication fork stalling, like MMS and bleomycin (reviewed in reference 57), the highest level of Rad53 phosphorylation and the most pronounced delay in cell cycle progression in mec1{Delta} cells were induced by UV treatment, while MMS and bleomycin were more efficient than UV in causing these effects in wild-type cells (Fig. 1A and B). These results suggest that UV-induced DNA alterations might be more suitable than those caused by MMS and bleomycin of eliciting a Mec1-independent checkpoint response.

Besides inducing formation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts in DNA, UV irradiation can also generate strand breaks and cross-links between duplex DNA molecules (24), which constitute some of the most serious damage to DNA. Since previous studies have shown that DNA interstrand cross-links (ICLs) are capable of eliciting a checkpoint-dependent G2/M arrest in both S. pombe and humans (2, 43), we verified the ability of ICLs to induce Rad53 phosphorylation both in the presence and absence of Mec1. To this end, exponentially growing wild-type and mec1{Delta} cells were arrested in G1 with {alpha}-factor and released in the presence of the bifunctional alkylating agent nitrogen mustard HN2, which forms both monoadducts and cross-links (43, 83).

As shown in Fig. 1D (left), partial Rad53 phosphorylation took place in mec1{Delta} cells 60 min after release from the G1 block in the presence of HN2, concomitantly with completion of S phase (data not shown), while its phosphorylation occurred in similarly treated wild-type cells 30 min after release as soon as cells initiated DNA replication (data not shown). Conversely, Rad53 phosphorylation was not observed when the same cell cultures were released in the presence of a monofunctional HN2 analogue, HN1, which causes only monoadduct formation (43, 83) (Fig. 1D, left). Although we could not rule out the possibility that HN2 formed monoadducts more efficiently than HN1 under our experimental conditions, these results suggest that HN2-dependent Rad53 phosphorylation might be induced by ICLs and not by monoadducts. Accordingly, HN2 treatment induced a delay in the G2/M transition in both wild-type and mec1{Delta} cells, while HN1 did not affect cell cycle progression at all (data not shown). Moreover, ICLs seem to trigger Tel1 activation in the absence of Mec1, since Mre11 phosphorylation was strongly induced by HN2 treatment in mec1{Delta} cells, while its level, if any, was very low in wild-type cells under the same conditions (Fig. 1D, right).

UV irradiation in the absence of Mec1 leads to Scc1-dependent metaphase arrest. We analyzed the in vivo consequences of UV-induced DNA damage in the absence of Mec1 in more detail by monitoring the kinetics of progression through the cell cycle, separation of sister chromatids, and nuclear division of mec1{Delta} cells released from {alpha}-factor arrest either undamaged or after UV irradiation. All strains carried the tetR-GFP/tetO system that allows detection of the separation of sister chromatids on one arm of chromosome V (60). The kinetics of cell cycle progression, sister chromatid separation, and nuclear division after {alpha}-factor release were similar in untreated wild-type and mec1{Delta} cells (Fig. 2A, top, and data not shown). After UV irradiation, wild-type cells progressed through the S phase slower than the untreated cultures did, due to G1/S checkpoint activation, completing DNA replication within 150 min (Fig. 2A, bottom). As expected, when the checkpoint is turned off, the cells completed the cell cycle and started to undergo sister chromatid separation (Fig. 2B) and nuclear division (Fig. 2C) 150 and 210 min after UV irradiation, respectively, and arresting with 1C DNA content after cell division, due to the addition of {alpha}-factor. Conversely, UV-treated mec1{Delta} cells, which progressed through S phase much faster than similarly treated wild-type cells and completed DNA replication within 75 min (Fig. 2A, bottom), failed to undergo anaphase and arrested mostly with duplicated but unseparated chromosomes (Fig. 2B and C).



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  		FIG. 2. Scc1 cleavage relieves the inhibition of the metaphase-to-anaphase transition in UV-irradiated mec1{Delta} cells. Logarithmically growing wild-type (wt) (DMP4290/27A), mec1{Delta} (DMP4290/13C), SCC1-TEV (DMP4263/12B), and mec1{Delta} SCC1-TEV (DMP4293/10D) cells, carrying the tetR-GFP/tetO and GAL-TEV constructs, were arrested in G1 with {alpha}-factor in YEP+Raf and released from the pheromone block at time zero in YEP+Raf+Gal to induce the TEV protease production or were UV irradiated (30 J/m2) prior to the release in YEP+Raf+Gal. As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Samples of untreated and UV-treated cell cultures were collected at the indicated times after {alpha}-factor release to analyze the DNA content by FACS in unirradiated (A, top) and UV-irradiated (A, bottom) cell cultures. The kinetics of sister chromatid separation using the tetR-GFP/tetO system was determined by fluorescence microscopy (B), and the kinetics of nuclear division was determined after propidium iodide staining (C). Rad53 protein was detected by Western blot analysis with anti-Rad53 antibodies in UV-treated cell extracts (D). exp, exponentially growing cells.

 
Sister chromatids are held together by cohesin, a multisubunit complex (reviewed in reference 100), and proteolytic cleavage of the Scc1 cohesin subunit at the metaphase-to-anaphase transition is essential for the separation of sister chromatids (102). If the metaphase arrest in UV-irradiated mec1{Delta} cells were due to some physical impediment, such as incompletely replicated DNA molecules not allowing sister chromatid separation, cleavage of Scc1 should not be sufficient to trigger anaphase in these cells. We took advantage of the fact that one Scc1 cleavage site can be exchanged with the recognition sequence for the TEV protease (101), and we generated isogenic wild-type and mec1{Delta} strains carrying both the SCC1-TEV allele at the SCC1 chromosomal locus and a galactose-induced GAL-TEV fusion at the TRP1 locus. Cell cultures were then arrested in G1 with {alpha}-factor and released from the {alpha}-factor block, either unirradiated or after UV irradiation, in medium containing galactose to induce TEV protease production (Fig. 2). As expected, both mec1{Delta} and mec1{Delta} SCC1-TEV cells completed DNA replication about 75 min after UV irradiation, faster than wild-type and SCC1-TEV cells under the same conditions (Fig. 2A, bottom). UV-irradiated mec1{Delta} cells then remained arrested with unseparated sister chromatids and single nuclei until the end of the experiment (Fig. 2B and C), whereas in mec1{Delta} SSC1-TEV cells, sister chromatids started to separate and nuclei started to divide 105 and 180 min after UV irradiation, respectively. It is worth pointing out that cell division in galactose-induced SCC1-TEV and mec1{Delta} SSC1-TEV cells was delayed compared to cell division in wild-type cells, as indicated by the accumulation of cells with 2C DNA content (Fig. 2A) and separated sister chromatids (Fig. 2B and data not shown), and that this delay was independent of UV treatment. This cell cycle delay might be due to activation of the spindle assembly checkpoint triggered by the lack of tension at kinetochores caused by Scc1 cleavage, since Scc1 cleavage has been shown to delay cell division in a Mad2-dependent manner (89, 92). Since Rad53 phosphorylation persisted in galactose-induced, UV-treated SCC1-TEV cells longer than in wild-type cells under the same conditions (Fig. 2D), it is also possible that Rad53 contributes to the cell division delay observed in these cells.

As shown in Fig. 2D, Scc1 cleavage did not seem to affect Rad53 phosphorylation or activation, since both the level and kinetics of Rad53 phosphorylation were similar in mec1{Delta} SCC1-TEV and mec1{Delta} cells after UV irradiation and release in medium containing galactose.

A Tel1/MRX-dependent checkpoint inhibits the metaphase-to-anaphase transition in UV-irradiated mec1{Delta} cells. Since the inability to undergo anaphase of UV-irradiated mec1{Delta} cells requires the cohesins to hold sister chromatids together, this cell cycle arrest likely involves the activation of a checkpoint. Therefore, we asked whether Tel1 and the MRX complex, which were very good candidates for mediating such a checkpoint response, were required for mec1{Delta} cells to arrest in metaphase after UV irradiation in G1. Exponentially growing cells of mec1{Delta} tel1{Delta} and mec1{Delta} mre11{Delta} double mutant strains, together with the appropriate control strains, were arrested in G1, UV irradiated, and allowed to proceed through the cell cycle; {alpha}-factor was added back to the cultures when 95% of the cells had budded (Fig. 3). Unlike UV-treated mec1{Delta} cells, which did not undergo sister chromatid separation and nuclear division throughout the experiment, mec1{Delta} tel1{Delta} and mec1{Delta} mre11{Delta} double mutant cells started to exhibit sister chromatid separation and nuclear division about 105 and 150 min after UV irradiation, respectively (Fig. 3A); cell division followed (data not shown).

Both Tel1 and Mre11 were required in mec1{Delta} cells for UV-induced phosphorylation not only of Rad53 but also of the Rad9 adaptor and of the G2/M checkpoint kinase Chk1. In fact, phosphorylated forms of Rad53, Rad9, and Chk1, which were detected in mec1{Delta} cells 60 to 75 min after UV irradiation (Fig. 3B) concomitantly with completion of DNA replication (data not shown), were absent throughout the experiment in mec1{Delta} tel1{Delta} and mec1{Delta} mre11{Delta} double mutant cells under the same conditions. Thus, both these phosphorylation events and the inhibition of the metaphase-to-anaphase transition in UV-irradiated mec1{Delta} cells involve the activation of a Tel1/MRX-dependent checkpoint. Tel1 was also responsible for the phosphorylation of Mre11, which was detected in mec1{Delta} cells 30 min after UV irradiation (Fig. 3B), indicating that Tel1 is activated in the absence of Mec1 after UV-induced damage.

If Chk1 and Rad53, which act independently from one another to inhibit the G2/M transition after DNA damage in G2 (85), were responsible for the Tel1-dependent metaphase arrest in UV-irradiated mec1{Delta} cells, a rad53{Delta} chk1{Delta} double mutant should be unable to carry on the Tel1-dependent response and should therefore undergo anaphase even in the absence of Mec1 after UV irradiation in G1 and release into the cell cycle. As shown in Fig. 4A, both mec1{Delta} rad53{Delta} chk1{Delta} and rad53{Delta} chk1{Delta} cell cultures completed DNA replication about 90 min after UV irradiation in G1 and release into the cell cycle, similar to mec1{Delta} cells under the same conditions. They then started to exhibit nuclear separation about 120 min after UV irradiation (Fig. 4B), followed by cell division and arrest with 1C DNA content, due to {alpha}-factor readdition, while UV-treated mec1{Delta} cells remained arrested with undivided nuclei throughout the experiment (Fig. 4A and B). It is interesting that the kinetics of cell cycle progression and nuclear division in mec1{Delta} rad53{Delta} chk1{Delta} and rad53{Delta} chk1{Delta} cell cultures were similar to those of mec1{Delta} tel1{Delta} cells (Fig. 4A and B), indicating that Tel1 inhibits the metaphase-to-anaphase transition acting exclusively through Rad53 and Chk1.



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  		FIG. 4. Effects of RAD53, CHK1, and MAD2 deletions on cell cycle progression after UV irradiation in G1. Logarithmically growing wild-type (wt) (K699), mec1{Delta} (YLL490), mec1{Delta} rad53{Delta} (DMP3249/14D), mec1{Delta} chk1{Delta} (DMP3274/1B), mec1{Delta} rad53{Delta} chk1{Delta} (DMP4359/6B), rad53{Delta} chk1{Delta} (DMP4359/4C), mad2{Delta} (SP1070), rad53{Delta} chk1{Delta} mad2{Delta} (DMP4359/9A), mec1{Delta} mad2{Delta} (DMP3262/3C), and mec1{Delta} tel1{Delta} (DMP4062/5C) strains were arrested in G1 with {alpha}-factor and released from the pheromone block at time zero in YEPD or were UV irradiated (30 J/m2) prior to the release in YEPD. As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Samples of untreated and UV-treated cell cultures were collected at the indicated times after {alpha}-factor release to analyze the DNA content by FACS (A). The kinetics of nuclear division after propidium iodide staining (B) and the kinetics of Rad53 phosphorylation by Western blot analysis with anti-Rad53 antibodies (C) were also determined. exp, exponentially growing cells.

 
Both Rad53 and Chk1 contributed to the metaphase arrest of UV-irradiated mec1{Delta} cells, since mec1{Delta} rad53{Delta} and mec1{Delta} chk1{Delta} cell cultures underwent nuclear division with similar kinetics and less efficiently than mec1{Delta} rad53{Delta} chk1{Delta} cells after UV irradiation (Fig. 4A and B). Thus, Chk1 and Rad53 are required to inhibit the metaphase-to-anaphase transition in UV-irradiated mec1{Delta} cells.

The spindle assembly checkpoint protein Mad2 contributes to the G2/M arrest of UV-irradiated mec1{Delta} cells. Some delay in nuclear division was still apparent in cells carrying the mec1{Delta} tel1{Delta}, mec1{Delta} mre11{Delta}, mec1{Delta} rad53{Delta} chk1{Delta}, and rad53{Delta} chk1{Delta} deletions, when they were released into the cell cycle after UV irradiation in G1 (Fig. 3 and 4). These observations suggested that although the Tel1-dependent checkpoint seemed to have a major role in the UV-induced G2/M block in the absence of Mec1, other mechanisms might contribute to this arrest. We reasoned that replication of damaged DNA in the absence of Mec1 could lead to changes in kinetochores, which are assembled at centromeres and mediate spindle microtubule attachment. These changes might in turn activate the Mad2-dependent spindle assembly checkpoint that prevents the onset of anaphase in response to changes in the structure or tension of kinetochores (reviewed in reference 64).

As shown in Fig. 4, Mad2 does not play a role in slowing down progression through the cell cycle in response to UV treatment when the DNA damage checkpoint is fully functional, since wild-type and mad2{Delta} cells exhibited delays in cell cycle progression and phosphorylated Rad53 with very similar kinetics after UV irradiation in G1 and release into the cell cycle. However, the absence of Mad2 advanced nuclear division of rad53{Delta} chk1{Delta} double mutants and partially relieved the metaphase arrest of mec1{Delta} cells after UV irradiation (Fig. 4A and B). In fact, UV-irradiated rad53{Delta} chk1{Delta} mad2{Delta} cells released from G1 arrest underwent nuclear division earlier than UV-treated rad53{Delta} chk1{Delta} cells did (Fig. 4B), while both cell cultures completed DNA replication at the same time (Fig. 4A). Moreover, unlike mec1{Delta} cells, which arrested with undivided nuclei after completion of UV-damaged DNA replication, mad2{Delta} mec1{Delta} cells underwent nuclear division, although much later than the untreated cultures, followed by mitotic exit and cell division (Fig. 4A and B). Rad53 phosphorylation was not affected by the absence of Mad2, since Rad53 was phosphorylated in mec1{Delta} mad2{Delta} cells after UV irradiation and release into the cell cycle (Fig. 4C).

Pds1/securin inactivation bypasses the metaphase arrest of UV-irradiated mec1{Delta} mutant. At the onset of anaphase, a caspase-related protease (separase) destroys the link between sister chromatids by cleaving the cohesin subunit Scc1 (reviewed in reference 100). During most of the cell cycle, separase is kept inactive by binding to an inhibitory protein called Pds1/securin, which is destroyed by ubiquitin-mediated proteolysis shortly before the metaphase-to-anaphase transition (14, 25, 114). One of the mechanisms by which both the DNA damage and spindle assembly checkpoints prevent the metaphase-to-anaphase transition is the inhibition of Pds1 ubiquitination by the ubiquitin ligase anaphase-promoting complex (APC), thus stabilizing the securin. In particular, the spindle assembly checkpoint proteins Bub3, Mad2, and Mad3 physically interact with the APC regulatory subunit Cdc20, required for Pds1 ubiquitination, likely leading to Cdc20 inhibition (reviewed in reference 64). Conversely, Rad53 activation seems to affect the Pds1-Cdc20 interaction, whereas Chk1 is responsible for Pds1 phosphorylation, which in turn appears to inhibit the ubiquitination reaction itself (1, 13).

Therefore, we asked whether Pds1 was involved in the metaphase arrest of UV-irradiated mec1{Delta} cells. When cell cultures were UV irradiated in G1 and allowed to proceed through the cell cycle, UV-treated mec1{Delta} pds1{Delta} cells, which completed DNA replication faster than similarly treated wild-type and pds1{Delta} cells (data not shown), started to exhibit sister chromatid separation (Fig. 5A, top) and nuclear division (Fig. 5A, middle) about 90 and 150 min after release, respectively, while mec1{Delta} cells under the same conditions were still arrested with undivided sister chromatids and nuclei at the end of the experiment. Once the nuclei divided, mec1{Delta} pds1{Delta} cell cultures underwent cell division as indicated by fluorescence-activated cell sorting (FACS) analysis (data not shown) and by the decrease in the percentage of binucleate cells about 300 min after UV irradiation (Fig. 5A, middle). As expected, the absence of Pds1 did not prevent the Tel1-dependent Rad53 phosphorylation, which took place with similar kinetics in mec1{Delta} and mec1{Delta} pds1{Delta} UV-irradiated cells (Fig. 5A, bottom).



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  		FIG. 5. The Pds1 securin mediates the G2/M arrest and undergoes Tel1-dependent phosphorylation and stabilization in UV-treated mec1{Delta} cells. (A) Logarithmically growing cell cultures of wild-type (wt) (DMP4290/27A), mec1{Delta} (DMP4290/13C), pds1{Delta} (SP2894), and mec1{Delta} pds1{Delta} (DMP4290/27B) strains were arrested in G1 with {alpha}-factor and released from the pheromone block at time zero in YEPD or were UV irradiated (30 J/m2) prior to the release in YEPD. As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Samples of UV-treated cell cultures were collected at the indicated times after {alpha}-factor release to analyze the DNA content by FACS (not shown). The kinetics of sister chromatid separation (top) and nuclear division (middle) were determined as described in the legends to Fig. 2B and C. The kinetics of Rad53 phosphorylation by Western blot analysis with anti-Rad53 antibodies (bottom) were also determined. The same procedure was applied to the unirradiated cell cultures (not shown, except for untreated wild type in panel A). (B) Logarithmically growing wild-type (DMP3390/17B), tel1{Delta} (DMP4292/4D), mec1{Delta} (DMP4291/4C), and mec1{Delta} tel1{Delta} (DMP4305/5C) cells, all expressing a Pds1-HA fusion, were treated as described above for panel A. (B) Samples of untreated and UV-treated cell cultures were collected at the indicated times after {alpha}-factor release to determine Pds1 level and phosphorylation by Western blot analysis with antihemagglutinin antibodies (top) and to analyze the DNA content by FACS (bottom). Cell samples taken immediately before UV treatment were taken at time zero. exp, exponentially growing cells.

 
We then monitored the phosphorylation and level of Pds1 of G1-arrested, UV-irradiated wild-type, tel1{Delta}, mec1{Delta}, and mec1{Delta} tel1{Delta} cells carrying a fully functional PDS1-HA tagged allele after the cells were allowed to proceed through the cell cycle. As previously reported (1), Pds1 appeared as a triplet in exponentially growing undamaged cells, with the fastest and constitutive migrating band exhibiting greater intensity than the two slower bands, one of which was weakly detected (Fig. 5B, top). After UV irradiation, the intensities of the two slower bands were expected to increase due to Mec1-dependent phosphorylation (1). As expected, Pds1 was absent in all {alpha}-factor-arrested cell cultures and increased in level when cells progressed through S phase. As shown in Fig. 5B, there was no change in the intensities of the two slower migrating bands of Pds1 in UV-irradiated mec1{Delta} tel1{Delta} double mutant cells. Conversely, the intensities of the two slower bands increased in similarly treated wild-type, tel1{Delta}, and mec1{Delta} cells (Fig. 5B, top), indicating that Tel1 contributed to the DNA damage-induced Pds1 phosphorylation. Moreover, UV-irradiated mec1{Delta} cells, which arrested with 2C DNA content (Fig. 5B, bottom), maintained constant Pds1 levels until the end of the experiment, while the securin was degraded after completion of DNA replication in wild-type, tel1{Delta}, and mec1{Delta} tel1{Delta} cells (Fig. 5B, top). Altogether, these data indicate that Pds1 is necessary for the Tel1-dependent metaphase arrest of UV-irradiated mec1{Delta} cells, where its phosphorylation and stabilization depend on Tel1, suggesting that its persistence may be responsible for the inability of these cells to undergo anaphase.

Signals triggering Tel1/MRX-dependent phosphorylation events in response to UV irradiation. The pathway primarily responsible for removing UV-induced DNA lesions is the nucleotide excision repair (NER) pathway, which is also necessary for checkpoint activation after UV irradiation in both the G1 and G2 phases of the cell cycle (30, 69). In the absence of a functional NER pathway, not only is the removal of UV-induced lesions impaired (79), but replication fork stalling and accumulation of recombination intermediates are also observed (69). Therefore, NER inactivation by deletion of the RAD14 gene, acting at early stages of the pathway, might influence the generation of checkpoint signals leading to Tel1 activation in mec1{Delta} cells. Indeed, when G1-arrested cells were UV irradiated with a very low UV dose (5 J/m2) and then allowed to proceed through the cell cycle, Rad53 phosphorylation was detected in mec1{Delta} rad14{Delta} cells about 75 min after UV irradiation (Fig. 6A, bottom), concomitantly with completion of S phase in the presence of irreparable DNA damage (Fig. 6A, middle). Conversely, since small amounts of UV-induced DNA lesions are rapidly repaired when the NER pathway is functional, the extent of Rad53 phosphorylation under the same conditions was very low in wild-type cells and even lower in mec1{Delta} cells (Fig. 6A, bottom). As expected, this low UV dose led rad14{Delta} cells to arrest in S phase with high levels of phosphorylated Rad53 (Fig. 6A, middle and bottom) due to the activation of a Mec1-dependent checkpoint (69). Wild-type, mec1{Delta}, and rad14{Delta} mec1{Delta} cells under the same conditions completed DNA replication with similar kinetics (Fig. 6A, middle). Strikingly, rad14{Delta} mec1{Delta} cells then arrested with 2C DNA content (Fig. 6A, middle), persistent Rad53 phosphorylation (Fig. 6A, bottom), and undivided nuclei (data not shown), while wild-type and mec1{Delta} cells underwent nuclear and cell division, as indicated by the reappearance of cells with 1C DNA content (Fig. 6A, middle). Both Rad53 phosphorylation and the metaphase arrest of rad14{Delta} mec1{Delta} cells were dependent on Tel1, since similarly treated rad14{Delta} mec1{Delta} tel1{Delta} triple mutant cells did not phosphorylate Rad53 and underwent cell division (Fig. 6A). Thus, RAD14 deletion appears to amplify the UV-induced signals leading to Tel1 activation in the absence of Mec1. Since the absence of Rad14 causes the complete loss of NER proteins at the damaged site, possibly by affecting the structure of the complexes involved in lesion recognition (79), these data also indicate that NER processing of UV-induced lesions is not required to activate the Tel1-dependent checkpoint.



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  		FIG. 6. Effects of NER inactivation and defective ssDNA signaling on Tel1-dependent checkpoint response. (A) Logarithmically growing wild-type (wt) (K699), rad14{Delta} (YLL355), mec1{Delta} (YLL490), rad14{Delta} mec1{Delta} (DMP3942/6C), and rad14{Delta} mec1{Delta} tel1{Delta} (DMP3942/20B) cells were arrested in G1 with {alpha}-factor and released from the pheromone block at time zero in YEPD, either untreated or after irradiation with 5 J/m2 UV dose prior to the release in YEPD. As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Cell samples were collected at the indicated times after {alpha}-factor release to analyze the DNA content by FACS in unirradiated (top) and UV-irradiated (middle) cell cultures and to evaluate Rad53 phosphorylation (bottom) in UV-treated cell extracts as described in the legend to Fig. 1B. (B) Logarithmically growing wild-type (DMP4290/27A), tel1{Delta} (DMP4062/19A), mec1{Delta} (DMP4290/13C), and rfa1-t11 (DMP4313/11C) cells were synchronized with {alpha}-factor ({alpha}f) and resuspended in YEPD containing 3 µg of {alpha}-factor per ml after UV irradiation (30 J/m2) (+ {alpha}-factor). Protein extracts prepared from cell samples collected at the indicated times were subjected to Western blot analysis with anti-Rad53 antibodies. (C) G1-arrested wild-type (DMP4290/27A), rfa1-t11 (DMP4313/11C), mec1{Delta} (DMP4290/13C), and rfa1-t11 mec1{Delta} (DMP4313/7B) cells were UV irradiated (30 J/m2) and released from the G1 block at time zero. As in Fig. 1, when 95% cells had budded after release, 3 µg of {alpha}-factor per ml was added back to all cultures. Cell samples were collected at the indicated times after {alpha}-factor release to evaluate Rad53 phosphorylation as described above for panel B. Cell samples taken immediately before UV treatment were taken at time zero. exp, exponentially growing cells.

 
Although it has been shown that RPA-coated ssDNA represents one of the signals activating the Mec1 kinase in response to DSBs induced by HO endonuclease (27, 45, 54, 115), it is still unclear whether UV-induced lesions in G1 need to be processed to ssDNA in order to be sensed by the checkpoint. To address this question, we used the rfa1-t11 mutant, because its alteration in the 70-kDa subunit of the RPA complex renders cells sensitive to UV radiation and impairs loading of checkpoint proteins onto damaged DNA, thus delaying checkpoint activation (103, 115). As shown in Fig. 6B, when G1-arrested cells were kept in G1 by {alpha}-factor addition after UV irradiation, rfa1-t11 cells exhibited defective phosphorylation of Rad53, which was normally phosphorylated in similarly treated wild-type cells, suggesting that ssDNA generation is necessary for the checkpoint machinery to detect UV-induced DNA lesions in G1. Moreover, Rad53 phosphorylation under these conditions was completely abolished in mec1{Delta} cells, while it still took place in tel1{Delta} cells (Fig. 6B), indicating that Mec1 is strictly required to respond to UV-induced lesions during the G1 phase.

In order to determine whether RPA-coated ssDNA is responsible for the Tel1-dependent Rad53 phosphorylation observed in UV-irradiated mec1{Delta} cells after the cells were allowed to proceed through the cell cycle, we released UV-irradiated mec1{Delta} rfa1-t11 double mutant cells and the appropriate control strains from G1 arrest. As shown in Fig. 6C, phosphorylated Rad53 appeared immediately in wild-type cells and 30 min after {alpha}-factor release in rfa1-t11 cells, while it was detected only 75 min after release in both rfa1-t11 mec1{Delta} and mec1{Delta} cells, concomitantly with S-phase completion (data not shown). Importantly, the level of phosphorylated Rad53 was significantly lower in the rfa1-t11 mec1{Delta} double mutant cells than in mec1{Delta} cells (Fig. 6C), suggesting that RPA-coated ssDNA may be part of the signals activating the Tel1-dependent G2/M checkpoint response.

Tel1-dependent checkpoint activation after UV irradiation requires DNA replication and occurs concomitantly with completion of the S phase. As shown in Fig. 7A, Rad53 phosphorylation could also be detected concomitantly with S-phase completion when mec1{Delta} cells were allowed to proceed through the cell cycle after UV irradiation in G2. Under these conditions, phosphorylated Rad53 appeared in mec1{Delta} cells about 105 min after release from the G2 block, when cells completed the subsequent DNA replication (Fig. 7A). This response was dependent on Tel1, since similarly treated mec1{Delta} tel1{Delta} cells did not undergo Rad53 phosphorylation at any time after release (data not shown). Moreover, these Tel1-dependent phosphorylation events required entry into the cell cycle. In fact, no phosphorylated Rad53 was detected when G2-arrested mec1{Delta} cells were held in G2 by the addition of nocodazole after UV irradiation (Fig. 7B), while wild-type cells were able to phosphorylate Rad53 immediately after UV irradiation in G2, independently of progression through the cell cycle (Fig. 7A and B).



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  		FIG. 7. Rad53 phosphorylation in mec1{Delta} cells requires DNA replication of UV-damaged DNA. (A and B) Logarithmically growing wild-type (wt) (K699) and mec1{Delta} (YLL490) cells were synchronized in G2 with nocodazole (noc) and UV irradiated (30 J/m2). Half of each culture was released from the G2 block in YEPD (A) or held in G2 by the addition of nocodazole (B). Cell samples were collected at the indicated times after UV irradiation to analyze the DNA content by FACS (A, top) and to evaluate Rad53 phosphorylation (A, bottom, and B) as described in the legend to Fig. 1B. (C) Logarithmically growing wild-type (K699), mec1{Delta} (YLL490), GAL-CDC6 (DMP4083/11C), and mec1{Delta} GAL-CDC6 (DMP4083/6B) cells, carrying a GAL-CDC6 fusion as the only source of Cdc6 protein, were grown in YEP+Gal and arrested in G2 with nocodazole. All arrested cultures were then transferred to YEPD in the presence of nocodazole for 1 h and then released from the G2 block into YEPD containing {alpha}-factor (G1 block). Finally, they were released from the {alpha}-factor block into YEPD, either unirradiated or after UV irradiation with 30 J/m2. Samples were taken at the indicated times after {alpha}-factor release ({alpha}f) to analyze the DNA content by FACS (C, top) and to evaluate Rad53 phosphorylation (C, bottom) in UV-treated cell extracts as described in the legend to Fig. 1. exp, exponentially growing cells.

 
The observation that Rad53 phosphorylation in UV-treated mec1{Delta} cells occurred concomitantly with the completion of damaged DNA replication, independently of whether the damage was generated in G1 or G2, suggested that S-phase entry and/or progression might be required for Tel1-dependent checkpoint activation. To further explore this possibility, we blocked initiation of DNA replication by preventing prereplicative complex (pre-RC) assembly through depletion of the Cdc6 protein (12, 77). To this end, isogenic wild-type and mec1{Delta} strains, both carrying an integrated copy of a galactose-induced GAL-CDC6 fusion (78) as the sole Cdc6 source, were blocked in G2 with nocodazole in medium containing galactose and transferred to medium containing nocodazole and glucose for 1 h to completely repress GAL-CDC6. Cell cultures were then released from the nocodazole arrest by transfer into fresh medium containing glucose in the presence of {alpha}-factor, followed by release from {alpha}-factor under glucose-repressed conditions of the G1-arrested cells, either unirradiated or after UV irradiation. As expected, GAL-CDC6 and mec1{Delta} GAL-CDC6 cells did not undergo DNA replication under these conditions independently of UV irradiation (Fig. 7C, top and middle), and the appearance of cells with less than 1C DNA content about 105 to 120 min after {alpha}-factor release indicated that they eventually underwent mitosis in the absence of DNA replication as previously described (77). Rad53 phosphorylation, which took place immediately after UV irradiation in wild-type and GAL-CDC6 cells and concomitantly with S-phase completion in UV-treated mec1{Delta} cells, was not detected in UV-irradiated mec1{Delta} GAL-CDC6 cells throughout the experiment (Fig. 7C, bottom).

Thus, UV-induced DNA damage requires entry into S phase to trigger Tel1-mediated Rad53 phosphorylation, suggesting that UV-damaged DNA needs to be replicated in order to be able to activate the Tel1-dependent checkpoint.

Links between cyclin-CDK complexes and Tel1-dependent Rad53 phosphorylation. In both budding yeast (S. cerevisiae) and fission yeast (S. pombe), entry and progression into S and M phases require activation of the CDK1/Cdc28/Cdc2 cyclin-dependent kinase bound to S-phase- and M-phase-specific B-type cyclins (reviewed in reference 63). In budding yeast, the S-phase Clb5-CDK1 and Clb6-CDK1 complexes trigger the firing of origins that had previously formed pre-RCs, while activation of M-phase Clb1-, Clb2-, Clb3-, and Clb4-CDK1 complexes promote the formation of a mitotic spindle and APC activation. The observation that Tel1-dependent checkpoint activation in UV-irradiated mec1{Delta} cells occurs concomitantly with completion of S phase suggests that it may require Clb-CDK1 complexes specifically active at the end of S phase. If this were the case, ectopic Clb-CDK1 activation in G1 by overexpression of the major mitotic cyclin gene CLB2 might be expected to trigger premature Tel1-dependent Rad53 phosphorylation after UV irradiation.

To explore this possibility, G1-arrested wild-type and mec1{Delta} cells, either lacking or carrying one (GAL-CLB2) or seven (7XGAL-CLB2) integrated copies of a galactose-induced GAL-CLB2 fusion (96), were UV irradiated and allowed to proceed through cell cycle in the presence of galactose to induce CLB2 expression from the GAL promoter. In all untreated cell cultures, CLB2 overexpression did not induce Rad53 phosphorylation (Fig. 8A, bottom). Phosphorylated Rad53 appeared immediately after UV irradiation in all strains expressing functional Mec1 (Fig. 8A, bottom), independent of the different kinetics of S-phase entry and progression, which were delayed by GAL-CLB2 expression (Fig. 8A, top). Conversely, all UV-irradiated mec1{Delta} strains completed DNA replication about 75 to 90 min after release, faster than the MEC1 strains (Fig. 8A, top). Strikingly, phosphorylated Rad53 was detected about 60 and 30 min after release in mec1{Delta} GAL-CLB2 and mec1{Delta} 7XGAL-CLB2 cells, respectively, while it was detected in similarly treated mec1{Delta} cells only after 90 min (Fig. 8A, bottom), when cells had completed DNA replication and reached the G2 phase (Fig. 8A, top). The advanced Rad53 phosphorylation induced by high levels of Clb2 was still dependent on Tel1, since both tel1{Delta} mec1{Delta} GAL-CLB2 and tel1{Delta} mec1{Delta} 7XGAL-CLB2 cells did not phosphorylate Rad53 in response to UV irradiation after induction of CLB2 overexpression (data not shown). Thus, ectopic CLB2 overexpression triggers UV-induced Rad53 phosphorylation in the absence of Mec1 even if cells have not yet completed replication of damaged DNA, indicating that these phosphorylation events are likely stimulated by a mitotic CDK.



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  		FIG. 8. Perturbations of Clb/CDK activity influence checkpoint response to UV irradiation. (A) Wild-type (wt) (YLL1072), GAL-CLB2 (DMP4340/5C), 7XGAL-CLB2 (DMP4341/10B), mec1{Delta} (DMP4225/4C), GAL-CLB2 mec1{Delta} (DMP4340/1C), and 7XGAL-CLB2 mec1{Delta} (DMP4341/6C) cells, exponentially growing in YEP+Raf, were arrested in G1 with {alpha}-factor, UV irradiated, and then released from {alpha}-factor in YEP+Raf+Gal to induce CLB2 overexpression (+ gal). Cell samples were collected at the indicated times after {alpha}-factor release to analyze the DNA content in UV-treated cell cultures by FACS (top) and to analyze Rad53 phosphorylation in both untreated and UV-treated cell cultures (bottom) as described in the legend to Fig. 1. (B) Logarithmically growing wild-type (DMP4337/20C) and mec1{Delta} (DMP4337/14C) cells, all carrying three copies of the GAL-SIC1{Delta}NT fusion integrated in the genome (GAL-SIC1), were arrested in G1 with {alpha}-factor and released from the pheromone block in medium containing raffinose and nocodazole (15 µg/ml) (+raf +noc) (top) either untreated (not shown) or after UV irradiation (30 J/m2). Galactose was added to half of each G2 nocodazole-arrested cell culture 210 min after {alpha}-factor release to induce GAL-SIC1{Delta}NT expression (+gal +noc) (B, bottom). Cell samples were collected at the indicated times after {alpha}-factor release for further analysis. The DNA content in both untreated (not shown) and UV-treated cell cultures was analyzed by FACS (B, left). Rad53 and Mre11 phosphorylation in both untreated (not shown) and UV-treated cell cultures was analyzed as described in the legends to Fig. 1B and C (B, right). No significant differences were detected in the FACS profiles in the different unirradiated cell cultures, which did not reveal any Rad53 and Mre11 phosphorylation throughout the experiment. exp, exponentially growing cells.

 
If unscheduled Tel1-dependent checkpoint activation caused by Clb2 overproduction required Clb/CDK1 kinase activity, Clb/CDK1 inhibition should influence UV-induced Rad53 and/or Mre11 phosphorylation. Previous studies have shown that overexpression of a truncated form of the Clb/CDK1 inhibitor Sic1 (Sic1{Delta}NT), lacking 50 N-terminal amino acids that target Sic1 for degradation, efficiently inhibits Clb/CDK1 activity (18, 71). Therefore, we analyzed the effects caused by overexpressing the galactose-induced GAL-SIC1{Delta}NT allele on UV-induced Rad53 and Mre11 phosphorylation in wild-type and mec1{Delta} cells (Fig. 8B). To this end, cell cultures exponentially growing in raffinose were arrested in G1 with {alpha}-factor, UV irradiated, and first released from G1 in medium containing raffinose in the presence of nocodazole, to allow DNA replication and Clb/CDK1 activation, followed by G2 arrest. After DNA replication was completed and cells were arrested in G2, SIC1{Delta}NT expression from the GAL promoter was induced in half of each cell culture by the addition of galactose. As shown in Fig. 8B, Rad53 was phosphorylated immediately after UV irradiation in wild-type cells, and 120 min later, concomitantly with S-phase completion, in mec1{Delta} cells. Phosphorylated Rad53 was still apparent after 300 min in both cell types in the absence of galactose (Fig. 8B, top), while it started to decrease with similar kinetics in both strains about 15 min after GAL-SIC1{Delta}NT induction (Fig. 8B, bottom). Thus, Clb/CDK1 activity is required to maintain Rad53 phosphorylation and activation both in the presence and absence of Mec1; therefore, some components of the Mec1- and Tel1-dependent checkpoint networks are likely activated by these CDKs. Interestingly, Clb/CDK1 inhibition did not affect Mre11 phosphorylation, which, as expected, was detected in UV-treated mec1{Delta} cells 30 min after release, concomitantly with S-phase entry, and persisted throughout the experiment independent of GAL-SIC1{Delta}NT induction (Fig. 8B).


   DISCUSSION
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 Discussion
References
 
In S. cerevisiae, Mec1 plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while Tel1 functions in the DNA damage response are inferred from the finding that its absence increases the sensitivity of mec1 mutants to genotoxic agents (62, 84), it impairs the checkpoint response to phleomycin treatment during S phase (67), and it allows the G2/M transition after DNA damage in a dominant-negative MEC3 mutant (29). Moreover, altering DSB processing in mec1{Delta} cells by deletion of SAE2 or non-null mutations in the RAD50 and MRE11 genes triggers Tel1/MRX-dependent Rad53 phosphorylation and interaction with Rad9 after MMS and hydroxyurea treatment, suggesting that accumulation of DNA lesions that specifically activate Tel1 might take place under these conditions (104). In this work, we have tried to gain insights into the roles of Tel1 in DNA damage checkpoint activation, as well as into the nature of the signals detected by this kinase.

Checkpoint-dependent metaphase arrest after UV irradiation in the absence of Mec1. We found that UV irradiation in the absence of Mec1 leads to metaphase arrest and concomitant phosphorylation of the adaptor protein Rad9 and of the downstream checkpoint kinases Rad53 and Chk1. All these events require both Tel1 and Mre11, indicating that a Tel1/MRX-dependent checkpoint, whose activation results in the inhibition of the metaphase-to-anaphase transition, can be activated in response to UV irradiation in the absence of Mec1.

Although Tel1 and MRX can be activated in response to UV irradiation in the absence of Mec1, tel1{Delta} cells show neither sensitivity to DNA-damaging agents nor defects in Rad53 phosphorylation after UV irradiation in any cell cycle stage, suggesting that activation of the Tel1 kinase becomes apparent only in the absence of Mec1. Although Tel1 associates with DSBs in an Xrs2-dependent manner in the presence of Mec1 (68), it may be excluded from the sites of damage by Mec1 when processing of UV-induced lesions generates ssDNA, which is known to be recognized by the Mec1-Ddc2 complex (115). Consistent with this hypothesis, Tel1 and Mec1 are localized to telomeric ends in a reciprocal manner, with Mec1 preferentially bound to chromosomal ends when ssDNA was generated (97). In this view, Tel1 might be recruited to sites of UV-induced lesions only in the absence of Mec1, thus leading to Mre11, Rad9, Rad53, and Chk1 phosphorylation and checkpoint activation. Since Tel1-dependent Rad53 phosphorylation also occurs in mec1kd mutants with defective kinase (our unpublished observation), the absence of Mec1 kinase activity appears to be sufficient to allow Tel1/MRX activation, suggesting that Mec1 may regulate Tel1 association with damaged DNA through phosphorylation events.

However, since Mec1 is required to maintain the integrity of the replication forks under stress conditions, as well as for processive DNA replication in the presence of hydroxyurea and MMS (52, 91, 98), we cannot exclude the possibility that replication of damaged templates in the absence of functional Mec1 results in the production of potentially lethal events that trigger a Tel1-dependent checkpoint response. Indeed, the checkpoint pathway plays a role during DNA replication even during unperturbed conditions. In fact, defects in the checkpoint genes acting during S phase in the presence of DNA damage increased the rates of gross chromosome rearrangements, suggesting that one normal role of S-phase checkpoint functions is to facilitate nonmutagenic repair of the DNA damage that normally occurs during DNA replication (40, 65, 66).

We show that the Rad53 and Chk1 kinases are required for the Tel1-dependent metaphase arrest of UV-irradiated mec1{Delta} cells, since the kinetics of nuclear division of UV-treated mec1{Delta} tel1{Delta}, rad53{Delta} chk1{Delta}, and mec1{Delta} rad53{Delta} chk1{Delta} cells were similar. Nonetheless, UV-irradiated mec1{Delta} tel1{Delta}, rad53{Delta} chk1