![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
Are Involved in the Dissolution of Sister Chromatids
Molecular Cell Biology Laboratory, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan,1 Ludwig-Maximilians-Universitat, Lehrstuhl fur Tieranatomie II, Veterinarstrasse 13, D-80539 Munchen, Germany,2 Biosystems Science, School of Advanced Sciences, The Graduate University for Advanced Studies, Shonan Village, Hayama, Kanagawa 240-0193, Japan,3 Department of Medical Genetics and Radiation Biology, Graduate School of Medicine, Osaka University, Suita, Osaka 565-0871, Japan,4 Shujitsu University, School of Pharmacy, Nishigawara, Okayama 703-8516, Japan,5 Tohoku University 21st Century COE Program "Comprehensive Research and Education Center for Planning of Drug Development and Clinical Evaluation," Sendai, Miyagi 980-8578, Japan6
Received 24 April 2006/ Returned for modification 26 May 2006/ Accepted 2 June 2006
 |
ABSTRACT |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
(Top3). However, the physiological relevance of the interaction between BLM and Top3 within the cell remains unclear. We show here that Top3 depletion causes accumulation of cells in G2 phase, enlargement of nuclei, and chromosome gaps and breaks that occur at the same position in sister chromatids. The transition from metaphase to anaphase is also inhibited. All of these phenomena except cell lethality are suppressed by BLM gene disruption. Taken together with the biochemical properties of BLM and Top3, these data indicate that BLM and Top3 execute the dissolution of sister chromatids. |
INTRODUCTION |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
and Top3ß (8, 22, 23). Knocking out the Top3 gene in mice results in embryonic lethality (17), while knocking out Top3ß does not affect development but reduces the life span (15). Various Top3 and RecQ helicase molecules have been reported to interact physically, including Top3 and BLM (35), one of the RecQ family helicases in higher eukaryotic cells (3). BLM is a causative gene for Bloom's syndrome (3), which is an autosomal disorder characterized by predisposition to a wide variety of cancers (6). Biochemical analyses have suggested that BLM and Top3 together affect the in vitro resolution of a recombination intermediate containing a double Holliday junction (HJ) via a double-junction dissolution mechanism (34). However, the phenotypes of cells that lack Top3 have not been characterized precisely, since TOP3 knockout is lethal. Furthermore, the phenotypes of Top3-depleted cells before they die have not been examined. Moreover, the physiological relevance of the interaction between BLM and Top3 within the cell remains unclear. Therefore, elucidating higher eukaryotic Top3 function may enhance our understanding of the physiological roles of BLM.
In this study, to assess the function of Top3 and its interactions with BLM, we constructed cells whose expression of Top3 can be switched off by doxycycline hydrochloride (Dox) treatment. To our knowledge, we present the first evidence to support the hypothesis that vertebrate Top3 together with the BLM helicase executes the dissolution of sister chromatids during DNA replication.
|
MATERIALS AND METHODS |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
and TOP3ß cDNAs were obtained by PCR from a 
ZAPII chicken cDNA library using primers designed from the human TOP3 and TOP3ß gene sequences. The terminal regions of the cDNAs were obtained from chicken testis RNA by 3' or 5' rapid amplification of cDNA ends. Genomic DNA fragments of the TOP3 and -ß genes were amplified by long-range PCR using genomic DNA from DT40 cells. Targeting constructs used to disrupt TOP3 were made by replacing the region encoding the active site of Top3 with a neomycin or histidinol selection marker cassette. Targeting constructs for TOP3ß disruption were made in an analogous manner using a puromycin or blastidin selection marker cassette. Targeting constructs used to disrupt WRN and BLM have been described previously (12, 29). To construct an expression plasmid carrying mouse TOP3 cDNA (22) with the tet-off promoter, a cDNA encoding mouse FLAG-tagged Top3 was inserted into the pUHG10-3 vector. Gene disruption. DT40 cells (1 x 107) were electroporated with a Gene Pulser (Bio-Rad, Hercules, CA) at 550 V and 25 µF in the presence of 30 µg linearized targeting constructs. Drug-resistant colonies were selected in 96-well plates with medium containing 2 mg/ml neomycin, 1 mg/ml histidinol, 0.5 µg/ml puromycin, or 30 µg/ml blastidin. The disruption of the targeted gene(s) was checked by Southern blotting, genomic PCR, and reverse transcriptase PCR. The genotypes of all of the cell lines used in this study are listed in Table 1.
|
polyclonal antibody, followed by a horseradish peroxidase-conjugated anti-rabbit immunoglobulin G (IgG) (New England Biolabs) secondary antibody. Bands were visualized using ECL detection reagents (Amersham Pharmacia Biotech). Growth curve. Cells (2 x 104) were inoculated and cultured at 39.5°C for the specified time periods. Cell samples were counted, and the growth rates were estimated from these data.
Cell cycle analysis. Cells (1 x 106) were washed with PBS and then processed with a Cycle TEST PLUS DNA reagent kit (Becton Dickinson). Cells were filtered through a nylon mesh and analyzed by FACScan (Becton Dickinson). The data obtained were processed by Cell Quest (Becton Dickinson).
Immunocytochemistry. Immunofluorescent staining of whole cells was performed as follows. Cells were collected onto slides with a cytocentrifuge, fixed in 3% paraformaldehyde in PBS for 15 min at room temperature, permeabilized in 0.5% Triton X-100 in PBS for 15 min at room temperature, rinsed three times in 0.5% bovine serum albumin, and incubated for 1 h at 37°C with rabbit anti-MCM4, mouse monoclonal anti-BrdU, and rabbit anti-Ser10-phosphorylated histone H3. Binding of primary antibodies was then detected using fluorescein isothiocyanate (FITC)-conjugated goat or rabbit anti-mouse IgG or Cy3-conjugated goat anti-rabbit IgG. Nuclei were counterstained with 0.2 µg/ml DAPI (4',6'-diamidino-2-phenylindole).
Detection of chromosome aberrations. Cells were cultured in the presence of 0.1 µg/ml colcemid for 2 h. The cells were harvested and treated with 75 mM KCl for 11 min at room temperature and fixed with methanol/acetic acid (3:1) for 30 min. The cell suspension was dropped onto ice-cold wet glass slides and air dried. The cells were stained with 2% Giemsa solution at pH 6.8 for 25 min and examined by light microscopy.
FISH analysis. Chromosomal aberrations were examined using fluorescent in situ hybridization (FISH) analysis. The macrochromosomes, 1, 2, 3, 4, 5, and Z, and other smaller chromosomes were distinguished by FISH analysis after three-colored painting of chromosomes, as described previously (7). Chromosomes were counterstained with 0.2 µg/ml DAPI.
Detection of early apoptotic cells by flow cytometry. Apoptotic cells were detected using Vybrant apoptosis assay kit no. 3 (Molecular Probes). Cells (1 x 106) were washed with ice-cold PBS and suspended in 100 µl annexin-binding buffer, and 5 µl of FITC-conjugated annexin V and 1 µl of 100 µg/ml PI were added to the cell suspension. After a 15-min incubation at room temperature, 400 µl annexin-binding buffer was added to the cell suspension, and cells were filtered through nylon mesh and analyzed by FACScan. The obtained data were processed with Cell Quest software (Becton Dickinson).
Nucleotide sequence accession numbers.
The chicken TOP3 and -ß cDNA sequences have been deposited in GenBank with accession numbers AB215104 and AB215105, respectively.
|
RESULTS |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
is an essential component of vertebrate cells.
To understand the physiological roles of vertebrate Top3s, we generated TOP3 and -ß knockout cells using chicken DT40 cells (Fig. 1a and c) (33). The TOP3ß–/– DT40 cells were constructed by transfecting two TOP3ß targeting vectors sequentially into wild-type DT40 cells. The TOP3–/– cells were generated by first disrupting one TOP3 genomic locus, transfecting the cells with a plasmid expressing the FLAG-tagged mouse TOP3 gene from the tet-off promoter, and then disrupting the second TOP3 genomic locus. Treatment of these cells with Dox suppresses the expression of the mTop3 protein and results in cells with the TOP3–/– genotype. This circumvents the lethality problem engendered by disruption of both alleles of the TOP3 gene. Gene disruption was confirmed by Southern blotting (Fig. 1b and d).
|
–/– and TOP3–/–/TOP3ß–/– cells grew at the same rate, which was slightly lower than that of wild-type cells. However, both TOP3–/– and TOP3–/–/TOP3ß–/– cells ceased growing within 3 days after Dox addition (Fig. 2A, panel b) The amount of mTop3 in the cells was markedly decreased after Dox treatment (Fig. 2B).
|
-depleted cells arrest primarily in G2 phase.
Flow cytometric analysis indicated that the cell cycle distribution of TOP3–/– cells was largely unchanged over 2 days following the addition of Dox (Fig. 3A). However, on the third day, the peak corresponding to G2/M phase became prominent and cells containing less DNA than G1 cells appeared. These changes increased markedly by the fourth day. To investigate the point of cell cycle arrest more precisely, the proportion of cells in different cell cycle stages was quantified using cell cycle marker-specific antibodies. Thus, cells in G1 to S phase, S phase, and late G2 phase were identified by the presence of nuclear staining of MCM4, BrdU, and Ser-10-phosphorylated histone H3, respectively. There was a remarkable increase in the proportion of cells that did not stain with any of these antibodies 3 days after the addition of Dox (Fig. 3B). These data suggest that depletion of Top3 caused a large population of the cells to arrest in G2 phase before histone H3 was phosphorylated. Moreover, the nuclei of a significant number of the cells were enlarged after the 3-day incubation with Dox and disrupted nuclei were also observed. On the fourth day, there was a marked increase in the number of cells with disrupted nuclei (Fig. 3C).
|
.
Although the number of M-phase cells was decreased by the depletion of Top3, a small but appreciable number of metaphase cells continued to be detected on the third and fourth days after Dox addition. When we examined the chromosomes of these cells, we found a significant increase in chromosome-type gaps and breaks (Table 2 and Fig. 4A, panels b and c). An increase in chromatid-type gaps and breaks was also observed in the Top3-depleted cells but was much smaller than the increase in chromosome-type aberrations. Top3 deficiency also led to the emergence of metaphase cells with highly aberrant karyotypes which could not be scored (Fig. 4A, panel d, and Table 2). In these cells, the 12 macrochromosomes found in typical metaphase DT40 cells (Fig. 4A, panel a) could not be identified. Chromosome painting revealed the fragmentation of the macrochromosomes in these Top3-depleted metaphase cells (Fig. 4B). Similar chromosome-type gaps and breaks have been observed in Rad51- or Mre11-depleted cells (25, 36), but almost no "unscorable" metaphase cells were detected in these cells (Table 3). The extent of chromosomal aberrations found in Top3-depleted cells was much higher than that found in Rad51- or Mre11-depleted cells.
|
|
|
-depleted cells, inhibition of the transition from G2 to M phase and from metaphase to anaphase is released by the protein kinase inhibitor 2-aminopurine.
We next examined the transition from metaphase to anaphase in Top3-depleted cells by scoring anaphase cells that had condensed chromosomes and dephoshorylated histone H3. The Top3 depletion markedly decreased the ratio of cells in anaphase to the total number of mitotic cells, suggesting that the metaphase-to-anaphase transition was inhibited (Fig. 5A). This result is consistent with the dramatic increase in chromosome-type gaps and breaks, which indicates that double-strand breaks are occurring at the same position in sister chromatids. These data suggest a possibility that the decatenation of sister chromatids is defective in Top3-depleted cells, reflecting an impediment at a very late stage of DNA replication. If the decatenation of sister chromatids is impaired, the decatenation checkpoint should be activated in Top3-depleted cells. Treatment with 2-aminopurine (2-AP), an inhibitor of protein kinases, has been reported to override G2 arrest induced by the failure of DNA topoisomerase II (Topo II)-dependent decatenation (1). To assess whether TOP3–/– cells are arrested in G2 by a 2-AP-sensitive checkpoint mechanism, TOP3–/– cells were treated with 2-AP after a 3-day incubation with Dox. The proportion of cells in M phase increased remarkably within 1 h after the addition of 2-AP and then declined after 3 h (Fig. 5B). In addition, the number of cells containing irregular-shaped, but not enlarged, nuclei increased 3 h after the addition of 2-AP (data not shown). This result further supports the hypothesis that Top3 depletion may result in an ineffective decatenation of sister chromatids. The reduced metaphase-to-anaphase transition of Dox-treated TOP3–/– cells may also be explained by a defect in the decatenation of sister chromatids. The 2-AP treatment decreased the frequency of enlarged nuclei induced by Dox treatment (Fig. 5C), presumably due to entry into M phase, as described above.
|
–/–/BLM–/–, TOP3ß–/–/BLM–/–, and TOP3–/–/WRN–/– double mutant cells.
To elucidate the functional relevance of Top3s and RecQ helicases, we generated TOP3–/–/BLM–/–, TOP3ß–/–/BLM–/–, and TOP3–/–/WRN–/– double mutant cells. WRN, which encodes a RecQ family helicase, is the causative gene of Werner syndrome, which is characterized by premature aging (37). Disruption of BLM and WRN was confirmed by PCR (Fig. 1e and f). While TOP3–/–/BLM–/– and TOP3–/–/WRN–/– cells proliferated at a slightly lower rate than TOP3–/– cells in the absence of Dox, the viable cell numbers of all three Top3-deficient cell lines declined after the addition of Dox. However, the cessation of growth of the TOP3–/–/BLM–/– cells was slightly delayed compared with that of the TOP3–/– or TOP3–/–/WRN–/– cells (Fig. 2A, panel c). This suggests that unlike WRN, BLM functionally interacts with Top3. TOP3ß–/–/BLM–/– cells grew at the same rate as BLM–/– cells (Fig. 2A, panel a). These data indicate that BLM but not WRN functionally interacts with Top3, while BLM and Top3ß do not functionally interact.
Effect of disruption of the BLM gene on the phenotypes of TOP3–/–-depleted cells.
Since our data showed that the disruption of BLM did not suppress the lethality of Top3-depleted cells but did delay the decline of viable cell numbers, we examined in detail the effect of disruption of BLM on the phenotypes of Top3-depleted cells. The mechanism of cell death in TOP3–/–, TOP3–/–/BLM–/–, or TOP3–/–/WRN–/– cells was examined by flow cytometry. In the case of TOP3–/–/BLM–/– cells, the number of cells that could be stained with an anti-annexin V antibody was markedly increased 4 days after the addition of Dox. In contrast, Dox treatment of TOP3–/– and TOP3–/–/WRN–/– cells did not lead to a significant increase of annexin V-positive cells (Fig. 6A). These observations indicate that Top3-depleted BLM–/– cells died from apoptosis, while Top3-depleted cells and Top3-depleted WRN–/– cells seem to die by a nonapoptotic pathway.
|
-depleted cells was also suppressed by the additional disruption of BLM but was not affected by the disruption of WRN (Fig. 6B). Both the number of S-phase cells and the number of cells with enlarged nuclei after the addition of Dox were quantified. While TOP3–/– cells showed a decrease in the number of DNA-synthesizing cells and the emergence of enlarged nuclei after Dox treatment for 48 h, TOP3–/–/BLM–/– cells showed a decrease in S-phase cells only after 78 h and no enlarged nuclei even after 96 h (Fig. 6C). The disruption of BLM also suppressed the G2 arrest (data not shown) and metaphase arrest observed in Top3-depleted cells (Fig. 5A). Moreover, the disruption of BLM in Top3-depleted cells remarkably reduced the frequency of chromosome-type aberrations and, more importantly, nearly completely suppressed the emergence of "unscorable" metaphase cells which contained chromosomal aberrations too severe to allow us to identify individual chromosomes (Table 2). |
DISCUSSION |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
depletion causes accumulation of cells in G2 phase, enlargement of nuclei, chromosome gaps and breaks that occur at the same position in sister chromatids, and the emergence of "unscorable" metaphase cells. Additionally, the transition from metaphase to anaphase is inhibited. All of these phenotypes were suppressed by BLM gene disruption in Top3-depleted cells.
DNA damage or the failure of sister chromatid dissolution (described below) may cause cells to arrest in G2 phase, and prolonged arrest in G2 may subsequently trigger the enlargement of nuclei, similar to that seen with Mre11-depleted cells (36). However, the size of the nuclei is extremely enlarged in Top3-depleted cells, and thus, it seems likely that packaging of interphase chromatin is somehow affected by the absence of Top3. This issue must be addressed in future studies.
The result of chromosome aberrations in Top3-depleted cells is the appearance of chromosome-type gaps and breaks, resulting in a high frequency of "unscorable" metaphase cells. As far as we know, the appearance of "unscorable" metaphase cells is a phenotype unique to Top3-depleted cells because almost no "unscorable" metaphase cells were detected in Rad51- or Mre11-depleted cells (Table 3). Based on the results obtained in this study, we present two possible mechanisms of BLM and Top3 function, as follows.
Dissolution of sister chromatids. It has been reported that catenanes are formed after DNA replication and are decatenated by Topo II (10, 26). In addition, silencing of human Topo II by small intefering RNA leads to a defect in the dissolution of sister chromatids in human cells (21). Thus, it seems likely that Topo II plays a major role in the dissolution of sister chromatids in eukaryotic cells.
A decade ago, an alternative process for the dissolution of sister chromatids was proposed. An in vitro study using Escherichia coli proteins indicated that Topo III efficiently decatenates precatenanes and that when this decatenation does not occur, the daughter molecules (catenanes) remain catenated after DNA replication is completed (9). In addition, it has been proposed that S. cerevisiae Sgs1 creates a deleterious topological substrate that Top3 preferentially resolves and that such substrates should be considered precatenanes (5). There is no direct evidence to prove the involvement of Top3 in the dissolution of sister chromatids in the cell. However, our data obtained in this study seem to strongly support the hypothesis that both Top3 and BLM are involved in the dissolution of sister chromatids in vertebrate cells.
If this is the case, the termination intermediates that normally arise when replication forks converge are effectively processed by Top3 and BLM, resulting in the dissolution of sister chromatids. Thus, it is speculated that in the absence of Top3, the termination intermediates created by BLM are not resolved properly to result in activation of the decatenation checkpoint and/or the formation of double-strand breaks in both sister chromatids, which are observed as chromosome-type aberrations and "unscorable" metaphase cells (Fig. 7A). In the absence of BLM, however, catenanes are formed after DNA replication and can be decatenated by Topo II.
|
and BLM resolve sister chromatids is compatible with the above model that Sgs1 and Top3 collapse the pseudo double HJs to restore the replication fork. However, this function of Sgs1 is appreciated only under damage-induced conditions, and the phenomena observed in this study occurred under non-damage-induced conditions. In assessing all of these data, we prefer the former scenario as a model for the mechanism of action of BLM and Top3 in the dissolution of sister chromatids but do not necessarily exclude the second scenario.
Why the lethality of Top3-depleted cells cannot be suppressed by disruption of BLM.
Previous studies demonstrated that the lethality of the top3 mutation in fission yeast was suppressed by deletion of the rqh1 gene (31). However, disruption of the BLM gene did not rescue the lethality of the Top3 depletion in chicken DT40 cells, although several phenotypes observed in the Top3-depleted cells were suppressed by disruption of BLM. This discrepancy may be due to the fact that vertebrate cells carry multiple RecQ helicases (4) which presumably together conduct the functions of the sole RecQ helicases found in unicellular organisms. We have previously suggested that both RECQL1 and RECQL5 may partially replace the function of BLM when it is absent in DT40 cells (30). In addition, human RECQL1 or mouse RECQL5 is required for efficient suppression of sister chromatid exchange even in the presence of their BLM counterparts (11, 16). Given that human RECQL1 interacts with Top3 (14) and human RECQL5 interacts with both Top3 and Top3ß (24), it seems likely that vertebrate RECQL1, RECQL5, and BLM perform some overlapping functions. Thus, it is speculated that the partial replacement of BLM function by RECQL1 and/or RECQL5 is responsible for the incomplete suppression by BLM deletion of the lethality induced by the loss of TOP3. In spite of the suppression of several phenotypes observed in the Top3-depleted cells by disruption of BLM, TOP3–/–/BLM–/– cells are still inviable, while BLM–/– cells are viable. Thus, it seems likely that Top3 has additional roles separate from those of BLM. Indeed, we previously showed that budding yeast Top3 has roles independent of those of Sgs1 (20). Taken together, the inability to suppress the lethality of Top3-depleted cells may be due to the combined effect of the presence of multiple RecQs and the defect in additional roles of Top3 independent of those of BLM.
In summary, this is the first detailed report of the phenotype of Top3-depleted higher eukaryotic cells and has revealed possible roles played by BLM and Top3 in replication. The results obtained in this study help us to understand the function of Top3 and BLM at the molecular level and account for the chromosome instability of Bloom's syndrome cells that arises from defects in BLM function.
|
ACKNOWLEDGMENTS |
|---|
This work was supported by Grants-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, Sports and Culture of Japan and by Health Sciences Research grants from the Ministry of Health and Welfare of Japan.
|
FOOTNOTES |
|---|
|
REFERENCES |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
2. Duno, M., B. Thomsen, O. Westergaard, L. Krejci, and C. Bendixen. 2000. Genetic analysis of the Saccharomyces cerevisiae Sgs1 helicase defines an essential function for the Sgs1-Top3 complex in the absence of SRS2 or TOP1. Mol. Gen. Genet. 264:89-97.[CrossRef][Medline]
3. Ellis, N. A., J. Groden, T. Z. Ye, J. Straughen, D. J. Lennon, S. Ciocci, M. Proytcheva, and J. German. 1995. The Bloom's syndrome gene product is homologous to RecQ helicases. Cell 83:655-666.[CrossRef][Medline]
4. Enomoto, T. 2001. Functions of RecQ family helicases: possible involvement of Bloom's and Werner's syndrome gene products in guarding genome integrity during DNA replication. J. Biochem. 129:501-507.
5. Gangloff, S., J. P. McDonald, C. Bendixen, L. Arthur, and R. Rothstein. 1994. The yeast type I topoisomerase Top3 interacts with Sgs1, a DNA helicase homolog: a potential eukaryotic reverse gyrase. Mol. Cell. Biol. 14:8391-8398.
6. German, J. 1993. Bloom syndrome: a Mendelian prototype of somatic mutational disease. Medicine 72:393-406.[Medline]
7. Habermann, F. A., M. Cremer, J. Walter, G. Kreth, J. von Hase, K. Bauer, J. Wienberg, C. Cremer, T. Cremer, and I. Solovei. 2001. Arrangements of macro- and microchromosomes in chicken cells. Chromosome Res. 9:569-584.[CrossRef][Medline]
8. Hanai, R., P. R. Caron, and J. C. Wang. 1996. Human TOP3: a single-copy gene encoding DNA topoisomerase III. Proc. Natl. Acad. Sci. USA 93:3653-3657.
9. Hiasa, H., R. J. DiGate, and K. J. Marians. 1994. Decatenating activity of Escherichia coli DNA gyrase and topoisomerases I and III during oriC and pBR322 DNA replication in vitro. J. Biol. Chem. 269:2093-2099.
10. Holm, C., T. Stearns, and D. Botstein. 1989. DNA topoisomerase II must act at mitosis to prevent nondisjunction and chromosome breakage. Mol. Cell. Biol. 9:159-168.[Medline]
11. Hu, Y., X. Lu, E. Barnes, M. Yan, H. Lou, and G. Luo. 2005. Recql5 and Blm RecQ DNA helicases have nonredundant roles in suppressing crossovers. Mol. Cell. Biol. 25:3431-3442.
12. Imamura, O., K. Fujita, C. Itoh, S. Takeda, Y. Furuichi, and T. Matsumoto. 2002. Werner and Bloom helicases are involved in DNA repair in a complementary fashion. Oncogene 21:954-963.[CrossRef][Medline]
13. Ira, G., A. Malkova, G. Liberi, M. Foiani, and J. E. Haber. 2003. Srs2 and Sgs1-Top3 suppress crossovers during double-strand break repair in yeast. Cell 115:401-411.[CrossRef][Medline]
14. Johnson, F. B., D. B. Lombard, N. F. Neff, M. A. Mastrangelo, W. Dewolf, N. A. Ellis, R. A. Marciniak, Y. Yin, R. Jaenisch, and L. Guarente. 2000. Association of the Bloom syndrome protein with topoisomerase III in somatic and meiotic cells. Cancer Res. 60:1162-1167.
15. Kwan, K. Y., and J. C. Wang. 2001. Mice lacking DNA topoisomerase IIIß develop to maturity but show a reduced mean lifespan. Proc. Natl. Acad. Sci. USA 98:5717-5721.
16. LeRoy, G., R. Carroll, S. Kyin, M. Seki, and M. D. Cole. 2005. Identification of RecQL1 as a Holliday junction processing enzyme in human cell lines. Nucleic Acids Res. 33:251-6257.
17. Li, W., and J. C. Wang. 1998. Mammalian DNA topoisomerase III is essential in early embryogenesis. Proc. Natl. Acad. Sci. USA 95:010-1013.
18. Liberi, G., G. Maffioletti, C. Lucca, I. Chiolo, A. Baryshnikova, C. Cotta-Ramusino, M. Lopes, A. Pellicioli, J. E. Haber, and M. Foiani. 2005. Rad51-dependent DNA structures accumulate at damaged replication forks in sgs1 mutants defective in the yeast ortholog of BLM RecQ helicase. Genes Dev. 19:39-350.
19. Mullen, J. R., V. Kaliraman, S. S. Ibrahim, and S. J. Brill. 2001. Requirement for three novel protein complexes in the absence of the Sgs1 DNA helicase in Saccharomyces cerevisiae. Genetics 157:03-118.
20. Onodera, R., M. Seki, A. Ui, Y. Satoh, A. Miyajima, F. Onoda, and T. Enomoto. 2002. Functional and physical interaction between Sgs1 and Top3 and Sgs1-independent function of Top3 in DNA recombination repair. Genes Genet. Syst. 77:1-21.[CrossRef][Medline]
21. Sakaguchi, A., and A. Kikuchi. 2004. Functional compatibility between isoform and ß of type II DNA topoisomerase. J. Cell Sci. 117:047-1054.
22. Seki, T., M. Seki, T. Katada, and T. Enomoto. 1998. Isolation of a cDNA encoding mouse DNA topoisomerase III which is highly expressed at the mRNA level in the testis. Biochim. Biophys. Acta 1396:27-131.[Medline]
23. Seki, T., M. Seki, R. Onodera, T. Katada, and T. Enomoto. 1998. Cloning of cDNA encoding a novel mouse DNA topoisomerase III (Topo IIIß) possessing negatively supercoiled DNA relaxing activity, whose message is highly expressed in the testis. J. Biol. Chem. 273:28553-28556.
24. Shimamoto, A., K. Nishikawa, S. Kitao, and Y. Furuichi. 2000. RecQ5ß, a large isomer of RecQ5 DNA helicase, localizes in the nucleoplasm and interacts with topoisomerases 3 and 3ß. Nucleic Acids Res. 28:1647-1655.
25. Sonoda, E., M. S. Sasaki, J. M. Buerstedde, O. Bezzubova, A. Shinohara, H. Ogawa, M. Takata, Y. Yamaguchi-Iwai, and S. Takeda. 1998. Rad51-deficient vertebrate cells accumulate chromosomal breaks prior to cell death. EMBO J. 17:598-608.[CrossRef][Medline]
26. Uemura, T., H. Ohkura, Y. Adachi, K. Morino, K. Shiozaki, and M. Yanagida. 1987. DNA topoisomerase II is required for condensation and separation of mitotic chromosomes in S. pombe. Cell 50:917-925.[CrossRef][Medline]
27. Ui, A., Y. Satoh, F. Onoda, A. Miyajima, M. Seki, and T. Enomoto. 2001. The N-terminal region of Sgs1, which interacts with Top3, is required for complementation of MMS sensitivity and suppression of hyper-recombination in sgs1 disruptants. Mol. Genet. Genomics 265:837-850.[CrossRef][Medline]
28. Wallis, J. W., G. Chrebet, G. Brodsky, M. Rolfe, and R. Rothstein. 1989. A hyper-recombination mutation in S. cerevisiae identifies a novel eukaryotic topoisomerase. Cell 58:409-419.[CrossRef][Medline]
29. Wang, W., M. Seki, Y. Narita, E. Sonoda, S. Takeda, K. Yamada, T. Masuko, T. Katada, and T. Enomoto. 2000. Possible association of BLM in decreasing DNA double strand breaks during DNA replication. EMBO J. 19:3428-3435.[CrossRef][Medline]
30. Wang, W., M. Seki, Y. Narita, T. Nakagawa, A. Yoshimura, M. Otsuki, Y. Kawabe, S. Tada, H. Yagi, Y. Ishii, and T. Enomoto. 2003. Functional relation among RecQ family helicases RecQL1, RecQL5, and BLM in cell growth and sister chromatid exchange formation. Mol. Cell. Biol. 23:3527-3535.
31. Win, T. Z., A. Goodwin, I. D. Hickson, C. J. Norbury, and S. W. Wang. 2004. Requirement for Schizosaccharomyces pombe Top3 in the maintenance of chromosome integrity. J. Cell Sci. 117:4769-4778.
32. Win, T. Z., H. W. Mankouri, I. D. Hickson, and S. W. Wang. 2005. A role for the fission yeast Rqh1 helicase in chromosome segregation. J. Cell Sci. 118:5777-5784.
33. Winding, P., and M. W. Berchtold. 2001. The chicken B cell line DT40: a novel tool for gene disruption experiments. J. Immunol. Methods 249:1-16.[CrossRef][Medline]
34. Wu, L., and I. D. Hickson. 2003. The Bloom's syndrome helicase suppresses crossing over during homologous recombination. Nature 426:870-874.[CrossRef][Medline]
35. Wu, L., S. L. Davies, P. S. North, H. Goulaouic, J. F. Riou, H. Turley, K. C. Gatter, and I. D. Hickson. 2000. The Bloom's syndrome gene product interacts with topoisomerase III. J. Biol. Chem. 275:9636-9644.
36. Yamaguchi-Iwai, Y., E. Sonoda, M. S. Sasaki, C. Morrison, T. Haraguchi, Y. Hiraoka, Y. M. Yamashita, T. Yagi, M. Takata, C. Price, N. Kakazu, and S. Takeda. 1999. Mre11 is essential for the maintenance of chromosomal DNA in vertebrate cells. EMBO J. 18:6619-6629.[CrossRef][Medline]
37. Yu, C. E., J. Oshima, Y. H. Fu, E. M. Wijsman, F. Hisama, R. Alisch, S. Matthews, J. Nakura, T. Miki, S. Ouais, and G. M. Martin. 1996. Positional cloning of the Werner's syndrome gene. Science 272:258-262.[Abstract]
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| J. Bacteriol. | J. Virol. | Eukaryot. Cell |
|---|
| Microbiol. Mol. Biol. Rev. | Clin. Vaccine Immunol. | All ASM Journals |
|---|
