![[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}
Dora B. Krimer, Jorge B. Schvartzman, and Pablo Hernández*
Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, 28040 Madrid, Spain
Received 7 July 2003/ Returned for modification 11 August 2003/ Accepted 25 September 2003
 |
ABSTRACT |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
|
INTRODUCTION |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
In Saccharomyces cerevisiae rDNA, protein Fob1 is required for RFB activity (19), although it is still unknown whether it arrests rDNA replication by binding to the RFB sites or through a different mechanism. Functional RFBs are required for HOT1 recombination, contraction and expansion of the rDNA repeat number, and the formation of extrachromosomal ribosomal circles (17-20), suggesting that RFB activity stimulates recombination occurring at the rDNA locus in this budding yeast (2). On the other hand, it has been recently shown that RFBs and HOT1 recombination are independent activities although they share cis-acting sequences (41).
In mouse rDNA, replication forks stall at the rRNA transcriptional terminator elements known as Sal boxes (27), which are the specific binding sites for transcription termination factor mTTF-1 (12, 21). This protein was able to arrest replication forks in an in vitro replication assay (10, 35). These in vivo and in vitro results suggest that Sal boxes and mTTF-1 block replication forks with the opposite polarity as they direct transcription termination. A protein factor that specifically binds to 27-bp repeated sequences located at the barrier has been proposed also to be involved in the RFBs of pea rDNA (28).
Contrary to what happens in the mouse, neither the rRNA transcription termination factor Reb1p nor its rDNA binding sequence seems to be involved in the RFB of S. cerevisiae (4, 41). These observations suggest that the molecular mechanism that regulates rDNA replication arrest diverged through evolution. In the present work, we found three independent closely spaced RFBs in the fission yeast rDNA. Two of these RFBs required both the transcription termination factor reb1p and its two binding sites near the 3' end of the 25S gene, whereas the other RFB functioned in the absence of these cis- and trans-acting factors. Therefore, Schizosaccharomyces pombe could be a transitional species in which the mechanisms operating in budding yeast and mammals coexist.
|
MATERIALS AND METHODS |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
::kanMX6+), and D8 (h- leu1-32 ura4-d18 ade6-M210 reb1+). Media and growth conditions used were as previously described (31). All pIRT2-derived plasmids containing S. pombe rDNA fragments were transformed by electroporation (34). Escherichia coli strain DH5
was used for recombinant DNA procedures.
Construction of plasmids containing rDNA sequences.
Plasmid pBL1263, containing a complete repeat of S. pombe rDNA (23), was the source of the sequences analyzed for RFB activity. Autonomously replicating plasmid pIRT2 (14) was used as a vector. For convenience, the HindIII insert in pBL1263 was first cloned into the HindIII site of pUC18 generating plasmid pHH10.4. Plasmids pIRT1.6(+) and pIRT1.6(-) were obtained by inserting the blunt-ended 1.6-kb XhoI-HindIII rDNA fragment from pHH10.4 into the SmaI site of pIRT2. The (+) plasmids contained the insert in the orientation expected to block fork progression. Construction of plasmid pRebs was carried out as follows. A PCR product was obtained using primer 1 (5'-CCCCTGCAGTTTTGAAGAGATAAAAGG-3') and primer 2 (5'-CCCGGATCCTTTTACTAGGATTTGTGC-3') on pIRT1.6(+). Each primer contained 18 nucleotides (underlined) that annealed a few nucleotides upstream from the 5' reb1p binding sequence (primer 1) or downstream from the 3' reb1p binding sequence (primer 2), a PstI (primer 1) or BamHI (primer 2) restriction site and a CCC tail. This fragment was PstI+BamHI digested and inserted into the polycloning site of pIRT2 near ars1. Thus, the 229-bp fragment cloned in pRebs contained, in addition to both reb1p binding sites and the sequence between them, 15 bp upstream from the 5' binding sequence and 14 bp downstream from the 3' binding sequence. For construction of plasmids pRebBS(+) and pRebBS(-), a double-stranded oligonucleotide containing the 17-bp reb1p binding site (5'-AGGTAAGGGTAATGCAC-3') (44) was inserted in both orientations into the PstI and BamHI sites of the pIRT2 polycloning site. To direct the ligation to the desired orientation, the oligonucleotide contained PstI and BamHI sticky ends. Appropriate insertion was checked by sequencing. For the construction of plasmid pRebsBS, a PCR product was obtained by using oligonucleotides 5'-AAGGCCTAAATCCTAGTAAAAGGATC-3' and 5'-AAGGCCTTTTCCCTTCAAAAAG-3'. These primers annealed divergently next to each side of the reb1p binding sequence closer to ars1 in pRebs (underlined nucleotides) and contained a StuI site at their 5' ends (boldface nucleotides). After digestion with StuI, the PCR product was ligase-mediated circularized. For the construction of pRebsSEP, first a StuI site was created by PCR between both reb1p binding sequences of plasmid pRebs. This StuI site was located 60 bp from the binding site closer to ars1 and at 106 bp from the other binding site. Then, the 787-bp EcoRV-NruI fragment from the pBR322 tetracycline resistance gene was cloned into this StuI site.
Deletion of reb1+ gene.
Deletion of the reb1+ gene was achieved by PCR-mediated replacement of the complete open reading frame by the kanMX6+ gene (1). PCRs were performed with two 84-nucleotide-long primers (5'-GATATTAGCG ATTGATAAGT TGAAGTGATT ACTCAATTAT AGTACTTCAA AAATATAATC CGCCAGGGTT TTCCCAGTCA CGAC-3' and 5'-ATTGTAAGGA CGTCAATTGG AGAATCCAGA AAGTACCACT TTAAAGTCAT CAATGGCTGA AGCGGATAAC AATTTCACAC AGGA-3'), where the first 60 nucleotides (underlined) corresponded to sequences flanking the reb1+ open reading frame. The remaining 24 nucleotides of each primer corresponded to sequences located at either side of the pBluescript SK(+) polycloning site, where kanMX6+ was cloned (a gift from S. Moreno). Transformation of S. pombe 117x118 diploid strain with the PCR fragment and selection of G418-resistant diploids were performed as described previously (1). Genomic DNA from selected transformants was digested with EcoRV, electrophoresed, and hybridized with a probe specific for kanMX6+ gene to confirm its integration. reb1+/reb1 diploids were induced to form spores to allow further analysis by tetrad separation. The four spores from selected asci were again checked for integration of the kanMX6+ gene as described above (see Fig. 5C).
|
|
RESULTS |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
100 copies of rRNA genes organized in two arrays near both ends of chromosome III (29, 33). Two 17-bp binding sequences for the transcription termination factor reb1p, separated by 166 bp, are present in the nontranscribed spacer (NTS) close to the 3' end of the 25S gene (Fig. 1) (44). Replication of S. pombe rDNA was analyzed by 2D agarose gel electrophoresis. DNA from exponentially growing strain 972 h- cells was digested with the restriction enzymes indicated, separated in 2D gels, transferred, and hybridized with probes specific for a series of overlapping fragments covering the rDNA repeat (Fig. 1, fragments A through E). Analysis of fragment A showed a spot corresponding to an accumulated Y-shaped replication intermediate (Fig. 1A, arrow), confirming previous observations of a replication barrier in S. pombe rDNA (26, 37). However, the elongated appearance of this signal suggested that forks stalled at several sites rather than at a single site. 2D gel analysis of the overlapping fragment B supported this possibility. Three independent spots were identified on the descending portion of the simple-Y arc (Fig. 1B), indicating that replication stalled at three alternative sites, herein called RFB1, RFB2, and RFB3. Equivalent spots on the ascending portion of the arc were absent, indicating that, as in other species, these three RFBs were polar, arresting only forks moving against the direction of transcription (leftwards in the map of Fig. 1). The intensities of the spots of accumulated replication intermediates were clearly different. The strongest signal corresponded to the first pausing site that leftward moving forks encounter (RFB1). The middle site, RFB2, gave the weakest signal, and RFB3 produced a signal of intermediate intensity. Replication analysis of fragment C (HindIII-SalI) showed no spots of accumulated intermediates (Fig. 1C), indicating that DNA sequences involved in fork stalling were located to the left of the HindIII site. Two additional fragments, corresponding to the coding region, revealed no replication impediment, as uniform simple-Y arcs were observed (Fig. 1D and E).
|
All three S. pombe RFBs are active in autonomously replicating plasmids with the same polarity and relative efficiency as in the chromosome. As described above, S. pombe RFBs are located in the NTS, 5' to the HindIII site. Therefore, the 1.6-kb XhoI-HindIII restriction fragment of S. pombe rDNA (Fig. 2A) was cloned in both orientations close to the ars1 replication origin of vector pIRT2 (Fig. 2B). This fragment contained the NTS portion lying to the left of the HindIII site and the 3' end of the 25S gene. In pIRT1.6(+) the inserted sequence is replicated in the direction in which the barriers are active in the chromosome, whereas in pIRT1.6(-) the insert is replicated in the opposite direction. Replication of these plasmids in strain 35 was analyzed by 2D gel electrophoresis after double digestion with PvuII and EcoRV, using a specific probe to detect the fragment containing the insert. Locating the insert close to ars1 ensured that the clockwise-moving fork would reach the RFBs before the fork moving counterclockwise entered the fragment analyzed. If the RFBs were active, simple-Y-shaped intermediates of this fragment would accumulate while the counterclockwise fork replicated the other fragment. The results obtained are shown in Fig. 2C and D. All three RFBs observed in the chromosomal context were also detected in pIRT1.6(+), visualized as three spots of accumulated intermediates on the Y arc (Fig. 2C). The relative intensities of these spots fitted well with those observed in the chromosome. No arrest sites were detected in pIRT1.6(-) (Fig. 2D), indicating that all three RFBs retained the same polarity as in the chromosomal context. No additional barriers were detected within the rightward HindIII-BamHI fragment of the NTS (not shown).
|
Location of RFB1. The spots of accumulated intermediates showed in Fig. 2 appeared on the simple-Y arc to the right of the inflexion point. This indicated that RFBs were located closer to the HindIII site. Therefore, to locate the DNA sequence required for RFB1, we analyzed a fragment spanning 383 bp next to the HindIII site (Fig. 3A). This fragment was cloned into pIRT2 and two restriction fragments of the resulting plasmid (p3'Rebs) (Fig. 3B) were analyzed. 2D gels showed a single strong spot of a specific Y-shaped intermediate corresponding to RFB1 (Fig. 3C and D). Replication termination structures were also visible (Fig. 3C and D), indicating termination at the barrier.
|
BS and only the spot closer to the inflexion of the Y arc remained (Fig. 4E). These results indicated that reb1p binding sites are essential cis-acting signals for RFB2 and RFB3 and that they function independently. When both binding sites were placed further apart by inserting a 787-bp sequence between them (Fig. 4C), the two fork arrest positions also appeared to be separated from each other as indicated by the new relative locations of the spots in the corresponding 2D gel (compare panels F and D of Fig. 4). In addition, we found that the 17-bp sequence was not only necessary but also sufficient for fork arrest. A synthetic sequence identical to the reb1p binding site inserted into pIRT2 (Fig. 4G) induced fork arrest in the (+) orientation (Fig. 4H) and had no effect in the opposite (-) orientation (Fig. 4I).
|
To analyze the function of S. pombe reb1p in RFB activity, we constructed a heterozygous reb1+/reb1 diploid strain. The null allele was obtained by PCR-mediated gene replacement of reb1+ by the selectable gene marker kanMX6+, which confers resistance to the antibiotic G418 (Fig. 5A). Dissections of three representative tetrads of the G418-resistant reb1+/reb1 diploids are shown in Fig. 5B. In the absence of G418 all four spores were viable, indicating that reb1+ is not an essential gene in S. pombe (Fig. 5B, left panel). However, two spores of each tetrad gave colonies slightly smaller than the other two. In all cases the smaller colonies were those generated by reb1::kanMX6+ spores, as they grew in the presence of G418 (Fig. 5B, right panel). Replacement of reb1+ by kanMX6+ was confirmed by Southern blotting (Fig. 5C) and PCR analysis (not shown). Therefore, deletion of reb1+ gene was not lethal, but mutant cells grew somewhat more slowly than wild-type cells. This small effect of reb1+ deletion on cell growth was confirmed by dilution assays (data not shown).
To address the involvement of reb1p in the barriers, replication of pIRT1.6(+) containing all three RFBs was analyzed in isogenic reb1+ and reb1 haploid strains. As shown in Fig. 5D and E, in the reb1 strain the spots corresponding to RFB2 and RFB3 disappeared and only the one generated by RFB1 remained. Similar results were obtained upon analysis of plasmids pRebs and pRebBS(+) (data not shown). This observation demonstrates that the transcription termination factor reb1p is required for RFB2 and RFB3.
|
DISCUSSION |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
In this work we have identified three RFBs in the S. pombe rDNA. Fork arrest at two of these RFBs (RFB2 and RFB3) is produced upon binding of the transcription termination protein reb1, in a fashion similar to what has been proposed to occur in mouse (27, 35). Removal of the reb1p binding sequence or deletion of reb1+ completely abolished replication blockage at RFB2 and RFB3 (Fig. 4E and 5E). Therefore, upon binding to its cognate sequence, reb1p inhibits both rRNA transcription and rDNA replication occurring with opposite directions, thus preventing head-on collision of both machineries. It cannot be ruled out that other factor(s) besides reb1p are also required for fork arrest at RFB2 and RFB3. On the other hand, fork arrest at RFB1 is independent of this transcription termination factor, since it remains active in a reb1 mutant strain (Fig. 5E). Moreover, we have found a reb1p-unrelated protein that specifically binds to a short sequence within the RFB1-containing fragment analyzed in Fig. 3 (unpublished data). Thus, S. pombe RFB1 seems to be similar to the two barriers found in budding yeast, as both of them are normal in strains with a temperature-sensitive allele of REB1 growing at restrictive temperature (41).
Therefore, two different and independent mechanisms operate in S. pombe rDNA to arrest replication forks. Remarkably, although different trans- and cis-acting factors are involved in these two mechanisms, both of them block replication progression in a polar fashion. This indicates that polarity of rDNA barriers is an essential feature in accomplishing their biological role, which may be to avoid or regulate head-on collision between transcription and replication.
Besides the RFBs present in the rDNA, another barrier, named RTS1, has been described for S. pombe. RTS1 is involved in the mating-type switching by determining the direction of replication at the mat1 locus (6, 7). As in the case of rDNA RFBs, RTS1 is a polar barrier that contains a cluster of three full-length and one truncated 60-bp imperfect direct repeats (7). Interestingly, each of these repeats includes a sequence that shows homology to the 17-bp reb1p binding sequence required for RFB2 and RFB3. Moreover, according to our findings, the orientation of these homologous sequences is in agreement with the reported polarity of RTS1 (7). This observation raises the possibility that reb1p also plays a role in fork arrest at the mat1 locus.
It is interesting that whereas REB1 is an essential gene in S. cerevisiae (16), deletion of reb1+ in S. pombe has only a weak effect on cell growth (Fig. 5B and D). Besides its function in RNA polymerase I transcription termination (22, 36), S. cerevisiae Reb1p regulates the expression of several unrelated RNA polymerase II transcribed genes (see reference 40 and references therein). In addition, Reb1p binding sites are also present in the subtelomeric X and Y' regions in budding yeast (8). Thus, the essential nature of Reb1p in S. cerevisiae could be due to the function(s) that this protein performs at these additional sites, but not at the rDNA locus. In agreement with this hypothesis, it was recently shown that deletion of the Reb1p binding site in all rDNA chromosomal repeats of S. cerevisiae has no effect on cell growth or rRNA synthesis (39). Altogether, these results suggest that, at least in these two species, in the absence of the cis- or trans-acting factors currently known to be involved in rRNA transcription termination, rRNA transcripts are terminated by an alternative unknown pathway and processed properly to form functional ribosomes. It would be worthy to investigate if factors involved in S. pombe RFB1 play a role in this alternative pathway.
|
ACKNOWLEDGMENTS |
|---|
This work was partially supported by Spanish grants 99/0850 from the Fondo de Investigación Sanitaria (Ministerio de Sanidad y Consumo), SAF2001-1740 from the Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica (Ministerio de Ciencia y Tecnología), PB98-048 from the Comisión Interministerial de Ciencia y Tecnología (CICYT), and a fellowship from the Consejería de Educación de la Comunidad de Madrid-Fondo Social Europeo to A.S.-G.
|
FOOTNOTES |
|---|
Present address: Department of Pathology, Medical School, Northwestern University, Chicago, IL 60610.
|
REFERENCES |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
2. Benguría, A., P. Hernández, D. B. Krimer, and J. B. Schvartzman. 2003. Sir2p suppresses recombination of replication forks stalled at the replication fork barrier of ribosomal DNA in Saccharomyces cerevisiae. Nucleic Acids Res. 31:893-898.
3. Brewer, B. J., and W. L. Fangman. 1988. A replication fork barrier at the 3' end of yeast ribosomal RNA genes. Cell 55:637-643.[CrossRef][Medline]
4. Brewer, B. J., D. Lockshon, and W. L. Fangman. 1992. The arrest of replication forks in the rDNA of yeast occurs independently of transcription. Cell 71:267-276.[CrossRef][Medline]
5. Caddle, M. S., and M. P. Calos. 1994. Specific initiation at an origin of replication from Schizosaccharomyces pombe. Mol. Cell. Biol. 14:1796-1805.
6. Dalgaard, J. Z., and A. J. S. Klar. 2000. swi1 and swi3 perform imprinting, pausing, and termination of DNA replication in S. pombe. Cell 102:745-751.[CrossRef][Medline]
7. Dalgaard, J. Z., and A. J. S. Klar. 2001. A DNA replication-arrest site RTS1 regulates imprinting by determining the direction of replication at mat1 in S. pombe. Genes Dev. 15:2060-2068.
8. Fourel, G., E. Revardel, C. E. Koering, and E. Gilson. 1999. Cohabitation of insulators and silencing elements in yeast subtelomeric regions. EMBO J. 18:2522-2537.[CrossRef][Medline]
9. Friedman, K. L., and B. J. Brewer. 1994. The analysis of replication intermediates by two-dimensional agarose gel electrophoresis. Methods Enzymol. 262:613-627.
10. Gerber, J.-K., E. Göegel, C. Berger, M. Wallisch, F. Müller, I. Grummt, and F. Grummt. 1997. Termination of mammalian rDNA replication: polar arrest of replication fork movement by transcription termination factor TTF-I. Cell 90:559-567.[CrossRef][Medline]
11. Gomez, M., and F. Antequera. 1999. Organization of DNA replication origins in the fission yeast genome. EMBO J. 18:5683-5690.[CrossRef][Medline]
12. Grummt, I., U. Maier, A. Ohrlein, N. Hassouna, and J. P. Bachellerie. 1985. Transcription of mouse rDNA terminates downstream of the 3' end of 28S RNA and involves interaction of factors with repeated sequences in the 3' spacer. Cell 43:801-810.[CrossRef][Medline]
13. Hernández, P., L. Martín-Parras, M. L. Martínez-Robles, and J. B. Schvartzman. 1993. Conserved features in the mode of replication of eukaryotic ribosomal RNA genes. EMBO J. 12:1475-1485.[Medline]
14. Hindley, J., G. Phear, M. Stein, and D. Beach. 1987. Suc11encodes a predicted 13-kilodalton protein that is essential for cell viability and is directly involved in the division cycle of Schizosaccharomyces pombe. Mol. Cell. Biol. 7:504-511.
15. Huberman, J. A., L. D. Spotila, K. A. Nawotka, S. M. El-Assouli, and L. R. Davis. 1987. The in vivo replication origin of the yeast 2 microns plasmid. Cell 51:473-481.[CrossRef][Medline]
16. Ju, Q. D., B. E. Morrow, and J. R. Warner. 1990. REB1, a yeast DNA-binding protein with many targets, is essential for growth and bears some resemblance to the oncogene myb. Mol. Cell. Biol. 10:5226-5234.
17. Kaeberlein, M., M. McVey, and L. Guarente. 1999. The SIR2/3/4 complex and SIR2 alone promote longevity in Saccharomyces cerevisiae by two different mechanisms. Genes Dev. 13:2570-2580.
18. Kobayashi, T., D. J. Heck, M. Nomura, and T. Horiuchi. 1998. Expansion and contraction of ribosomal DNA repeats in Saccharomyces cerevisiae: requirement of replication fork blocking (Fob1) protein and the role of RNA polymerase I. Genes Dev. 12:3821-3830.
19. Kobayashi, T., and T. Horiuchi. 1996. A yeast gene product, Fob1 protein, required for both replication fork blocking and recombinational hotspot activities. Genes Cells 1:465-474.[Abstract]
20. Kobayashi, T., M. Nomura, and T. Horiuchi. 2001. Identification of DNA cis elements essential for expansion of ribosomal DNA repeats in Saccharomyces cerevisiae. Mol. Cell. Biol. 21:136-147.
21. Kuhn, A., I. Bartsch, and I. Grummt. 1990. Specific interaction of the murine transcription termination factor TTF I with class-I RNA polymerases. Nature 344:559-562.[CrossRef][Medline]
22. Lang, W. H., and R. H. Reeder. 1993. The REB1 site is an essential component of a terminator for RNA polymerase I in Saccharomyces cerevisiae. Mol. Cell. Biol. 13:649-658.
23. Lapeyre, B., B. Michot, J. Feliu, and J. P. Bachellerie. 1993. Nucleotide sequence of the Schizosaccharomyces pombe 25S ribosomal RNA and its phylogenetic implications. Nucleic Acids Res. 21:3322.
24. Linskens, M. H. K., and J. A. Huberman. 1988. Organization of replication of ribosomal DNA in Saccharomyces cerevisiae. Mol. Cell. Biol. 8:4927-4935.
25. Little, R. D., T. H. K. Platt, and C. L. Schildkraut. 1993. Initiation and termination of DNA replication in human rRNA genes. Mol. Cell. Biol. 13:6600-6613.
26. López-Estraño, C., J. B. Schvartzman, and P. Hernández. 1997. The replication of ribosomal RNA genes in eukaryotes. Chromosomes Today 12:149-169.
27. López-Estraño, C., J. B. Schvartzman, D. B. Krimer, and P. Hernández. 1998. Co-localization of polar replication fork barriers and rRNA transcription terminators in mouse rDNA. J. Mol. Biol. 277:249-256.[CrossRef][Medline]
28. López-Estraño, C., J. B. Schvartzman, D. B. Krimer, and P. Hernández. 1999. Characterization of the pea rDNA replication fork barrier: putative cis-acting and trans-acting factors. Plant Mol. Biol. 40:99-110.[CrossRef][Medline]
29. Maleszka, R., and G. D. Clark-Walker. 1993. Yeasts have a four-fold variation in ribosomal DNA copy number. Yeast 9:53-58.[CrossRef][Medline]
30. Melekhovets, Y. F., P. S. Shwed, and R. N. Nazar. 1997. In vivo analyses of RNA polymerase I termination in Schizosaccharomyces pombe. Nucleic Acids Res. 25:5103-5109.
31. Moreno, S., A. Klar, and P. Nurse. 1991. Molecular genetic analysis of fission yeast Schizosaccharomyces pombe. Methods Enzymol. 194:795-823.[Medline]
32. Olavarrieta, L., P. Hernández, D. B. Krimer, and J. B. Schvartzman. 2002. DNA knotting caused by head-on collision of transcription and replication. J. Mol. Biol. 322:1-6.[CrossRef][Medline]
33. Pasero, P., and M. Marilley. 1993. Size variation of rDNA clusters in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Mol. Gen. Genet. 236:448-452.[CrossRef][Medline]
34. Prentice, H. L. 1992. High efficiency transformation of Schizosaccharomyces pombe by electroporation. Nucleic Acids Res. 20:621.
35. Pütter, V., and F. Grummt. 2002. Transcription termination factor TTF-I exhibits contrahelicase activity during DNA replication. EMBO Rep. 3:147-152.[CrossRef][Medline]
36. Reeder, R. H., P. Guevara, and J. G. Roan. 1999. Saccharomyces cerevisiae RNA polymerase I terminates transcription at the Reb1 terminator in vivo. Mol. Cell. Biol. 19:7369-7376.
37. Sanchez, J. A., S. M. Kim, and J. A. Huberman. 1998. Ribosomal DNA replication in the fission yeast, Schizosaccharomyces pombe. Exp. Cell Res. 238:220-230.[CrossRef][Medline]
38. Shwed, P. S., and R. N. Nazar. 1999. Terminator element mutations affect both the efficiency and position of RNA polymerase I termination in Schizosaccharomyces pombe. Nucleic Acids Res. 27:2883-2888.
39. Wai, H., K. Johzuka, L. Vu, K. Eliason, T. Kobayashi, T. Horiuchi, and M. Nomura. 2001. Yeast RNA polymerase I enhancer is dispensable for transcription of the chromosomal rRNA gene and cell growth, and its apparent transcription enhancement from ectopic promoters requires Fob1 protein. Mol. Cell. Biol. 21:5541-5553.
40. Wang, K. L., and J. R. Warner. 1998. Positive and negative autoregulation of REB1 transcription in Saccharomyces cerevisiae. Mol. Cell. Biol. 18:4368-4376.
41. Ward, T. R., M. L. Hoang, R. Prusty, C. K. Lau, R. L. Keil, W. L. Fangman, and B. J. Brewer. 2000. Ribosomal DNA replication fork barrier and HOT1 recombination hot spot: shared sequences but independent activities. Mol. Cell. Biol. 20:4948-4957.
42. Wiesendanger, B., R. Lucchini, T. Koller, and J. M. Sogo. 1994. Replication fork barriers in the Xenopus rDNA. Nucleic Acids Res. 22:5038-5046.
43. Zhang, Z., D. M. Macalpine, and G. M. Kapler. 1997. Developmental regulation of DNA replication: replication fork barriers and programmed gene amplification in Tetrahymena thermophila. Mol. Cell. Biol. 17:6147-6156.[Abstract]
44. Zhao, A., A. Guo, Z. Liu, and L. Pape. 1997. Molecular cloning and analysis of Schizosaccharomyces pombe Reb1p: sequence-specific recognition of two sites in the far upstream rDNA intergenic spacer. Nucleic Acids Res. 25:904-910.
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| J. Bacteriol. | J. Virol. | Eukaryot. Cell |
|---|
| Microbiol. Mol. Biol. Rev. | Clin. Vaccine Immunol. | All ASM Journals |
|---|
