Weitao Tao, Budd Martin, Hoopes Laura L Mays, Campbell Judith L
Braun Laboratories 147-75, California Institute of Technology, Pasadena, California 91125, USA.
J Biol Chem. 2003 Jun 20;278(25):22513-22. doi: 10.1074/jbc.M301610200. Epub 2003 Apr 9.
We have proposed that faulty processing of arrested replication forks leads to increases in recombination and chromosome instability in Saccharomyces cerevisiae and contributes to the shortened lifespan of dna2 mutants. Now we use the ribosomal DNA locus, which is a good model for all stages of DNA replication, to test this hypothesis. We show directly that DNA replication pausing at the ribosomal DNA replication fork barrier (RFB) is accompanied by the occurrence of double-strand breaks near the RFB. Both pausing and breakage are elevated in the early aging, hypomorphic dna2-2 helicase mutant. Deletion of FOB1, encoding the fork barrier protein, suppresses the elevated pausing and DSB formation, and represses initiation at rDNA ARSs. The dna2-2 mutation is synthetically lethal with deltarrm3, encoding another DNA helicase involved in rDNA replication. It does not appear to be the case that the rDNA is the only determinant of genome stability during the yeast lifespan however since strains carrying deletion of all chromosomal rDNA but with all rDNA supplied on a plasmid, have decreased rather than increased lifespan. We conclude that the replication-associated defects that we can measure in the rDNA are symbolic of similar events occurring either stochastically throughout the genome or at other regions where replication forks move slowly or stall, such as telomeres, centromeres, or replication slow zones.
我们曾提出,停滞的复制叉处理缺陷会导致酿酒酵母中重组增加和染色体不稳定,并导致dna2突变体寿命缩短。现在我们使用核糖体DNA位点(它是DNA复制所有阶段的良好模型)来检验这一假设。我们直接表明,核糖体DNA复制叉屏障(RFB)处的DNA复制暂停伴随着RFB附近双链断裂的出现。在早衰的低表达dna2 - 2解旋酶突变体中,暂停和断裂都有所增加。编码叉屏障蛋白的FOB1缺失可抑制增加的暂停和DSB形成,并抑制rDNA ARS处的起始。dna2 - 2突变与编码参与rDNA复制的另一种DNA解旋酶的deltarrm3合成致死。然而,rDNA似乎并不是酵母寿命期间基因组稳定性的唯一决定因素,因为携带所有染色体rDNA缺失但所有rDNA都由质粒提供的菌株寿命缩短而非延长。我们得出结论,我们在rDNA中能够测量到的与复制相关的缺陷,象征着在整个基因组中随机发生的类似事件,或者发生在复制叉移动缓慢或停滞的其他区域,如端粒、着丝粒或复制缓慢区。