Regulation of replication origins is important for complete duplication of the genome, but the effect of origin activation on the cellular response to replication stress is poorly understood. The budding yeast rRNA gene (rDNA) forms tandem repeats and undergoes replication fork arrest at the replication fork barrier (RFB), inducing DNA double-strand breaks (DSBs) and genome instability accompanied by copy number alterations. Here, we demonstrate that the S-phase cyclin Clb5 promotes rDNA stability. Absence of Clb5 led to reduced efficiency of replication initiation in rDNA but had little effect on the number of replication forks arrested at the RFB, suggesting that arrival of the converging fork is delayed and forks are more stably arrested at the RFB. Deletion of CLB5 affected neither DSB formation nor its repair at the RFB but led to homologous recombination-dependent rDNA instability. Therefore, arrested forks at the RFB may be subject to DSB-independent, recombination-dependent rDNA instability. The rDNA instability in clb5Δ was not completely suppressed by the absence of Fob1, which is responsible for fork arrest at the RFB. Thus, Clb5 establishes the proper interval for active replication origins and shortens the travel distance for DNA polymerases, which may reduce Fob1-independent DNA damage.

中文翻译：

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S期细胞周期蛋白Clb5通过维持rDNA中的复制起始效率来促进rRNA基因（rDNA）稳定性。

复制起点的调节对于基因组的完全复制很重要，但是对激活起点对复制应激的细胞反应的影响知之甚少。出芽的酵母rRNA基因（rDNA）形成串联重复序列，并在复制叉屏障（RFB）处经历复制叉停滞，诱导DNA双链断裂（DSB）和基因组不稳定，并伴随拷贝数改变。在这里，我们证明了S期细胞周期蛋白Clb5可以促进rDNA的稳定性。Clb5的缺乏导致rDNA复制起始效率降低，但对停在RFB处的复制叉的数量影响很小，这表明聚合叉的到达被延迟了，并且叉被稳定地停在了RFB处。删除CLB5既不影响DSB的形成，也不影响其在RFB的修复，但导致同源重组依赖的rDNA不稳定性。因此，在RFB处停滞的货叉可能会受到DSB独立，重组依赖的rDNA不稳定的影响。没有Fob1不能完全抑制clb5Δ中的rDNA不稳定性，这是造成RFB叉停的原因。因此，Clb5为主动复制起点建立了适当的间隔，并缩短了DNA聚合酶的传播距离，这可以减少不依赖Fob1的DNA损伤。