There are two major pathways for repairing DNA double-strand breaks (DSBs): homologous directed recombination (HDR) and non-homologous end-joining (NHEJ). While NHEJ functions throughout the cell cycle, HDR is only possible during S/G₂ phases, suggesting that there are cell cycle-specific mechanisms regulating the balance between the two repair systems. The regulation exerted by CDKs on HDR has been extensively demonstrated, and here we present evidence that the CDK Pho85, in association with the G₁ cyclin Pcl1, phosphorylates Yku80 on Ser 623 to regulate NHEJ activity. Cells bearing a non-phosphorylatable version of Yku80 show increased NHEJ and reduced HDR activity. Accordingly, yku80^S623A cells present diminished viability upon treatment with the DSB-producer bleomycin, specifically in the G₂ phase of the cell cycle. Interestingly, the mutation of the equivalent residue in human Ku80 increases sensitivity to bleomycin in several cancer cell lines, suggesting that this mechanism is conserved in humans. Altogether, our results reveal a new mechanism whereby G₁-CDKs mediate the choice between HDR and NHEJ repair pathways, putting the error prone NHEJ on a leash and enabling error free HDR in G₂ when homologous sequences are available.

修复DNA双链断裂（DSB）有两种主要途径：同源定向重组（HDR）和非同源末端连接（NHEJ）。尽管NHEJ在整个细胞周期中都起作用，但HDR仅在S / G ₂阶段才可能，这表明存在特定于细胞周期的机制来调节两个修复系统之间的平衡。已经广泛证明了CDK对HDR的调节作用，在这里我们提供证据表明CDK Pho85与G ₁细胞周期蛋白Pcl1一起磷酸化Ser 623上的Yku80，从而调节NHEJ活性。带有不可磷酸化版本的Yku80的细胞显示NHEJ增加，HDR活性降低。因此，yku80 ^S623A在用DSB生产者博来霉素处理后，特别是在细胞周期的G ₂期，细胞呈现出降低的生存能力。有趣的是，人类Ku80中等同残基的突变增加了几种癌细胞系对博来霉素的敏感性，表明该机制在人类中是保守的。总而言之，我们的结果揭示了一种新的机制，其中G ₁ -CDK介导HDR和NHEJ修复途径之间的选择，将易出错的NHEJ置于皮带上，并在同源序列可用时在G _2中实现无错误的HDR 。