Regulation of repair choice: Cdk1 suppresses recruitment of end joining factors at DNA breaks

Cell cycle plays a crucial role in regulating the pathway used to repair DNA double-strand breaks (DSBs). In Saccharomyces cerevisiae, homologous recombination is primarily limited to non-G1 cells as the formation of recombinogenic single-stranded DNA requires CDK1-dependent 5′ to 3′ resection of DNA ends. However, the effect of cell cycle on non-homologous end joining (NHEJ) is not yet clearly defined. Using an assay to quantitatively measure the contributions of each repair pathway to repair product formation and cellular survival after DSB induction, we found that NHEJ is most efficient at G1, and markedly repressed at G2. Repression of NHEJ at G2 is achieved by efficient end resection and by the reduced association of core NHEJ proteins with DNA breaks, both of which depend on the CDK1 activity. Importantly, repression of 5′ end resection by CDK1 inhibition at G2 alone did not fully restore either physical association of Ku/Dnl4-Lif1 with DSBs or NHEJ proficiency to the level at G1. Expression of excess Ku can partially offset the inhibition of end joining at G2. The results suggest that regulation of Ku/Dnl4-Lif1 affinity for DNA ends may contribute to the cell cycle-dependent modulation of NHEJ efficiency.