Distinct Cdk1 requirements during single-strand annealing, noncrossover and crossover recombination

Repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) in haploid cells is generally restricted to S/ G2 cell cycle phases, when DNA has been replicated and a sister chromatid is available as a repair template. This cell cycle specificity depends on cyclin-dependent protein kinases (Cdk1 in Saccharomyces cerevisiae), which initiate HR by promoting 59–39 nucleolytic degradation of the DSB ends. Whether Cdk1 regulates other HR steps is unknown. Here we show that yku70D cells, which accumulate single-stranded DNA (ssDNA) at the DSB ends independently of Cdk1 activity, are able to repair a DSB by single-strand annealing (SSA) in the G1 cell cycle phase, when Cdk1 activity is low. This ability to perform SSA depends on DSB resection, because both resection and SSA are enhanced by the lack of Rad9 in yku70D G1 cells. Furthermore, we found that interchromosomal noncrossover recombinants are generated in yku70D and yku70D rad9D G1 cells, indicating that DSB resection bypasses Cdk1 requirement also for carrying out these recombination events. By contrast, yku70D and yku70D rad9D cells are specifically defective in interchromosomal crossover recombination when Cdk1 activity is low. Thus, Cdk1 promotes DSB repair by single-strand annealing and noncrossover recombination by acting mostly at the resection level, whereas additional events require Cdk1-dependent regulation in order to generate crossover outcomes.