Cell-Cycle-Specific Repair of DNA Double-Strand Breaks in Saccharomyces cerevisiae

It has been established in yeast that the response to ionizing radiation is cell-cycle dependent. These observations can be interpreted in terms of variations in the ability to repair DNA double-strand breaks (DSB). In the present work, DSB repair was examined directly in synchronous yeast cultures irradiated in either the G₂ or the G₁ phase. DSB repair in G₂ cells was rapid and appeared to be complete in less than 2 hr. Based on the absence of DNA synthesis during repair, it was concluded that DSB repair does not require a normal round of S phase DNA replication. In G₁ cells no DSB repair was observed and the nuclear DNA was degraded during incubation after irradiation. Following irradiation, the G₁ cells produced buds with little delay and accumulated at the next mitosis. However, the extent of S phase DNA synthesis normally associated with budding was substantially decreased. The data are consistent with the idea that cell-cycle-dependent changes in survival following ionizing radiation reflect differential capacity for DSB repair, and that the repair observed in G₂ may involve recombination between sister chromatids.