Table 2.

rad55Δ has an intact cell cycle arrest in response to DNA damagea

Strain% Lethality% ArrestArrest/lethality
Wild type21180.86
rad9 4050.13
rad51 54440.81
rad55 58460.79
rad57 48340.71
  • a The following strains (Table 1) used were: FF18984 (wild-type), WDHY1020 (rad9Δ), WDHY994 (rad51Δ), WDHY837 (rad55Δ), and WDHY839 (rad57Δ). The presence of the G2 DNA damage checkpoint-induced cell cycle arrest was monitored as described in Materials and Methods using an established assay (89, 90). The ratio of percent arrested cells (microcolonies with either a large-budded cell or with two adjacent large-budded cells) to the percent of inviable cells provides a metric of the efficiency of cell cycle arrest. For wild-type cells this metric approaches 1.0 because essentially all cells with unrepairable DNA breaks die in the G2 phase; haploid cells in the G1 or postanaphase stages when irradiated cannot repair the DNA DSBs, arrest in the next G2 phase, and die as large-budded and two adjacent large-budded cells, respectively. Wild-type cells that can repair DSBs (S and G2 phase cells) form large microcolonies that are not counted. In contrast, checkpoint mutants cells that die after irradiation do not usually arrest immediately; rather, they continue to divide for a few generations. Therefore, in checkpoint mutants, the ratio of arrested cells to inviable cells is <<1.0 and typically is <0.3 (see reference 90).