Checkpoint controls in Schizosaccharomyces pombe: rad1.

‘Checkpoint’ controls ensure that the events of the cell cycle are completed in an orderly fashion. For example, such controls delay mitosis until DNA synthesis and repair of radiation‐induced DNA damage are complete. The rad series of radiosensitive fission yeast mutants was examined to identify strains deficient for the DNA damage‐responsive checkpoint control. Five were identified. A characterization of one (rad1‐1) and the wild‐type is presented. The rad1‐1 mutant does not arrest after irradiation, is sensitive to killing by radiation and is not arrested by hydroxyurea, and thus is also deficient for the DNA synthesis‐responsive checkpoint control. The radiosensitivity of the rad1‐1 mutant was greatly reduced when irradiated and maintained for 6 h in a non‐dividing (density inhibited) state, demonstrating that rad1‐1 is repair proficient and radiosensitive only through failure to delay. The checkpoint controls for which rad1 is required appear to regulate G2‐M progression through the activity of cdc2, here implicated in this role by the coincidence of the radiation transition point and the cdc2 execution point.