Characterization of G₁ checkpoint control in the yeast Saccharomyces cerevisiae following exposure to DNA-damaging agents

The delay of S-phase following treatment of yeast cells with DNA-damaging agents is an actively regulated response that requires functional RAD9 and RAD24 genes. An analysis of cell cycle arrest indicates the existence of (at least) two checkpoints for damaged DNA prior to S-phase; one at START (a G₁ checkpoint characterized by pheromone sensitivity of arrested cells) and one between the CDC4- and CDC7-mediated steps (termed the G₁/S checkpoint). When a dna1-1 mutant (that affects early events of replicon initiation) also carries a rad9 deletion mutation, it manifests a failure to arrest in G₁/S following incubation at the restrictive temperature. This failure to execute regulated G₁/S arrest is correlated with enhanced thermosensitivity of colony-forming ability. In an attempt to characterize the signal for RAD9 gene-dependent G₁ and G₁/S cell cycle arrest, we examined the influence of the continued presence of unexcised photoproducts. In mutants defective in nucleotide excision repair, cessation of S-phase was observed at much lower doses of UV radiation compared to excision-proficient cells. However, this response was not RAD9-dependent. We suggest that an intermediate of nucleotide excision repair, such as DNA strand breaks or single-stranded DNA tracts, is required to activate RAD9-dependent G₁ and G₁/S checkpoint controls.