CHARACTERIZATION OF G(1) 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-damag ing agents is an actively regulated response that requires functional RAD9 and RAD24 genes. An analysis of cell cycle arrest indicates the e xistence of (at least) two checkpoints for damaged DNA prior to S-phas e; one at START (a G(1) checkpoint characterized by pheromone sensitiv ity of arrested cells) and one between the CDC4- and CDC7-mediated ste ps (termed the G(1)/S checkpoint). When a dna1-1 mutant (that affects early events of replicon initiation) also carries a rad9 deletion muta tion, it manifests a failure to arrest in G(1)/S following incubation at the restrictive temperature. This failure to execute regulated G(1) /S arrest is correlated with enhanced thermosensitivity of colony-form ing ability. In an attempt to characterize the signal for RAD9 gene-de pendent G(1) and G(1)/S cell cycle arrest, we examined the influence o f the continued presence of unexcised photoproducts. In mutants defect ive in nucleotide excision repair, cessation of S-phase was observed a t much lower doses of UV radiation compared to excision-proficient cel ls. 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 G(1) and G(1)/S checkpoint controls.