Epistatic participation of REV1 and REV3 in the formation of UV-induced frameshift mutations in cell cycle-arrested yeast cells. 
Mutations arising in times of cell cycle arrest may provide a selective advantage for unicellular organisms adapting to environmental changes.  For multicellular organisms, however, they may pose a serious threat, in that such mutations in somatic cells contribute to carcinogenesis and ageing.  The budding yeast Saccharomyces cerevisiae presents a convenient model system for studying the incidence and the mechanisms of stationary-phase mutation in a eukaryotic organism.  Having studied the emergence of frameshift mutants after several days of starvation-induced cell cycle arrest, we previously reported that all (potentially error-prone) translesion synthesis (TLS) enzymes identified in S. cerevisiae did not contribute to the basal level of spontaneous stationary-phase mutations.  However, we observed that an increased frequency of stationary-phase frameshift mutations, brought about by a defective nucleotide excision repair (NER) pathway or by UV irradiation, was dependent on Rev3p, the catalytic subunit of the TLS polymerase zeta (Pol zeta).  Employing the same two conditions, we now examined the effect of deletions of the genes coding for polymerase eta (Pol eta) (RAD30) and Rev1p (REV1).  In a NER-deficient strain background, the increased incidence of stationary-phase mutations was only moderately influenced by a lack of Pol eta but completely reduced to wild type level by a knockout of the REV1 gene.  UV-induced stationary-phase mutations were abundant in wild type and rad30Delta strains, but substantially reduced in a rev1Delta as well as a rev3Delta strain.  The similarity of the rev1Delta and the rev3Delta phenotype and an epistatic relationship evident from experiments with a double-deficient strain suggests a participation of Rev1p and Rev3p in the same mutagenic pathway.  Based on these results, we propose that the response of cell cycle-arrested cells to an excess of exo- or endogenously induced DNA damage includes a novel replication-independent cooperative function of Rev1p and Pol zeta, which has the potential to generate mutations.