Suppression of genetic defects within the RAD6 pathway by srs2 is specific for error-free post-replication repair but not for damage-induced mutagenesis

srs2 was isolated during a screen for mutants that could suppress the UV-sensitive phenotype of rad6 and rad18 cells. Genetic analyses led to a proposal that Srs2 acts to prevent the channeling of DNA replication-blocking lesions into homologous recombination. The phenotypes associated with srs2indicate that the Srs2 protein acts to process lesions through RAD6-mediated post-replication repair (PRR) rather than recombination repair. The RAD6 pathway has been divided into three rather independent subpathways: two error-free (represented by RAD5 and POL30) and one error-prone (represented by REV3). In order to determine on which subpathways Srs2 acts, we performed comprehensive epistasis analyses; the experimental results indicate that the srs2 mutation completely suppresses both error-free PRR branches. Combined with UV-induced mutagenesis assays, we conclude that the Polzeta-mediated error-prone pathway is functional in the absence of Srs2; hence, Srs2 is not required for mutagenesis. Furthermore, we demonstrate that the helicase activity of Srs2 is probably required for the phenotypic suppression of error-free PRR defects. Taken together, our observations link error-free PRR to homologous recombination through the helicase activity of Srs2.