ROLE OF YEAST RTH1 NUCLEASE AND ITS HOMOLOGS IN MUTATION AVOIDANCE, DNA-REPAIR, AND DNA-REPLICATION

The RTH1(RAD27) gene of Saccharomyces ces cerevisiae encodes a structu re-specific endonuclease that cleaves 5'-ended single-stranded DNA at its junction with duplex DNA. Genetic and biochemical studies have ind icated a role of Rth 1 nuclease in the removal of RNA primers formed d uring DNA replication. The rth1 Delta mutation confers temperature-sen sitive lethality, and increases sensitivity to alkylating agents. The instability of repetitive DNA is greatly enhanced in the I rth1 Delta mutant. The conditional lethality of the rth1 Delta mutation indicates that another nuclease can function in DNA replication in the absence of RTH1. RAD2, a homolog of RTH1, is required for nucleotide-excision repair. Here, we examine three other homologs of RTH1/RAD2 - YEN1, EXO 1, and DIN7. Deletion of any of these genes in the rth1 Delta strain h as no effect on cell viability, suggesting the involvement of another, and as yet unidentified, nuclease in the maturation of Okazaki fragme nts. Our data also indicate that only RTH1 functions in the repair of alkylation damage. Deletions of YEN1, EXO1, DIN7, or RAD2, either sing ly or when combined with one another and with the rth1 Delta mutation, have no effect on the rate of instability of dinucleotide repeats or on the rate of formation of large duplications in the CAN1 gene. These data provide evidence of a high degree of spec specificity for the ro le of RTH1 in DNA replication and in base-excision repair, and for the requirement of RAD2 in nucleotide-excision repair. The possibility th at both Rth1 and Exo1 function in DNA mismatch repair is discussed.