The 3'-->5' exonucleases of both DNA polymerases delta and epsilon participate in correcting errors of DNA replication in Saccharomyces cerevisiae

Abstract

DNA polymerases II (epsilon) and III (delta) are the only nuclear DNA polymerases known to possess an intrinsic 3'-->5' exonuclease in Saccharomyces cerevisiae. We have investigated the spontaneous mutator phenotypes of DNA polymerase delta and epsilon 3'-->5' exonuclease-deficient mutants, pol3-01 and pol2-4, respectively. pol3-01 and pol2-4 increased spontaneous mutation rates by factors of the order of 10(2) and 10(1), respectively, measured as URA3 forward mutation and his7-2 reversion. Surprisingly, a double mutant pol2-4 pol3-01 haploid was inviable. This was probably due to accumulation of unedited errors, since a pol2-4/pol2-4 pol3-01/pol3-01 diploid was viable, with the spontaneous his7-2 reversion rate increased by about 2 x 10(3)-fold. Analysis of mutation rates of double mutants indicated that the 3'-->5' exonucleases of DNA polymerases delta and epsilon can act competitively and that, like the 3'-->5' exonuclease of DNA polymerase delta, the 3'-->5' exonuclease of DNA polymerase epsilon acts in series with the PMS1 mismatch correction system. Mutational spectra at a URA3 gene placed in both orientations near to a defined replication origin provided evidence that the 3'-->5' exonucleases of DNA polymerases delta and epsilon act on opposite DNA strands, but were in sufficient to distinguish conclusively between different models of DNA replication.