Identification of a mutant DNA polymerase delta in Saccharomyces cerevisiae with an antimutator phenotype for frameshift mutations

We propose that 3 beta -turn-beta structure, which plays a critical role in exonucleolytic proofreading in the bacteriophage T4 DNA polymerase, is als o present in the Saccharomyces cerevisiae DNA pol delta. Site-directed muta genesis was used to test this proposal by introducing a mutation into the y east POL3 gene in the region that encodes the putative beta -turn-beta stru cture. The mutant DNA pol delta has a serine substitution in place of glyci ne at position 447. DNA replication fidelity of tile G447S-DNA pol delta wa s determined in vivo by using reversion and forward assays. An antimutator phenotype for frameshift mutations in short homopolymeric tracts was observ ed for the G447S-DNA pol delta in the absence of postreplication mismatch r epair, which was produced by inactivation of the MSH2 gene. Because the G44 7S substitution reduced frameshift but not base substitution mutagenesis, s ome aspect of DNA polymerase proofreading appears to contribute to producti on of frameshifts. Possible roles of DNA polymerase proofreading in framesh ift mutagenesis are discussed.