Mutator alleles of yeast DNA polymerase ζ

The yeast REV3 gene encodes the catalytic subunit of DNA polymerase zeta (pol ζ), a B family polymerase that performs mutagenic DNA synthesis in cells. To probe pol ζ mutagenic functions, we generated six mutator alleles of REV3 with amino acid replacements for Leu979, a highly conserved residue inferred to be at the pol ζ active site. Replacing Leu979 with Gly, Val, Asn, Lys, Met or Phe resulted in yeast strains with elevated UV-induced mutant frequencies. While four of these strains had reduced survival following UV irradiation, the rev3-L979F and rev3-L979M strains had normal survival, suggesting retention of pol ζ catalytic activity. UV mutagenesis in the rev3-L979F background was increased when photoproduct bypass by pol η was eliminated by deletion of RAD30. The rev3-L979F mutation had little to no effect on mutagenesis in an ogg1Δ background, which cannot repair 8-oxo-guanine in DNA. UV-induced can1 mutants from rev3-L979F and rad30Δ rev3-L979F strains primarily contained base substitutions and complex mutations, suggesting error-prone bypass of UV photoproducts by L979F pol ζ. Spontaneous mutation rates in rev3-L979F and rev3-L979M strains are elevated by about two-fold overall and by two- to eight-fold for C to G transversions and complex mutations, both of which are known to be generated by wild-type pol ζ in vitro. These results indicate that Rev3p-Leu979 replacements reduce the fidelity of DNA synthesis by yeast pol ζ in vivo. In conjunction with earlier studies, the data establish that the conserved amino acid at the active site location occupied by Leu979 is critical for the fidelity of all four yeast B family polymerases. Reduced fidelity with retention of robust polymerase activity suggests that the homologous rev3-L979F allele may be useful for analyzing pol ζ functions in mammals, where REV3 deletion is lethal.