Mutator effects of overproducing DNA polymerase η (Rad30) and its catalytically inactive variant in yeast

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DNA polymerase η synthesizes DNA in vitro with low fidelity. Based on this, here we report the effects of deletion or increased expression of yeast RAD30 gene, encoding for polymerase η (Pol η), on spontaneous mutagenesis in vivo. Deletion of RAD30 did not affect spontaneous mutagenesis. Overproduction of Rad30p was slightly mutagenic in a wild-type yeast strain and moderately mutagenic in strains with inactive 3′→5′-exonuclease of DNA polymerase ϵ or DNA mismatch repair. These data suggest that excess Rad30p reduces replication fidelity in vivo and that the induced errors may be corrected by exonucleolytic proofreading and DNA mismatch repair. However, the magnitude of mutator effect (only up to 10-fold) suggests that the replication fork is protected from inaccurate synthesis by Pol η in the absence of DNA damage. Overproduction of catalytically inactive Rad30p was also mutagenic, suggesting that much of the mutator effect results from indirect perturbation of replication rather than from direct misincorporation by Pol η. Moreover, while excess wild-type Pol η primarily induced base substitutions in the msh6 and pms1 strains, excess inactive Rad30p induced both base substitutions and frameshifts. This suggests that more than one mutagenic mechanism is operating when RAD30 is overexpressed.