The 3'-->5' exonucleases of DNA polymerases delta and epsilon and the 5'-->3' exonuclease Exo1 have major roles in postreplication mutation avoidance in Saccharomyces cerevisiae

Abstract

Replication fidelity is controlled by DNA polymerase proofreading and postreplication mismatch repair. We have genetically characterized the roles of the 5'-->3' Exo1 and the 3'-->5' DNA polymerase exonucleases in mismatch repair in the yeast Saccharomyces cerevisiae by using various genetic backgrounds and highly sensitive mutation detection systems that are based on long and short homonucleotide runs. Genetic interactions were examined among DNA polymerase epsilon (pol2-4) and delta (pol3-01) mutants defective in 3'-->5' proofreading exonuclease, mutants defective in the 5'-->3' exonuclease Exo1, and mismatch repair mutants (msh2, msh3, or msh6). These three exonucleases play an important role in mutation avoidance. Surprisingly, the mutation rate in an exo1 pol3-01 mutant was comparable to that in an msh2 pol3-01 mutant, suggesting that they participate directly in postreplication mismatch repair as well as in other DNA metabolic processes.

FIG. 1

Model of interaction between 3′→5′ and 5′→3′ nucleases in the appearance and prevention of frameshift mutations. The small shaded circle represents DNA polymerase. As discussed in the text, frameshift intermediates escape proofreading in long homonucleotide runs. The DNA Polɛ and DNA Polδ 3′→5′ exonucleases as well as the Exo1 5′→3′ exonuclease, in conjunction with a DNA helicase similar to the helicase involved in E. coli MMR (5, 18), could participate in mismatch removal in opposite directions, so that multiple enzyme deficiencies cause synergistic mutator effects for frameshift mutations.