Single-Strand DNA-Specific Exonucleases in Escherichia coli: Roles in Repair and Mutation Avoidance

Mutations in the genes encoding single-strand DNA-specific exonucleases (ssExos) of Escherichia coli were examined for effects on mutation avoidance, UV repair, and conjugational recombination. Our results indicate complex and partially redundant roles for ssExos in these processes. Although biochemical experiments have implicated RecJ exonuclease, Exonuclease I (ExoI), and Exonuclease VII (ExoVII) in the methyl-directed mismatch repair pathway, the RecJ− ExoI− ExoVII− mutant did not exhibit a mutator phenotype in several assays for base substitution mutations. If these exonucleases do participate in mismatch excision, other exonucleases in E. coli can compensate for their loss. Frameshift mutations, however, were stimulated in the RecJ− ExoI− ExoVII− mutant. For acridine-induced frameshifts, this mutator effect was due to a synergistic effect of ExoI− and ExoVII− mutations, implicating both ExoI and ExoVII in avoidance of frameshift mutations. Although no single exonuclease mutant was especially sensitive to UV irradiation, the RecJ− ExoVII− double mutant was extremely sensitive. The addition of an ExoI− mutation augmented this sensitivity, suggesting that all three exonucleases play partially redundant roles in DNA repair. The ability to inherit genetic markers by conjugation was reduced modestly in the ExoI− RecJ− mutant, implying that the function of either ExoI or RecJ exonucleases enhances RecBCD-dependent homologous recombination.