Sites of transition mutations in mutS, Klenow-defective mutS and 5´ → 3´ exonuclease-defective mutS strains are different. The Klenow-defective or 5´ → 3´ exonuclease-defective mutS strains showed a marked increase in transition mutations. In the present study, rates of mutation were found to be higher in Klenow-defective mutS strains and 5´ → 3´ exonuclease-defective mutS strains than mutS or polA strains. PolI, encoded by the polA gene, possesses Klenow and 5´ → 3´ exonuclease domains. We argued that DNA polymerase I (PolI) corrects transition mismatches. We have previously demonstrated that the Escherichia coli strain mutS ΔpolA had a higher rate of transition and minus frameshift mutations than mutS or ΔpolA strains.
