Role of the 5́ → 3́ exonuclease and Klenow fragment of Escherichia coli DNA polymerase I in base mismatch repair

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. We argued that DNA polymerase I (PolI) corrects transition mismatches. PolI, encoded by the polA gene, possesses Klenow and 5́ → 3́ exonuclease domains. In the present study, rates of mutation were found to be higher in Klenow-defective mutS strains and 5́ → 3́ exonuclease-defective mutS strains than mutS or polA strains. The Klenow-defective or 5́ → 3́ exonuclease-defective mutS strains showed a marked increase in transition mutations. Sites of transition mutations in mutS, Klenow-defective mutS and 5́ → 3́ exonuclease-defective mutS strains are different. Thus, it is suggested that, in addition to mutS function, both the Klenow and 5́ → 3́ exonuclease domains are involved in the decrease of transition mutations. Transition hot and warm spots in mutS ⁺ polA ⁺ strains were found to differ from those in mutS and mutS ΔpolA strains. We thus argue that all the spontaneous transition mutations in the wild-type strain do not arise from transition mismatches left unrepaired by the MutS system or MutS PolI system.