Proofreading deficiency of Pol I increases the levels of spontaneous rpoB mutations in E. coli.

The fidelity role of DNA polymerase I in chromosomal DNA replication in E. coli was investigated using the rpoB forward target. These experiments indicated that in a strain carrying a proofreading-exonuclease-defective form of Pol I (polAexo mutant) the frequency of rpoB mutations increased by about 2-fold, consistent with a model that the fidelity of DNA polymerase I is important in controlling the overall fidelity of chromosomal DNA replication. DNA sequencing of rpoB mutants revealed that the Pol I exonuclease deficiency lead to an increase in a variety of base-substitution mutations. A polAexo mutator effect was also observed in strains defective in DNA mismatch repair and carrying the dnaE915 antimutator allele. Overall, the data are consistent with a proposed role of Pol I in the faithful completion of Okazaki fragment gaps at the replication fork.