Role of the dinB gene product in spontaneous mutation in Escherichia coli with an impaired replicative polymerase

We isolated several new mutator mutations of the Escherichia coli replicati ve polymerase dnaE subunit alpha and used them and a previously reported dn aE mutation to study spontaneous frameshift and base substitution mutations . Two of these dnaE strains produce many more mutants when grown on rich (L uria-Bertani) than on minimal medium. A differential effect of the medium w as not observed when these dnaE mutations were combined with a mismatch rep air mutation, The selection scheme for the dnaE mutations required that the y be able to complement a temperature-sensitive strain. However, the abilit y to complement is not related to the mutator effect for at least one of th e mutants. Comparison of the mutation rates for frameshift and base substit ution mutations in mutS and dnaE mutS strains suggests that the mismatch re pair proteins respond differently to the two types of change. Deletion of d inB from both chromosome and plasmid resulted in a four-to fivefold decreas e in the rate of frameshift and base substitution mutations in a dnaE mutS double mutant background. This reduction indicates that most mistakes in re plication occur as a result of the action of the auxiliary rather than the replicative polymerase in this dnaE mutant. Deletion of dinB from strains c arrying a wild-type dnaE had a measurable effect, suggesting that a tractio n of spontaneous mutations occur as a result of dinB polymerase action even in cells with a normal replicative polymerase.