Replication fidelity in E. coli: Differential leading and lagging strand effects for dnaE antimutator alleles

DNA Pol III holoenzyme (HE) is the major DNA replicase of Escherichia coli. It is a highly accurate enzyme responsible for simultaneously replicating the leading- and lagging DNA strands. Interestingly, the fidelity of replication for the two DNA strands is unequal, with a higher accuracy for lagging-strand replication. We have previously proposed this higher lagging-strand fidelity results from the more dissociative character of the lagging-strand polymerase. In support of this hypothesis, an E. coli mutant carrying a catalytic DNA polymerase subunit (DnaE915) characterized by decreased processivity yielded an antimutator phenotype (higher fidelity). The present work was undertaken to gain deeper insight into the factors that influence the fidelity of chromosomal DNA replication in E. coli. We used three different dnaE alleles (dnaE915, dnaE911, and dnaE941) that had previously been isolated as antimutators. We confirmed that each of the three dnaE alleles produced significant antimutator effects, but in addition showed that these antimutator effects proved largest for the normally less accurate leading strand. Additionally, in the presence of error-prone DNA polymerases, each of the three dnaE antimutator strains turned into mutators. The combined observations are fully supportive of our model in which the dissociative character of the DNA polymerase is an important determinant of in vivo replication fidelity. In this model, increased dissociation from terminal mismatches (i.e., potential mutations) leads to removal of the mismatches (antimutator effect), but in the presence of error-prone (or translesion) DNA polymerases the abandoned terminal mismatches become targets for error-prone extension (mutator effect). We also propose that these dnaE alleles are promising tools for studying polymerase exchanges at the replication fork.