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J. Bacteriol. doi:10.1128/JB.01463-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Role of Accessory DNA Polymerases in DNA replication in Escherichia coli: analysis of the dnaX36 Mutator Mutant

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709; and Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw 02-106, Poland

^* To whom correspondence should be addressed. Email: schaaper{at}niehs.nih.gov.

	Abstract

The dnaX36(TS) mutant of Escherichia coli confers a distinct mutator phenotype characterized by enhancement of transversion base substitutions and certain (-1) frameshift mutations. Here, we have further investigated the possible mechanism(s) underlying this mutator effect, focusing in particular on the role of the various E. coli DNA polymerases. The dnaX gene encodes the subunit of DNA polymerase III holoenzyme, the enzyme responsible for replication of the bacterial chromosome. The dnaX36 defect resides in the C-terminal domain V of , essential for interaction of with the (= polymerase) subunit, suggesting that the mutator phenotype is caused by an impaired or altered - interaction. We previously proposed that the mutator activity results from aberrant processing of terminal mismatches created by Pol III insertion errors. The present results, including lack of interaction of dnaX36 with mutM, mutY, and recA defects, support our assumption that dnaX36-mediated mutations originate as errors of replication rather than DNA damage-related events. Secondly, an important role is described for DNA polymerases II and IV in, respectively, preventing or producing the mutations. In the system used, a high fraction of the mutations is dependent on the action of Pol IV in a (dinB) gene dosage-dependent manner. However, an even larger but opposing role is deduced for Pol II, revealing Pol II to be a major editor of Pol III mediated replication errors. Overall, the results provide insight into the interplay of the various DNA polymerases, and of subunit, in securing high fidelity of replication.