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Role of Accessory DNA Polymerases in DNA Replication in Escherichia coli: Analysis of the dnaX36 Mutator Mutant

Damian Gawel,¹ Phuong T. Pham,^1, Iwona J. Fijalkowska,² Piotr Jonczyk,² and Roel M. Schaaper^1*

Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709,¹ Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw 02-106, Poland²

Received 10 September 2007/ Accepted 13 December 2007

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 (Pol 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. Second, an important role is described for DNA Pol II and Pol IV in preventing and producing, respectively, 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 a high fidelity of replication.

Published ahead of print on 21 December 2007.

Present address: Department of Molecular Biology, University of Southern California, Los Angeles, CA 90089.