Elevated expression of DNA polymerase II increases spontaneous mutagenesis in Escherichia coli

Escherichia coli DNA polymerase II (Pol-II), encoded by the SOS-regulated polB gene, belongs to the highly conserved group B (alpha-like) family of "high-fidelity" DNA polymerases. Elevated expression of polB gene was recently shown to result in a significant elevation of translesion DNA synthesis at 3, N(4)-ethenocytosine lesion with concomitant increase in mutagenesis. Here, I show that elevated expression of Pol-II leads to an approximately 100-fold increase in spontaneous mutagenesis in a manner that is independent of SOS, umuDC, dinB, recA, uvrA and mutS functions. Cells grow slowly and filament with elevated expression of Pol-II. Introduction of carboxy terminus ("beta interaction domain") mutations in polB eliminates elevated spontaneous mutagenesis, as well as defects in cell growth and morphology, suggesting that these abilities require the interaction of Pol-II with the beta processivity subunit of DNA polymerase III. Introduction of a mutation in the proofreading exo motif of polB elevates mutagenesis by a further 180-fold, suggesting that Pol-II can effectively compete with DNA polymerase III for DNA synthesis. Thus, Pol-II can contribute to spontaneous mutagenesis when its expression is elevated.