Molecular mechanisms of replicational fidelity in Escherichia coli.

DNA polymerase III holoenzyme is responsible for chromosomal DNA synthesis in Escherichia coli and seems to be a major determinant of the fidelity of replication of this organism. Among ten different subunits of the holoenzyme, the alpha subunit, encoded by the dnaE gene, has a polymerase activity, while the epsilon subunit, encoded by the dnaQ gene, is a proofreader with a 3'-5' exonuclease activity. Using poly(dA)/oligo(dT)20 as a template-primer, misincorporation of dGMP, dCMP, and dAMP by the alpha subunit and exonucleolytic editing of those mispairs by the epsilon subunit were investigated. When the polymerization reaction was performed with the alpha subunit, dCMP and dGMP but not dAMP were misincorporated. This would suggest that the polymerase might have a base-selecting function to avoid dA:dA mispairing. A subassembly of the DNA polymerase III consisting of alpha, epsilon, and theta subunits misincorporated only dGMP. This would imply that the proofreading function of the epsilon subunit may correct the dC:dA but not the dG:dA mispair. Addition of a protein encoded by the mutT gene, defects of which cause AT to CG transversions in vivo, diminished the misincorporation of dGMP onto poly(dA) template by the alpha subunit. A dGTPase activity was associated with the MutT protein. The significance of the dGTPase activity in the prevention of dG:dA mispairing is discussed.