A specific role of MutT protein: to prevent dG.dA mispairing in DNA replication

Occurrence of the transversion mutation A.T to C.G is specifically enhanced in Escherichia coli mutT mutants. With the aid of the cloned mutT gene, the MutT protein, which has a molecular mass of 15 kilodaltons, was overproduced and purified to near homogeneity. The protein catalyzes hydrolysis of dGTP to dGMP. dGDP and GTP were also hydrolyzed by the protein, but at a lower rate than seen with dGTP. No other deoxynucleoside triphosphates were hydrolyzed. Using poly(dA).(dT)20 as a template-primer, we investigated the misincorporation of dGMP, dCMP, and dAMP by the alpha subunit and the core of E. coli DNA polymerase III. When the polymerization reaction was performed with the alpha subunit, both dCMP and dGMP were misincorporated. The core, composed of alpha, epsilon, and theta subunits, misincorporated only dGMP. This would imply that the proofreading function of the epsilon subunit of DNA polymerase III may correct the dC.dA mispair but not the dG.dA mispair. Misincorporation of dAMP was not observed in reactions with the alpha subunit or core. The misincorporation of dGMP, but not dCMP, was almost completely suppressed by adding purified MutT protein to the reaction mixture. Under these conditions, only a portion of dGTP present in the reaction mixture was degraded. It is therefore likely that the MutT protein may prevent dGMP misincorporation by degrading a specific form of dGTP, probably the syn form, which can pair with deoxyadenosine.