MUTANT HPAII METHYLTRANSFERASE FUNCTIONS AS A MUTATOR ENZYME

DNA (cytosine-5)-methyltransferases can cause deamination of cytosine when the cofactor S-adenosylmethioine (AdoMet) is limiting and thus fu nction as sequence-specific C-->U mutator enzymes. Here we explored wh ether mutations causing inactivation of the cofactor binding activity of the Hpall methyltransferase, thus mimicking conditions of limiting AdoMet concentration, could convert a DNA methyltransferase to a C-->U mutator enzyme. We created two mutator enzymes from the Hpall methylt ransferase (F38S and G40D) which both showed enhanced cytosine deamina tion activities in vitro and in vivo. Interestingly, the G:U mispairs generated by these enzymes were not repaired completely in bacteria eq uipped with uracil-DNA glycosylase-initiated repair machinery, giving rise to a potent mutator phenotype. This is the first report showing t he creation of mutator enzymes from a DNA methyltansferase and the dem onstration of their mutagenicity in living cells.