Human DNA (cytosme-5)methyftransferase selectively methylates duplex DNA containing mispairs

Abstract

The presence of the C.C mispair in a defined duplex oligodeoxy-nucleotide enhanced its capacity to serve as a substrate for highly purified human DNA methyltransferase. Analysis of tritiated reaction products showed that the C.C mispair acted as a “methylation acceptor” in that it was itself rapidly methylated. The m⁵ C.G base pair also enhanced the capacity of the oligodeoxynucleotide to serve as a substrate for the enzyme. However, this complementary base pair was found to act as a “methylation director”. That is, the presence of the m⁵C in one strand induced the enzyme to rapidly methylate at the cytosine residue on the opposite strand in an adjacent C.G base pair.