HpaII methyltransferase is mutagenic in Escherichia coli

A genetic reversion assay to study the increase in C-to-T mutations at CG sites in Escherichia coli by HpaII methyltransferase (MTase) is described. HpaII and SssI MTases can also convert cytosine to uracil in vitro. Thus to define the methylation at C-5 of the cytosine in DNA as being intrinsically mutagenic the occurrence of mutagenic hot spots at all methylated cytosines in DNA must be taken to be true.