Site-directed mutagenesis defines the catalytic aspartate in the active site of the atypical DNA: m4C methyltransferase M.NgoMXV.

M.NgoMXV is one of the few atypical DNA:m4C methyltransferases that does not possess a serine residue in its predicted active site. We previously reported a homology model of M.NgoMXV and argued that the aspartate side chain at a corresponding position, similarly to some DNA:m6 A-specific enzymes, is essential for the methyltransferase activity (Radlinska et al., 1999). Here we report the corrected amino acid sequence of M.NgoMXV and the analysis of substitution of D68 with alanine or serine, which both render the enzyme totally inactive.