The cytosine N-4-methyltransferase M.PvuII also modifies adenine residues

Methylation of DNA occurs at the C-5 and N-4 positions of cytosine and N-6 of adenine. The chemistry of methylation is similar among methyltransferase s specific for cytosine-N-4 and adenine-N-6. Moreover these enzymes have si milar structures and active sites. Previously it has been demonstrated that the DNA-(adenine-N-6)- methyltransferases M.EcoRV, M.EcoRI, E. coli dam an d both domains of M.FokI also modify cytosine residues at the N-4 position [Jeltsch et al., J. Biol. Chem. 274 (1999), 19538-19544]. Here we show that the cytosine-N-4 methyltransferase M.PvuII, which modifies the second cyto sine in CAGCTG sequences, also methylates adenine residues in CAGATG/CAGCTQ substrates in which the target cytosine is replaced by adenine in one stra nd of the recognition sequence. Therefore, adenine-N-6 and cytosine-N-4 met hyltransferases have overlapping target base specificities. These results d emonstrate that the target base recognition by N-specific DNA methyltransfe rases is relaxed in many cases. Furthermore, it shows that the catalytic me chanisms of adenine-N-6 and cytosine-N-4 methyltransferases are very simila r.