The adenine-specific DNA methyltransferase M.TaqI transfers a methyl group from S-adenosylmethionine to N6 of the adenine residue in the DNA sequence 5'-TCGA-3'. In the crystal structure of M.TaqI in complex with S-adenosylmethionine the enzyme is folded into two domains: An N-terminal catalytic domain, whose fold is conserved among S-adenosyl-methionine dependent methyltransferases, and a DNA recognition domain which possesses a unique fold. In the active site, two aromatic residues, Tyr 108 and Phe 196, are postulated to bind the flipped-out target DNA adenine which becomes methylated. By lowering the energy of the positively charged transition state via cationic-pi interactions, these two residues probably hold a key role in catalysis.