question: What is the catalytic mechanism of DNA (cytosine-5) methyltransferases? 
answer: The catalytic mechanism of the DNA (Cytosine-5-)-methyltransferase involves nucleophilic attack of the C6 of the substrate cytosine by the single conserved cysteine of the enzyme, followed by C5 nucleophilic replacement of the methyl group of the cofactor S-adenosyl-L-methionine (AdoMet) to produce 5-methyl-6-Cys-81-S-5,6-dihydrocytosine. It has been also demonstrated that Phe and Glu, which are found in the catalytic motifs I and II of the enzyme are important for AdoMet binding and catalysis.