The product of the dcm gene is the only DNA cytosine-C5 methyltransferase of 
Escherichia coli K-12; it catalyses transfer of a methyl group from S-adenosyl 
methionine (SAM) to the C-5 position of the inner cytosine residue of the 
cognate sequence CCA/TGG. Sequence-specific, covalent crosslinking of the enzyme 
to synthetic oligonucleotides containing 5-fluoro-2'-deoxycytidine is 
demonstrated. This reaction is abolished if serine replaces the cysteine at 
residue #177 of the enzyme. These results lend strong support to a catalytic 
mechanism in which an enzyme sulfhydryl group undergoes Michael addition to the 
C5-C6 double bond, thus activating position C-5 of the substrate DNA cytosine 
residue for electrophilic attack by the methyl donor SAM. The enzyme is capable 
of self-methylation in a DNA-independent reaction requiring SAM and the presence 
of cysteine at position #177.