The DNA (cytosine-5)-methyltransferase (m5C-MTase) M.BspRI is able to accept the 
methyl group from the methyl donor S-adenosyl-L-methionine (AdoMet) in the 
absence of DNA. Transfer of the methyl group to the enzyme is a slow reaction 
relative to DNA methylation. Self-methylation is dependent on the native 
conformation of the enzyme and is inhibited by S-adenosyl-L-homocysteine, DNA 
and sulfhydryl reagents. Amino acid sequencing of proteolytic peptides obtained 
from M.BspRI, which had been methylated with [methyl-3H]AdoMet, and thin layer 
chromatography of the modified amino acid identified two cysteines, Cys156 and 
Cys181 that bind the methyl group in form of S-methylcysteine. One of the 
acceptor residues, Cys156 is the highly conserved cysteine which plays the role 
of the catalytic nucleophile of m5C-MTases.