A methylase, recognizes the double-stranded sequence 5'-GGCC-3', methylates C-3 on both strands, and protects the DNA from cleavage by the BspRI endonuclease. a 2'-deoxycytidine in DNA + S-adenosyl-L-methionine = a 5-methyl-2'-deoxycytidine in DNA + H(+) + S-adenosyl-L-homocysteine Monomer. In the absence of DNA, can self-methylate two cysteine residues. Belongs to the class I-like SAM-binding methyltransferase superfamily. C5-methyltransferase family.