A methylase, recognizes the double-stranded sequence 5'-GGNNCC-3', methylates C-5 on both strands, and protects the DNA from cleavage by the NgoBV endonuclease. a 2'-deoxycytidine in DNA + S-adenosyl-L-methionine = a 5-methyl-2'-deoxycytidine in DNA + H(+) + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. C5-methyltransferase family.