A methyltransferase that methylates C-1 within the sequences 5'-GGCC-3' and 5'-GCNGC-3' (PubMed:12654995). Modification confers resistance against restriction enzymes that recognize these sequences (Probable). 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.