A beta subtype methylase, recognizes the double-stranded sequence 5'-GGATCC-3', methylates C-5 on both strands, and protects the DNA from cleavage by the BamHI endonuclease. a 2'-deoxycytidine in DNA + S-adenosyl-L-methionine = an N(4)-methyl-2'-deoxycytidine in DNA + H(+) + S-adenosyl-L-homocysteine The protein is very stable, after 18 months at -20 degrees Celsius it retains 90% of its activity. The protein sequnce was determined following expression in E.coli. Belongs to the N(4)/N(6)-methyltransferase family. Truncated N-terminus.