A methylase, recognizes the double-stranded sequence 5'-YTCGAR-3', methylates C-3 on both strands, and protects the DNA from cleavage by the BsuMI endonuclease. a 2'-deoxycytidine in DNA + S-adenosyl-L-methionine = a 5-methyl-2'-deoxycytidine in DNA + H(+) + S-adenosyl-L-homocysteine Somewhat inhibited by MgCl(2) and spermidine, strongly inhibited by MnCl(2). Optimum pH is 8.0. Rapidly loses activity at 37 degrees Celsius in the absence of DNA. Monomer (PubMed:3150363). May form a complex with YdiP, also seems to be active alone (Probable). Not expressed during sporulation. Constitutively expressed during exponential growth. Encoded in an operon with ydiP and in a second with groES, groEL, ydiM and ydiN. This second operon is heat-inducible. Essential for growth, it can be disrupted once one of the components of the corresponding BsuMI restriction endonuclease complex (AC O34303, O34885, O35025, YdjA, YdiS and YdiR respectively) has been disrupted (PubMed:11751814). Triple deletions ydiO-ydiP-ydiR, ydiO-ydiP-ydiS and ydiO-ydiP-ydjA lead to loss of susceptibility to MspJI, which only digests C-methylated DNA (PubMed:32324221). Belongs to the class I-like SAM-binding methyltransferase superfamily. C5-methyltransferase family. The characterized enzyme was reported to be a monomer of approximately 45 kDa; it is not clear whether this corresponds to YdiO, YdiP or to another activity altogether.