Involved in mitochondrial ribosome large subunit biogenesis. a cytidine in rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in rRNA + H(+) + S-adenosyl-L-homocysteine a cytidine in tRNA + S-adenosyl-L-methionine = a 5-methylcytidine in tRNA + H(+) + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.