S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the C(5) position of a cytosine in 28S rRNA. a cytidine in 28S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 28S rRNA + H(+) + S-adenosyl-L-homocysteine Increased lifespan and stress resistance. Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.