S-adenosyl-L-methionine-dependent methyltransferase that specifically methylates the C(5) position of cytosine 2268 (m5C2268) in 25S rRNA. a cytidine in 25S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 25S rRNA + H(+) + S-adenosyl-L-homocysteine Reduced methylation of the C(5) position of cytosine 2268 (m5C2268) in 25S rRNA. Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.