S-adenosyl-L-methionine-dependent methyltransferase which methylates the carbon-5 position of cytosine 2381 to 5-methylcytosine (m5C2381) in 26S rRNA (PubMed:25635753, PubMed:33289480, PubMed:33283887). Plays a role in the production of mature 5S, 5.8S, 18S and 26S rRNAs and promotes the processing of the internally transcribed spacer 2 (ITS2), which separates the 5.8S and 26S rRNAs on large pre-rRNA precursors (PubMed:33289480). May play a role in the translation of leucine and proline codons (Probable). May play a role in maintaining ribosomal frameshifting in response to osmotic stress (PubMed:30977983). Not required for global translation (PubMed:33289480). a cytidine in 26S rRNA + S-adenosyl-L-methionine = a 5-methylcytidine in 26S rRNA + H(+) + S-adenosyl-L-homocysteine RNAi-mediated knockdown increases lifespan and stress resistance (PubMed:25635753). RNAi-mediated knockdown results in a higher frequency of gene products that arise as a consequence of increased ribosomal frameshifting in response to osmotic stress induced by high salt concentrations in adults (PubMed:30977983). RNAi-mediated knockdown does not result in fertility defects (PubMed:33283887). Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.