Mitochondrial methyltransferase which methylates cytosine to 5-methylcytosine (m5C) in rRNAs and tRNAs at multiple sites (PubMed:33283887). May play a role in the translation of leucine and proline codons (Probable). 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 Expression increases during the larval stages to adulthood. RNAi-mediated knockdown does not result in fertility defects. Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family.