RNA cytosine C(5)-methyltransferase that methylates cytosine to 5-methylcytosine (m5C) in various RNAs, such as tRNAs, mRNAs and some long non-coding RNAs (lncRNAs) (PubMed:17071714, PubMed:22995836, PubMed:31358969, PubMed:31199786). Involved in various processes, such as epidermal stem cell differentiation, testis differentiation and maternal to zygotic transition during early development: acts by increasing protein synthesis; cytosine C(5)-methylation promoting tRNA stability and preventing mRNA decay (PubMed:31199786). Methylates cytosine to 5-methylcytosine (m5C) at positions 34 and 48 of intron-containing tRNA(Leu)(CAA) precursors, and at positions 48, 49 and 50 of tRNA(Gly)(GCC) precursors (PubMed:17071714, PubMed:22995836, PubMed:31199786). tRNA methylation is required generation of RNA fragments derived from tRNAs (tRFs) (PubMed:31199786). Also mediates C(5)-methylation of mitochondrial tRNAs (PubMed:31276587). Catalyzes cytosine C(5)-methylation of mRNAs, leading to stabilize them and prevent mRNA decay: mRNA stabilization involves YBX1 that specifically recognizes and binds m5C-modified transcripts (PubMed:22395603, PubMed:31358969, PubMed:34556860). Cytosine C(5)-methylation of mRNAs also regulates mRNA export: methylated transcripts are specifically recognized by THOC4/ALYREF, which mediates mRNA nucleo-cytoplasmic shuttling (PubMed:28418038). Also mediates cytosine C(5)-methylation of non-coding RNAs, such as vault RNAs (vtRNAs), promoting their processing into regulatory small RNAs (PubMed:23871666). Cytosine C(5)-methylation of vtRNA VTRNA1.1 promotes its processing into small-vault RNA4 (svRNA4) and regulates epidermal differentiation (PubMed:31186410). May act downstream of Myc to regulate epidermal cell growth and proliferation (By similarity). Required for proper spindle assembly and chromosome segregation, independently of its methyltransferase activity (PubMed:19596847). cytidine(48) in tRNA + S-adenosyl-L-methionine = 5-methylcytidine(48) in tRNA + H(+) + S-adenosyl-L-homocysteine cytidine(49) in tRNA + S-adenosyl-L-methionine = 5-methylcytidine(49) in tRNA + H(+) + S-adenosyl-L-homocysteine cytidine(50) in tRNA + S-adenosyl-L-methionine = 5-methylcytidine(50) in tRNA + H(+) + S-adenosyl-L-homocysteine cytidine(34) in tRNA precursor + S-adenosyl-L-methionine = 5-methylcytidine(34) in tRNA precursor + H(+) + S-adenosyl-L-homocysteine a cytidine in mRNA + S-adenosyl-L-methionine = a 5-methylcytidine in mRNA + H(+) + S-adenosyl-L-homocysteine Inhibited by magnesium ions. Interacts with NPM1 and NCL during interphase; interaction is disrupted following phosphorylation at Ser-139. Concentrated in the nucleolus during interphase and translocates to the spindle during mitosis as an RNA-protein complex that includes 18S ribosomal RNA (PubMed:19596847). In testis, localizes to the chromatoid body (By similarity). Expressed in adult and fetal brain and in lymphoblastoid cells. Phosphorylated at Ser-139 by AURKB during mitosis, leading to abolish methyltransferase activity and the interaction with NPM1. The disease is caused by variants affecting the gene represented in this entry. Belongs to the class I-like SAM-binding methyltransferase superfamily. RsmB/NOP family. TRM4 subfamily. Truncated N-terminus. Truncated N-terminus.