Mitochondrial S-adenosyl-L-methionine-dependent methyltransferase that mediates N(3)-methylcytidine modification of residue 32 of the tRNA anticodon loop of mitochondrial tRNA(Ser)(UCN) and tRNA(Thr) (PubMed:34774131, PubMed:35017528). N(3)-methylcytidine methylation modification regulates mitochondrial translation efficiency and is required for activity of the respiratory chain (PubMed:34774131, PubMed:35017528). N(3)-methylcytidine methylation of mitochondrial tRNA(Ser)(UCN) requires the formation of N(6)-dimethylallyladenosine(37) (i6A37) by TRIT1 as prerequisite (PubMed:34774131, PubMed:35017528). May also mediate N(3)-methylcytidine modification of mRNAs (PubMed:28655767). The existence of N(3)-methylcytidine modification on mRNAs is however unclear, and additional evidences are required to confirm the role of the N(3)-methylcytidine-specific mRNA methyltransferase activity of METTL8 in vivo (PubMed:33313824, PubMed:34774131).
