NSUN4

Mitochondrial RNA cytosine C(5)-methyltransferase that methylates cytosine to 5-methylcytosine (m5C) in various RNAs, such as rRNAs, mRNAs and some long non-coding RNAs (lncRNAs). Involved in mitochondrial ribosome small subunit (SSU) maturation by catalyzing methylation of mitochondrial 12S rRNA; the function is independent of MTERFD2/MTERF4 and assembled mitochondrial ribosome large subunit (LSU). Targeted to LSU by MTERFD2/MTERF4 and probably is involved in a final step in ribosome biogenesis to ensure that SSU and LSU are assembled. In vitro can methylate 16S rRNA of the LSU; the methylation is enhanced by MTERFD/MTERF4. Also acts as a regulator of innate immunity by marking double-stranded mitochondrial RNAs(mt-dsRNAs) generated in response to stress: catalyzes m5C modification on mitochondrial RNAs, such as a mRNAs and lncRNAs, with a preference for the termini of light-strand lncRNAs, promoting their degradation and cytosolic release. Modified light-strand lncRNAs are then recognized by C1QBP reader and recruited to the mitochondrial degradosome complex, which promotes their degradation..

Function from UniProtKB (Q96CB9), read 2026-09-23. CC BY 4.0.