12S rRNA N(4)-cytidine methyltransferase METTL15
rRNA base methylation
The human mitochondrial 12S rRNA m<sup>4</sup>C methyltransferase METTL15 is required for mitochondrial function.
Mitochondrial DNA gene expression is coordinately regulated both pre- and post-transcriptionally, and its perturbation can lead to human pathologies. Mitochondrial rRNAs (mt-rRNAs) undergo a series of nucleotide modifications after release from polycistronic mitochondrial RNA precursors, which is essential for mitochondrial ribosomal biogenesis. Cytosine <i>N</i><sup>4</sup>-methylation (m<sup>4</sup>C) at position 839 (m<sup>4</sup>C839) of the 12S small subunit mt-rRNA was identified decades ago; however, its biogenesis and function have not been elucidated in detail. Here, using several approaches, including immunofluorescence, RNA immunoprecipitation and methylation assays, and bisulfite mapping, we demonstrate that human methyltransferase-like 15 (METTL15), encoded by a nuclear gene, is responsible for 12S mt-rRNA methylation at m<sup>4</sup>C839 both <i>in vivo</i> and <i>in vitro</i> We tracked the evolutionary history of RNA m<sup>4</sup>C methyltransferases and identified a difference in substrate preference between METTL15 and its bacterial ortholog rsmH. Additionally, unlike the very modest impact of a loss of m<sup>4</sup>C methylation in bacterial small subunit rRNA on the ribosome, we found that METTL15 depletion results in impaired translation of mitochondrial protein-coding mRNAs and decreases mitochondrial respiration capacity. Our findings reveal that human METTL15 is required for mitochondrial function, delineate the evolution of methyltransferase substrate specificities and modification patterns in rRNA, and highlight a differential impact of m<sup>4</sup>C methylation on prokaryotic ribosomes and eukaryotic mitochondrial ribosomes.