Mitochondrial Ribosomal RNA (rRNA) Methyltransferase Family Members Are Positioned to Modify Nascent rRNA in Foci near the Mitochondrial DNA Nucleoid

We have identified RNMTL1, MRM1, and MRM2 (FtsJ2) as members of the RNA methyltransferase family that may be responsible for the three known 2 '-O-ribose modifications of the 16 S rRNA core of the large mitochondrial ribosome subunit. These proteins are confined to foci located in the vicinity of mtDNA nucleoids. They show distinct patterns of association with mtDNA nucleoids and/or mitochondrial ribosomes in cell fractionation studies. We focused on the role of the least studied protein in this set, RNMTL1, to show that this protein interacts with the large ribosomal subunit as well as with a series of non-ribosomal proteins that may be involved in coupling of the rate of rRNA transcription and ribosome assembly in mitochondria. siRNA-directed silencing of RNMTL1 resulted in a significant inhibition of translation on mitochondrial ribosomes. Our results are consistent with a role for RNMTL1 in methylation of G(1370) of human 16 S rRNA.