Probable S-adenosyl-L-methionine-dependent RNA methyltransferase (PubMed:19351663, PubMed:24651469). May provide the activity for methylation of the rRNA of the small mitochondrial subunit, which is required for the assembly and stability of the mitochondrial ribosome (PubMed:22689777). Has no protein methyltransferase activity (PubMed:24651469). Associates with mitochondrial ribosomes. Present with 1900 molecules/cell in log phase SD medium. Although reported constituent of the mitochondrial small ribosomal subunit (PubMed:11278769), it has not been identified in the structure of the yeast mitoribosome (PubMed:28154081), indicating that RSM22 only transiently interacts with the mitoribosome. Belongs to the methyltransferase superfamily. Rsm22 family.