Mitochondrial methyltransferase which uses S-adenosyl methionine to dimethylate two highly conserved adjacent adenosine residues (A1583 and A1584) within the loop of helix 45 at the 3-prime end of 12S rRNA, thereby regulating the assembly or stability of the small subunit of the mitochondrial ribosome (PubMed:12496758, PubMed:25305075, PubMed:31251801). Also required for basal transcription of mitochondrial DNA, probably via its interaction with POLRMT and TFAM. Stimulates transcription independently of the methyltransferase activity (PubMed:11809803, PubMed:12068295, PubMed:12897151). {ECO:0000269|PubMed:11809803, ECO:0000269|PubMed:12068295, ECO:0000269|PubMed:12496758, ECO:0000269|PubMed:12897151, ECO:0000269|PubMed:25305075, ECO:0000269|PubMed:31251801}. This is the function of ENSG00000029639 (TFB1M, transcription factor B1, mitochondrial).