Human mtDNA is transcribed by a dedicated mitochondrial RNA polymerase (POLRMT), which displays significant sequence similarity to the monomeric RNA polymerases found in bacteriophages. In contrast to the phage T7 RNA polymerase, POLRMT cannot interact with promoter DNA and initiate transcription on its own, but requires the presence of the mitochondrial transcription factor A (TFAM), and either transcription factor B1 (TFB1M) or B2 (TFB2M). The 4 proteins of the basal mitochondrial transcription machinery have been purified in recombinant form and used to reconstitute transcription in vitro with a promoter containing DNA fragment (Falkenberg et al., 2002). Although both TFB1M and TFB2M can support in vitro transcription with POLRMT, TFB2M is at least two orders of magnitude more active than TFB1M and the physiological role of TFB1M in mitochondrial transcription has not yet been completely defined. The TFB1M and TFB2M display primary sequence similarity to a family of rRNA methyltransferases, which dimethylates two adjacent adenosine bases near the 3' end of the small subunit rRNA during ribosome biogenesis (Falkenberg et al., 2002; McCulloch et al., 2002). Human TFB1M is, in fact, a dual function protein, which not only support mitochondrial transcription in vitro, but also acts as a rRNA methyltransferase (Seidel-Rogol et al., 2003). The methyltransferase activity is not required for transcription, since point mutations in conserved methyltransferase motifs of TFB1M revealed that it stimulates transcription in vitro independently of S-adenosylmethionine binding and rRNA methyltransferase activity.