The 39-kDa mitochondrial transcription termination factor (mTERF), binds to a 28-base pair region of mtDNA (3237-3249) located within the tRNALeu(UUR) gene, at a position immediately downstream of the rRNA 16S gene (Fernandez-Silva et al., 1997; Kruse et al., 1989; Daga et al., 1993). mTERF binding to the termination sequence block transcription bidirectionally in a partially purified human mitochondrial system (Shang and Clayton, 1994). A polar termination activity has been also observed for the sea urchin homologue of mTERF, referred to as mtDBP with respect of phage RNA polymerases (Fernandez Silva et al. 2001).