DNA-dependent RNA polymerase catalyzes the transcription of mitochondrial DNA into RNA using the four ribonucleoside triphosphates as substrates (PubMed:21278163). Component of the mitochondrial transcription initiation complex, composed at least of TFB2M, TFAM and POLRMT that is required for basal transcription of mitochondrial DNA (PubMed:29149603). In this complex, TFAM recruits POLRMT to a specific promoter whereas TFB2M induces structural changes in POLRMT to enable promoter opening and trapping of the DNA non-template strand (PubMed:29149603). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Homodimer (PubMed:29149603). Component of the mitochondrial transcription initiation complex, composed at least of TFB2M, TFAM and POLRMT (PubMed:29149603). In this complex TFAM recruits POLRMT to the promoter whereas TFB2M induces structural changes in POLRMT to enable promoter opening and trapping of the DNA non-template strand (PubMed:29149603). Upon metabolic stress, forms a complex composed of FOXO3, SIRT3 and mitochondrial RNA polymerase POLRMT; the complex is recruited to mtDNA in a SIRT3-dependent manner (PubMed:23283301). Also forms a complex composed of FOXO3, SIRT3, TFAM and POLRMT (PubMed:29445193). Interacts with TFB1M and TFB2M, leading to the stimulation of transcription (PubMed:12068295). Interacts with TEFM (PubMed:21278163). Interacts with MTRES1 (PubMed:31226201). Belongs to the phage and mitochondrial RNA polymerase family.