DNA-dependent RNA polymerase catalyzes the transcription of mitochondrial DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of the mitochondrial transcription initiation complex, composed at least of TFB2M, TFAM and POLRMT that is required for basal transcription of mitochondrial DNA. 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. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Homodimer. Component of the mitochondrial transcription initiation complex, composed at least of TFB2M, TFAM and POLRMT. 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 (By similarity). 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. Interacts with TFB1M and TFB2M, leading to the stimulation of transcription. Interacts with TEFM. Interacts with MTRES1 (By similarity). Belongs to the phage and mitochondrial RNA polymerase family.