Transcription elongation factor which increases mitochondrial RNA polymerase processivity (PubMed:21278163, PubMed:36823193). Regulates transcription of the mitochondrial genome, including genes important for the oxidative phosphorylation machinery (PubMed:21278163, PubMed:36823193). {ECO:0000269|PubMed:21278163, ECO:0000269|PubMed:36823193}. This is the function of TEFM (transcription elongation factor, mitochondrial, ENSG00000172171).