Transcription termination factor. Binds to a 28 bp region within the tRNA(Leu(uur)) gene at a position immediately adjacent to and downstream of the 16S rRNA gene; this region comprises a tridecamer sequence critical for directing accurate termination. Binds DNA along the major grove and promotes DNA bending and partial unwinding. Promotes base flipping. Transcription termination activity appears to be polarized with highest specificity for transcripts initiated on the light strand. Monomer. Predominantly expressed in heart and liver, with extremely low levels in other tissues (PubMed:15582606). Expressed strongly in the heart and at lower levels in brain, liver and kidney (PubMed:23562081). Contains nine structural repeats of about 35 residues, where each repeat contains three helices. The repeats form a left-handed superhelical assembly with a solenoid structure that wraps itself around DNA (By similarity). Phosphoprotein with mostly four phosphate groups. While the DNA-binding activity is unaffected by the phosphorylation state, only the phosphorylated form of the protein is active for termination activity. Functioning seems to be regulated by phosphorylation (By similarity). Double knockout of Mterf1a and Mterf1b results in viable animals with no gross phenotype, and normal oxidative phosphorylation capacity. Steady-state mitochondrial DNA levels are normal. There are subtle effects on levels of mitochondrial transcripts: transcripts initiated at the light strand promoter and also downstream of the MTERF binding site are increased, levels of 7S RNA are reduced, while levels of other mitochondrial transcripts are normal. Belongs to the mTERF family.