Polymerase that creates the 3' poly(A) tail of mitochondrial transcripts. Can use all four nucleotides, but has higher activity with ATP and UTP (in vitro). Plays a role in replication-dependent histone mRNA degradation. May be involved in the terminal uridylation of mature histone mRNAs before their degradation is initiated. Might be responsible for the creation of some UAA stop codons which are not encoded in mtDNA. ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Homodimer. Ubiquitous, with stronger expression in tissues with high energy requirements: heart, brain, and skeletal muscle. The disease is caused by variants affecting the gene represented in this entry. MTPAP mutations result in a defect of mitochondrial mRNA maturation. Affected individuals exhibit a drastic decrease in poly(A) tail length of mitochondrial mRNA transcripts, including COX1 and RNA14 (PubMed:20970105). Belongs to the DNA polymerase type-B-like family.