Terminal uridylyltransferase which is involved in the post-transcriptional editing of mitochondrial RNA, a process involving the addition and deletion of uridine (U) nucleotides in the pre-mRNA. Specifically, catalyzes the addition of Us to the 3'-hydroxyl group of guided RNA (gRNA), with a preference for RNAs terminating in 6 Us, but also can add Us to RNAs terminating in 6 adenines (A), 6 cytosines (C), or 6 guanines (G) (PubMed:11893335, PubMed:15060068). Can mediate RNA-independent UTP polymerization in vitro (PubMed:15060068). Can mediate pyrophosphate-dependent degradation of synthetic RNA ending with U residues in vitro (PubMed:15060068). RNA(n) + UTP = diphosphate + RNA(n)-3'-uridine ribonucleotide Binds 1 Mg(2+) or Mn(2+) per subunit (PubMed:11893335). The type of divalent cation used by the enzyme affects the nucleotide specificity; Mg(2+) induces predominantly uridine (U) incorporation while Mn(2+) induces also substantial incorporation of both adenine (A) and cytosine (C) (PubMed:11893335). Zinc-binding is required for catalytic activity. KM is 45-65 uM for UTP (at 27 degrees Celsius) (PubMed:15060068). KM is 18-30 uM for RNA (at 27 degrees Celsius) (PubMed:15060068). Homotetramer (PubMed:11893335, PubMed:15060068). Part of a 700kDa complex (PubMed:11893335). Interacts with p45 and p50 RNA ligases (PubMed:11893335). Belongs to the DNA polymerase type-B-like family.