Involved in the 3'-end maturation of mitochondrial mRNAs, rRNAs and tRNAs. Functions as a poly(A) mRNA 3'-5' degrading phosphorylase (By similarity). phosphate + RNA(n+1) = a ribonucleoside 5'-diphosphate + RNA(n) Belongs to the polyribonucleotide nucleotidyltransferase family.