Phosphorolytic 3'-5' exoribonuclease that plays an important role in tRNA 3'-end maturation. Removes nucleotide residues following the 3'-CCA terminus of tRNAs; can also add nucleotides to the ends of RNA molecules by using nucleoside diphosphates as substrates, but this may not be physiologically important. Probably plays a role in initiation of 16S rRNA degradation (leading to ribosome degradation) during starvation. phosphate + tRNA(n+1) = a ribonucleoside 5'-diphosphate + tRNA(n) Homohexameric ring arranged as a trimer of dimers. Belongs to the RNase PH family.