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. Plays a role in the secondary pathway of 23S rRNA 3' end maturation (PubMed:19880604). phosphate + tRNA(n+1) = a ribonucleoside 5'-diphosphate + tRNA(n) Homohexameric ring arranged as a trimer of dimers (PubMed:14767080). It has been suggested that the active form is the dimer which binds tRNA and that the hexameric form protects the substrate recognition loop (approximately residues 65-82) from proteolysis (PubMed:14767080). Correct processing of the 3' end of 23S rRNA no longer occurs in the absence of mrnC. Sulfate ions in the crystal structure may represent the inorganic phosphate substrate. Belongs to the RNase PH family.