Involved in mRNA degradation. Catalyzes the phosphorolysis of single-stranded polyribonucleotides processively in the 3'- to 5'-direction. Also involved, along with RNase II, in tRNA processing. RNases II and R contribute to rRNA degradation during starvation, while RNase R and PNPase are the major contributors to quality control of rRNA during steady state growth (PubMed:21135037). phosphate + RNA(n+1) = a ribonucleoside 5'-diphosphate + RNA(n) Magnesium. Can also use manganese. Component of the RNA degradosome, which is a multiprotein complex involved in RNA processing and mRNA degradation. Interacts with RNase E (rne). Homotrimer. The homotrimer forms a ring-like structure with a central channel, where RNA molecules can bind. RNA molecules bind between neighboring subunits. Has also been isolated in association with the inner membrane. Expression is negatively autoregulated at the translational level via an RNase III-dependent mechanism. Induced by cold shock (42 to 15 degrees Celsius) (at protein level) (PubMed:8898389). At low temperature, the destabilizing effect of PNPase on its own mRNA is less efficient, leading to a decrease in repression and an increase in the expression level. The S1 motif domain is important for trimerization and RNA-binding. Belongs to the polyribonucleotide nucleotidyltransferase family. Extended N-terminus. Extended N-terminus.