Polynucleotide Phosphorylase (PNPase) is a bifunctional enzyme with a phosphorolytic 3' to 5' exoribonuclease activity and a 3'-terminal oligonucleotide polymerase activity.^[2] That is, it dismantles the RNA chain starting at the 3' end and working toward the 5' end.^[1] It also synthesizes long, highly heteropolymeric tails in vivo. It accounts for all of the observed residual polyadenylation in strains of Escherichia coli missing the normal polyadenylation enzyme.^[1] Discovered by Marianne Grunberg-Manago working in Severo Ochoa's lab in 1955, the RNA-polymerization activity of PNPase was initially believed to be responsible for DNA-dependent synthesis of messenger RNA, a notion that was disproven by the late 1950s.^[3][4]

It is involved in mRNA processing and degradation in bacteria, plants,^[5] and animals.^[6]

In humans, the enzyme is encoded by the PNPT1 gene. In its active form, the protein forms a ring structure consisting of three PNPase molecules. Each PNPase molecule consists of two RNase PH domains, an S1 RNA binding domain and a K-homology domain. The protein is present in bacteria and in the chloroplasts^[2] and mitochondria^[7] of some eukaryotic cells. In eukaryotes and archaea, a structurally and evolutionary related complex exists, called the exosome complex.^[7]

The same abbreviation (PNPase) is also used for another, otherwise unrelated enzyme, Purine nucleoside phosphorylase.