Polynucleotide phosphorylase

Polynucleotide Phosphorylase (PNPase) is bifunctional enzyme with a phosphorolytic 3' to 5' exoribonuclease activity and a 3'-terminal oligonucleotide polymerase activity. It is involved with mRNA processing and degradation in bacteria, plants, and in humans.

In humans, the enzyme is encoded by the 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 an K-homology domain. The protein is present in eubacteria and in the chloroplasts and mitochondria of some eukaryotic cells. In eukaryotes and archaebacteria, a structurally and evolutionary related complex exists, called the exosome.

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