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polynucleotide phosphorylase
Polynucleotide Phosphorylase (PNPase) is a bifunctional enzyme with a phosphorolytic 3' to 5' exoribonuclease activity and a 3'-terminal oligonucleotide polymerase activity. That is, it dismantles the RNA chain starting at the 3' end and working toward the 5' end.〔 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.〔 Discovered by Ochoa in 1951, the RNA-polymerization activity of PNPase was initially believed to be responsible for DNA-dependent synthesis of messenger RNA, a notion that got disproved by the late 1950s (http://www.jbc.org/content/281/15/e12.full.pdf).
It is involved on 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 a K-homology domain. The protein is present in bacteria and in the chloroplasts〔 and mitochondria of some eukaryotic cells. In eukaryotes and archaea, a structurally and evolutionary related complex exists, called the ''exosome''.〔
The same abbreviation (''PNPase'') is also used for another, otherwise unrelated enzyme, Purine nucleoside phosphorylase.
==Model organisms==
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