Human PNPase I
Identifiers
Symbol	PNPASE
Alt. symbols	PNPase, OLD35, old-35
Entrez	87178
HUGO	23166
OMIM	610316
PDB	1E3P
RefSeq	NM_033109
UniProt	Q8TCS8
Other data
EC number	2.7.7.8
Locus	Chr. 2 p15

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 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 got disproved by the late 1950s.

It is involved on mRNA processing and degradation in bacteria, plants, and in humans.

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 and 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 have been used in the study of PNPT1 function. A conditional knockout mouse line, called Pnpt1^{tm1a(KOMP)Wtsi} was generated as part of the International Knockout Mouse Consortium program — a high-throughput mutagenesis project to generate and distribute animal models of disease to interested scientists.

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