Polyribonucleotide nucleotidyltransferase <p>The eukaryotic exosome and the prokaryotic degradosome are important protein complexes involved in RNA processing and maintaining appropriate RNA levels within the cell [<cite idref="PUB00033243"/>]. Both of these complexes contain exoribonucleases (exoRNases) which degrade RNA from the 3' end. The hydrolytic exoRNases produce nucleoside monophosphates, while the phosphorolytic exoRNases add orthophosphate at the cleaved bond to produce nucleoside monophosphates.</p><p>This entry represents polyribonucleotide nucleotidyltransferase (<db_xref db="EC" dbkey=""/>), also known as polynucleotide phosphorylase (PNPase), found in bacterial and eukaryotic organelle degradosomes. This enzyme can process single-stranded RNA, but is stalled by double-stranded structures such as stem-loops. Structural studies show that PNPase is a trimeric multidomain protein with a central channel [<cite idref="PUB00024411"/>]. Each subunit contains duplicated RNase PH-like domains which, though structurally homologous, are thought to be functionally distinct. The first domain is more divergent in sequence than than the second domain and is thought to be involved in the flexible binding of RNA substrate and the formation of the trimer channel structure. The second domain is thought to contain the catalytic site and show exoRNase activity. The catalytic mechanism of the enzyme is not yet known but it seems likely that single-stranded RNA would be threaded through the channel to be processed by the three active sites within the trimer, which would thus be restricted to a single substrate molecule per trimer. PNPase activity would thus be tightly regulated by secondary structural elements within the RNA [<cite idref="PUB00035567"/>].</p><p>More information about these proteins can be found at Protein of the Month: RNA Exosomes [<cite idref="PUB00035573"/>].</p>