InterPro domain: IPR015867

This entry represents a structural domain found in the nitrogen regulatory protein PII, in ATP phosphribosyltransferases (C-terminal domain), and in some bacterial hypothetical proteins. This domain consists of a ferredoxin-like alpha/beta sandwich, which forms trimeric structures with orthogonally packed beta-sheets around a three-fold axis.

PII is a tetrameric protein encoded by glnB that functions as a component of the adenylation cascade involved in the regulation of glutamine synthetase activity [ 1 ]. PII helps regulate the level of glutamine synthetase in response to nitrogen source availability. In nitrogen-limiting conditions, PII is uridylylated to form PII-UMP, which allows the deadenylation of glutamine synthetase, thus activating the enzyme. Conversely, in nitrogen excess, PI-UMP is deuridylated to PII, promoting the adenylation and deactivation of glutamine synthetase [ 2 ].

ATP phosphoribosyltransferase is the first enzyme of the histidine pathway. It is allosterically regulated, controlling the flow of intermediates through the pathway. The C-terminal domain is the regulatory region of the protein, which binds the allosteric inhibitor histidine [ 3 ].

1. Characterization of three different nitrogen-regulated promoter regions for the expression of glnB and glnA in Azospirillum brasilense. Mol. Gen. Genet. 224, 421-30
2. Characterization of pII family (GlnK1, GlnK2, and GlnB) protein uridylylation in response to nitrogen availability for Rhodopseudomonas palustris. Anal. Biochem. 357, 93-104
3. The structure of Escherichia coli ATP-phosphoribosyltransferase: identification of substrate binding sites and mode of AMP inhibition. J. Mol. Biol. 336, 131-44