<p>This entry represents a structural domain found in the nitrogen regulatory protein PII, in ATP phosphribosyltransferases (C-terminal domain), in the divalent ion tolerance protein CutA1, 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. </p><p>PII is a tetrameric protein encoded by glnB that functions as a component of the adenylation cascade involved in the regulation of GS activity [<cite idref="PUB00003738"/>]. 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 [<cite idref="PUB00035665"/>].</p><p>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 [<cite idref="PUB00022501"/>].</p><p>CutA1 functions in divalent ion tolerance in bacteria, plants and animals [<cite idref="PUB00014093"/>, <cite idref="PUB00007506"/>]. Divalent metal ions play key roles in all living organisms, serving as cofactors for many proteins involved in a variety of electron-transfer activities. In <taxon tax_id="562">Escherichia coli</taxon> it is thought to be involved in copper ion tolerance, excessive copper ions being toxic [<cite idref="PUB00035666"/>].</p>