<p>This entry represents a structural domain with an irregular fold that is found in a subset of coagulation factors, where it forms part of the Gla region.</p><p>The P-II protein (gene glnB) is a bacterial protein important for the control of glutamine synthetase [<cite idref="PUB00000687"/>, <cite idref="PUB00003730"/>, <cite idref="PUB00005249"/>]. In nitrogen-limiting conditions, when the ratio of glutamine to 2-ketoglutarate decreases, P-II is uridylylated on a tyrosine residue to form P-II-UMP. P-II-UMP allows the deadenylation of glutamine synthetase (GS), thus activating the enzyme. Conversely, in nitrogen excess, P-II-UMP is deuridylated and then promotes the adenylation of GS. P-II also indirectly controls the transcription of the GS gene (glnA) by preventing NR-II (ntrB) from phosphorylating NR-I (ntrC), the transcriptional activator of glnA. Once P-II is uridylylated, these events are reversed.</p><p>P-II is a protein of about 110 amino acid residues extremely well conserved. The tyrosine, which is uridylated, is located in the central part of the protein. In cyanobacteria, P-II seems to be phosphorylated on a serine residue rather than being uridylated. In the red alga, <taxon tax_id="2787">Porphyra purpurea</taxon>, there is a glnB homologue encoded in the chloroplast genome. Other proteins highly similar to glnB include <taxon tax_id="1423">Bacillus subtilis</taxon> protein nrgB [<cite idref="PUB00002236"/>]; and <taxon tax_id="562">Escherichia coli</taxon> hypothetical protein ybaI [<cite idref="PUB00001837"/>].</p>