This entry describes GlnD, the uridylyltransferase/uridylyl-removing enzyme for signal-transduction protein PII, and acts as the sensory component of the nitrogen regulation (ntr) system [PMID:11810255]. The ntr system modulates nitrogen metabolism in response to the prevailing nitrogen source and the requirements of the cell. During nitrogen fixation, ammonia and 2-oxoglutarate can be used to produce glutamate. The activity of the PII protein is stimulated by glutamine and inhibited by 2-oxoglutarate. Under glutamate-limiting conditions, PII is uridylylated by GlnD leading to the activation of glutamate synthetase and to the stimulation of NtrC-dependent promoters. Under high concentrations of fixed nitrogen, PII is de-uridylylated leading to the inactivation of the glutamate synthetase pathway and switching off NtrC-dependent promoters [PMID:11065377].

Not all homologues of PII share the property of uridylyltransferase modification on the characteristic Tyr residue (see INTERPRO: IPR002332), but the modification site is preserved in the PII homologue of all species with a member of this family.