Regulation of ammonium assimilation in cyanobacteria

Ammonium assimilation constitutes a central metabolic pathway in cyanobacteria that is linked to photosynthesis through the requirement of ATP and reducing power. Ammonium entry into the cell takes place in two ways: diffusion of its gaseous form ammonia (NH3), or operation of specific permeases for its charged form, ammonium (NH4+). Other nitrogen sources also provide ammonium as a result of their metabolization (mainly reduction). Ammonium is incorporated into carbon skeletons by the sequential action of two enzymes: glutamine synthetase (GS) and glutamate synthase (GOGAT), commonly known as the GS-GOGAT pathway. 2-Oxoglutarate synthesized by isocitrate dehydrogenase (IDH) is the carbon skeleton required for ammonium incorporation. Glutamate dehydrogenase (GDH) is also present in some cyanobacteria, but with a limited role in ammonium assimilation. Maintenance of carbon/nitrogen (C/N) balance needs a fine regulation of the GS-GOGAT pathway, GS being the target of this regulation at transcriptional and posttranscriptional levels. The transcription factor NtcA plays a central role in nitrogen regulation by activating the expression of many nitrogen-regulated genes. In addition, NtcA can also repress the synthesis of two inhibitory polypeptides (IF7 and IF17) that inactivate the GS by direct protein-protein interaction. The signalling of the C/N status is carried out by the intracellular concentration of 2-oxoglutarate, a metabolite that is able to modulate NtcA function.