Fig. 7: NirP1 is a regulator at a central position in the assimilation of inorganic nitrogen.

Cyanobacteria can assimilate N from simple compounds, such as ammonium, nitrate, nitrite, and urea. During nitrate assimilation, intracellular nitrate is reduced in two steps to nitrite and ammonium, which is then incorporated into glutamate in the GS/GOGAT cycle or into carbon skeletons during the synthesis of amino acids by transaminases. The reduction of nitrate to nitrite is catalyzed by the NR and the subsequent reduction to ammonium by the NiR. Transcription of the regulatory factor NirP1 is controlled by an unknown transcription factor (TF) and by NtcA. 2-OG is a corepressor of NtcA. Therefore, an increased 2-OG level leads to the repression of nirP1 transcription, while a low level of 2-OG leads to its activation. NirP1 binds to the enzyme nitrite reductase (NiR), leading to the inhibition of nitrite reduction. The accumulating nitrite is then rapidly exported by an unidentified export system. Transport systems for ammonium (AMT) and nitrate/nitrite (NRT) uptake are shown.