Active glutamine synthetase is required for ammonium‐ or glutamine‐promoted prevention of nitrate and nitrite reduction in the cyanobacterium Phormidium laminosum

Abstract

The filamentous non‐N2‐fixing cyanobacterium Phormidium laminosum (strain OH‐1‐p.Cl1) was able to utilize glutamine as the sole nitrogen source. The addition to ammonium‐grown cultures of the irreversible inhibitor of glutamine synthetase activity L‐methionine‐D, L‐sulfoximine (MSX) inhibited cell growth. Supplying glutamine to the culture restored cell growth. This re‐established growth was not due to interference by glutamine of MSX uptake by the cells, since glutamine synthetase (GS, EC 6.3.1.2) activity remained completely inhibited by MSX even when glutamine was simultaneously present. Both glutamine and ammonium exerted a negative effect on nitrate reductase (NR. EC 1.7.7.2) and nitrite reductase (NiR, EC 1.7.7.1) in vivo. This negative effect was reversed by MSX. When glutamine was added to MSX‐treated cells, intracellular glutamine level was high, but the activity of both reductases remained at a high level. These results suggest that the presence of the active form of glutamine synthetase is required for the in vivo prevention of nitrate assimilation caused by ammonium and glutamine.