Short-term effect of ammonia on nitrogenase activity ofAnabaena variabilis(ATCC29413)

Intact filaments of the cyanobacterium Anabaena variabilis switch off nitrogenase activity very rapidly upon addition of NH4Cl when incubated in an alkaline environment (pH 10.0) permitting a fast NH3-influx into the cells. When assayed in cell-free extracts (prepared from ammonia-treated filaments), nitrogenase remains inhibited in the presence of an ATP-regenerating system. Furthermore, l-methionine-d,l-sulfoximine, an inhibitor of glutamine synthetase, added to the filaments, prevents inactivation of nitrogenase by ammonia, showing that ammonia is not the compound directly responsible for nitrogenase switch-off.