Abstract
Addition of NH 4Cl at low concentrations to Azotobacter chroococcum cells caused an immediate cessation of nitrogenase activity, which was recovered once the added NH 4 + was exhausted from the medium. In the presence of inhibitors of ammonium assimilation, such as L-methionine-DL-sulfoximine, L-methionine sulfone or 6-diazo-5-oxo-L-norleucine, externally added NH 4 + had no effect on nitrogenase activity and the newly-fixed nitrogen was excreted into the medium as NH 4 +. It is concluded that, in A. chroococcum , NH 4 + must be assimilated to exert its short-term inhibitory effect on nitrogen fixation.
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