Abstract
Addition of NH4Cl at low concentrations to Azotobacter chroococcum cells caused an immediate cessation of nitrate uptake activity, which was restored when the added NH4+was exhausted from the medium or by adding an NH4+assimilation inhibitor, l-methionine-dl-sulfoximine (MSX) or l-methionine sulfone (MSF). In the presence of such inhibitors the newly-reduced nitrate was released into the medium as NH4+. When the artificial electron donor system ascorbate/N-methylphenazinium methylsulfate (PMS), which is a respiratory substrate that was known to support nitrate uptake by A. chroococcum while inhibiting glutamine synthetase activity, was the energy source, externally added NH4+had no effect on nitrate uptake. It is concluded that, in A. chroococcum cells, NH4+must be assimilated to exert its short-term inhibitory effect on nitrate uptake. A similar proposal was previously made to explain the short-term ammonium inhibition of N2 fixation in this bacterium.
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