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SUMMARY: Increasing concentrations of ammonium ions in the medium of nitrogen-fixing, sulphate-limited continuous cultures of Azotobacter chroococcum caused a proportionate repression of nitrogenase activity; free NH4+ could be detected in the extracellular culture fluid only when nitrogenase activity was wholly repressed. The NH4+ concentrations giving 50% or 100% repression were proportional to the population density. Nitrate ions repressed with similar stoichiometry; glutamate, glutamine and aspartate did not repress and were not metabolized; repressed and derepressed populations contained equal amounts and proportions of glutamate-forming enzymes. Repressed populations lacked both enzymatic components of nitrogenase. The intracellular free amino acid pools were typical of Gram-negative bacteria; an increase in the degree of repression was associated with an increase in the pool levels of ammonia, aspartate and glutamate. Nitrogen-fixing populations possessed a convoluted intracytoplasmic membrane system which was absent from ammonia-assimilating organisms, but the phospholipid contents of the two types of population were similar. All members of a half-repressed population possessed these membranes, but to a lesser extent that fully derepressed populations.When N2-fixing chemostat populations were abruptly exposed to repressive concentrations of ammonium succinate. repression occurred exponentially and nitrogenase activity disappeared from the culture faster than wash-out of stable enzyme. Repression was not alleviated by exogenous cyclic AMP. Derepression was complete, according to the acetylene test, within half a doubling time of disappearance of free ammonium ions from the culture.

A Chemostat Study of the Effect of Fixed Nitrogen Sources on Nitrogen Fixation, Membranes and Free Amino Acids in Azotobacter chroococcum