Summary: The synthesis of nitrogenase by intact cells of was repressed in N-free media supplemented with L-glutamine or L-asparagine, but was unaffected by the presence of L-glutamate, L-aspartate or L-histidine. Specific activities attained by cultures in supplemented media maintained under Ar-CO were 2 to 3 times higher than those in N-free medium under N-CO. A loss in total activity occurred both in cultures growing with N after maximum activity had been reached, and in cultures maintained under Ar when the gas phase was changed to N. There was a rapid loss in nitrogen-fixing activity when low concentrations of NH , L-glutamine or L-asparagine were added to cultures with high activities, but this could be recovered in the absence of demonstrable protein synthesis. During growth, the degree of inactivation brought about by 0·5 mM-inactivator increased to 80 to 90%, and NH excreted into the medium reached a maximum concentration towards the end of exponential growth.


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