SUMMARY: Rates of nitrogenase synthesis by were measured by pulse-labelling organisms with a mixture of C-labelled amino acids followed by sodium dodecyl sulphate gel electrophoresis and autoradiography. Populations from an NH+-repressed, SO -limited chemostat (0-46 mg dry wt m1), when released from NH+ repression, simultaneously synthesized detectable quantities of the three nitrogenase polypeptides 45 min before acetylene-reducing activity was observed. Exposure of populations synthesizing nitrogenase to air or NH+ (200 µg N m1) repressed synthesis of both component proteins simultaneously, the rate initially decreasing by half in 11 to 12 min; in the presence of NH+ a second slower phase with an approximate half-life of 30 min was observed. With 5% O in N the half-lives for the decreases in the rates of synthesis were 30 min for the Fe protein and 33 min for the Mo-Fe protein. Oxygen also repressed nitrogenase in a glutamine synthetase constitutive derivative of (strain SK24) which escapes NH+ repression. Regulation of nitrogenase by O may therefore be independent of glutamine synthetase.


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