Summary: Effects of oxygen upon derepression of nitrogenase were studied in , using oxyleghaemoglobin to supply and monitor very low dissolved O concentrations in a steady-state system. Expression of the gene was studied by using a fusion strain, which also carried the Nif plasmid pRD 1 so that the production of active nitrogenase could also be monitored. When compared with anaerobic treatments, very low concentrations of dissolved O inhibited derepression of both and pRD1 Fifty percent inhibition of derepression occurred at 0.1 μm-O. The apparent K of the dominant terminal oxidase was 0.08 μm-O. These results suggest that there is a close relationship between the terminal respiratory system of these bacteria and the repression of nitrogenase by O.


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