Effects of very low concentrations of dissolved O on nitrogenase activity in were studied in a stirred chamber system which enabled simultaneous measurements of steady-state O concentrations, O consumption and CH reduction. A strain carrying a chromosomal fusion as well as the Nif pRDl, expressed nitrogenase activity with 80 nM-O, a concentration known to inhibit expression by about 50%. Thus nitrogenase activity was no more sensitive to O than expression of . When compared with anaerobic treatments, dissolved O near 30 nM apparently stimulated nitrogenase derepression and enhanced the activity of nitrogenase synthesized anaerobically. Thus, in this organism, N fixation occurs in microaerobic as well as anaerobic conditions.


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