Synthesis and Activity of Nitrogenase in Klebsiella pneumoniae Exposed to Low Concentrations of Oxygen

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Abstract
Effects of very low concentrations of dissolved O2 on nitrogenase activity in K. pneumoniae were studied in a stirred chamber system which enabled simultaneous measurements of steady-state O2 concentrations, O2 consumption and C2H2 reduction. A strain carrying a chromosomal nifH::lac fusion and the Nif+ plasmid pRD1, expressed nitrogenase activity with 80 nM-O2, a concentration known to inhibit nifH::lac expression by .apprx. 50%. Nitrogenase activity in vivo was no more sensitive to O2 than expression of nifH::lac. When compared with anaerobic treatments, dissolved O2 near 30 nM apparently stimulated nitrogenase derepression and enhanced the activity of nitrogenase synthesized anaerobically. In this organism, N2 fixation occurs in microaerobic and anaerobic conditions.