Summary: A purpose-built oxystat has been used to study reversible inhibition of nitrogenase by O in the facultative anaerobe CH-reducing activity in samples from either an anaerobic glucose-limited or an O-limited diazotrophic chemostat culture was completely inhibited by exposure to a dissolved O concentration (DOC) of 1.5 μm or above. Subsequently, under anaerobic conditions, CH-reducing activity returned in the absence of protein synthesis. The amount of activity returning never reached 100% of the initial anaerobic activity before O treatment. The degree of reversibility was inversely proportional to the log of DOC during exposure and was decreased by increasing the time of exposure to O (about 60% reversibility occurred after a 20 min exposure to 6 μm-O). The failure to obtain complete recovery of activity was apparently not due to inactivation of the very O-sensitive pyruvate-flavodoxin oxidoreductase ( product) which provides electrons for nitrogenase activity Samples from the O-limited culture behaved similarly to those limited by glucose. Thus, “training” of the organism to use O during growth does not influence the tolerance of nitrogenase to O. Since the behaviour towards O reported here for differs from that known to occur in , the mechanism of protection of nitrogenase from O damage may differ in these organisms.


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