Oxygen inhibition of nitrogenase activity in Klebsiella pneumoniae

A purpose-built oxystat has been used to study reversible inhibition of nitrogenase by O2 in the facultative anaerobe Klebsiella pneumoniae. C2H2-reducing activity in samples from either an anaerobic glucose-limited or an O2-limited diazotrophic chemostat culture was completely inhibited by exposure to a dissolved O2 concentration (DOC) of 1.5 microM or above. Subsequently, under anaerobic conditions, C2H2-reducing activity returned in the absence of de novo protein synthesis. The amount of activity returning never reached 100% of the initial anaerobic activity before O2 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 O2 (about 60% reversibility occurred after a 20 min exposure to 6 microM-O2). The failure to obtain complete recovery of activity was apparently not due to inactivation of the very O2-sensitive pyruvate-flavodoxin oxidoreductase (nifJ product) which provides electrons for nitrogenase activity in vivo. Samples from the O2-limited culture behaved similarly to those limited by glucose. Thus, 'training' of the organism to use O2 during growth does not influence the tolerance of nitrogenase to O2. Since the behaviour towards O2 reported here for K. pneumoniae differs from that known to occur in Azotobacter, the mechanism of protection of nitrogenase from O2 damage may differ in these organisms.