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KZebsielZa pneumoniae had low levels of nitrogenase activity and antigenic cross-reacting material to both nitrogenase components when grown, or when the nifoperons were de- repressed, under Mo-deficient conditions. When molybdate (100 q) was added to Mo-de- prived cultures shortly after initiating derepression of nitrogenase, activity was rapidly restored and reached up to 60% of that of control cultures supplied with molybdate throughout de- repression. Restoration of activity was prevented by chloramphenicol (200 pg ml-l) but not by tetracycline (100 pg ml-l), although both compounds inhibited general protein synthesis at these concentrations. Expression of the nif structural genes was not rapidly stimulated by added molybdate, so the rapid stimulation of activity was due to the activation of a low level of apoprotein present in Mo-deprived cultures. After 16 h of Mo-deprivation, suspen- sions no longer responded rapidly to added molybdate, consistent with the finding that, when further protein synthesis was inhibited by the addition of chloramphenicol after 4 h Mo- deprivation, polypeptides of the MoFe protein (Kpl) but not the Fe protein (Kp2) of nitro- genase were degraded within the following 18 h.

microbiology

Nitrogen Fixation in Klebsiella pneumoniae: Nitrogenase Levels and the Effect of Added Molybdate on Nitrogenase Derepressed under Molybdenum Deprivation