Results and Discussion

Addition of ammonium chloride (0.2 mmol L"1) to nitrogenase derepressed A. brasilense FP2 (wild type) cultures caused almost complete inhibition of nitrogenase activity. The activity was fully recovered after approximately 15 minutes, following exhaustion of ammonium ions from the medium. The A. brasilense strain 7611 {glnZ mutant) was also fully derepressed for nitrogenase activity and the addition of ammonium ions caused a similar level of inhibition. However, recovery of the initial nitrogenase activity was partial (20-40%) even after 80 min of ammonium depletion. Since added ammonium was completely taken up after 10 min in both the wild type and glnZ strains this result suggests that the GlnZ protein is involved in the mechanism of reactivation of the ADP-ribosylated iron protein, but is not essential for nitrogenase switch-off by ammonium ions. Furthermore, full complementation of the glnZ mutant by the A. brasilense glnZ gene strongly supports this role for the GlnZ protein. In contrast, nitrogenase switch-off by anaerobic conditions was reversed completely by oxygenation and at the same rate in both the wild type and the glnZ mutant. This result indicates that GlnZ is neither involved in the mechanism of anaerobic inactivation nor in the aerobic reactivation of nitrogenase.

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