Effect of ammonium ions on nitrogenase activity in H. seropedicae wild-type, glnB and glnK strains. Nitrogenase switch-off/on of H. seropedicae wild-type, glnB and glnK carrying plasmid pLNΔNifA was performed as described. Cells were grown under nitrogenase de-repressing conditions when NH4Cl was added (arrow). Samples were analyzed 10, 20 and 30 minutes after acetylene injection to confirm linear nitrogenase activity. Panel A: addition of NH4Cl (0.3 mmol.L-1). Panel B: addition of NH4Cl (1 mmol.L-1). The results represent the average of experiments with three independent cultures and bars indicate the standard deviation.