What Robson Saw
He grew A. chroococcum in chemostats with Burk medium with mannitol, with or without ammonium for nitrogen, and stressed them with oxygen either by moving ammonium-grown cells to ammonium-free medium or by suddenly increasing aeration in nitrogen-fixing cells. He looked at levels of nitrogenase proteins (by labeling with radioactive sulfur isotopes) and nitrogenase activity.
At initial low oxygen levels, activity increased and radioisotype-labeled protein levels were relatively high (indicating high levels of nitrogenase protein synthesis). Upon oxygen shock, activity went to zero and protein synthesis levels dropped a lot. When the stress was relieved, both measures increased again.
With ammonium removed and then re-added, things were similar: nitrogenase activity went up and synthesis gradually increased to a plateau, but decreased when more ammonium was added. It did take a relatively longer time for nitrogenase activity to pick up after ammonium was removed, about 80 minutes.
Along with nitrogenase, flavodoxin and the small protein that protects nitrogenase from oxygen by temporarily inactivating it both showed up in radiolabeling. The former matched nitrogenase synthesis patterns, but the latter was fairly constant.
What This Means
It seems that oxygen stress can repress synthesis of nitrogenase even in Azotobacter, though radiolabeling might not be the best method for studying this question. This adds another layer to Azotobacter's protection of its enzymes from oxygen.