1. In intact Azotobacter vinelandii the influence of oxygen on the levels of oxidized nicotinamide adenine dinucleotides and adenine nucleotides in relation to nitrogenase activity was investigated. 2. The hypothesis that a high (NADH + NADPH)/(NAD+ + NADP+) is the driving force for the transport of reducing equivalents to nitrogenase in intact A. vinelandii was found to be invalid. On the contrary, with a decreasing ratio of reduced to oxidized pyridine nucleotides, the nitrogenase activity of the whole cells increases. 3. By measuring oxidative phosphorylation and using 9-amino acridine as a fluorescent probe, it could be demonstrated that respiration-coupled transport of reducing equivalents to the nitrogenase requires a high energy level of the plasma membrane or possibly coupled to it, a high pH gradient over the cytoplasmic membrane. Furthermore nitrogen fixation is controlled by the presence of oxygen and the ATP/ADP ratio. |