Oxygen, hydrogen and nitrogen fixation in Azotobacter

This paper discusses four aspects of the H2-uptake hydrogenase (Hup) — dependent respiration in Azotobacter. (1): Competition between isogenic Hup+ and Hup− strains of Azotobacter chroococcum containing the vanadium nitrogenase showed that the Hup+ strain dominated under carbon-, phosphate- or intrinsic N2-limited conditions but not under O2-, sulphate- or Fe-limitation. (2): A Cyd− mutant of Azotobacter vinelandii was capable of catalysing O2-dependent H3H uptake, indicating that H2-dependent respiration linked to a high affinity (O2) oxidase system. (3): High-O2-adapted A. chroococcum showed lower specific nitrogenase activity at low ambient O2 and 16°C than did low-O2-adapted A. chroococcum, suggesting that the high-affinity oxidases were inactivated at high ambient O2. (4): A second region of DNA, distinct from the main hup gene region, involved in H2-dependent respiration is described.