Abstract
This paper discusses four aspects of the H 2-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 N 2-limited conditions but not under O 2-, sulphate- or Fe-limitation. (2): A Cyd − mutant of Azotobacter vinelandii was capable of catalysing O 2-dependent H 3H uptake, indicating that H 2-dependent respiration linked to a high affinity (O 2) oxidase system. (3): High-O 2-adapted A. chroococcum showed lower specific nitrogenase activity at low ambient O 2 and 16°C than did low-O 2-adapted A. chroococcum, suggesting that the high-affinity oxidases were inactivated at high ambient O 2. (4): A second region of DNA, distinct from the main hup gene region, involved in H 2-dependent respiration is described.
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