SUMMARY: synthesized only -type and -type cytochromes under the wide range of growth conditions tested, and reaction with CO revealed two potential oxidases. The -type oxidase was produced only in the presence of O and appeared to be repressed by glucose. The -type oxidase was, by contrast, produced only in the absence of measurable O (< 1 μM), and was the only oxidase expressed in nitrogen-fixing conditions. It was extracted from the membrane, purified and shown to be similar to that from in being a heterodimer (subunits of 52000 and 35000), in containing two distinguishable haems and haem (one or two molecules per molecule of oxidase), and in being able to react with O to give a stable oxygenated intermediate. The purified -type cytochrome oxidase had a very high affinity for O ( 20 nM; measured by the spectral properties of leghaemoglobin). It is proposed that this provides a role for this oxidase in lowering the O concentration to allow nitrogenase synthesis and function, and to provide a terminal oxidase to permit electron-transport-coupled ATP synthesis which supports the increase in efficiency of nitrogen fixation observed under microaerobic conditions.


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