Summary: Cytochrome has been implicated in having an important role in microaerobic nitrogen fixation in the enteric bacterium , where it is expressed under all conditions that permit diazotrophy. In this paper the sequence of the genes encoding this terminal oxidase () of and the characterization of a mutant are reported. The deduced amino acid sequences support the proposal that His 19, His 186 and Met 393 provide three of the four axial ligands to the Fe of the three haems in the oxidase complex. The nitrogen-fixing ability of the mutant was severely impaired in the presence of low concentrations of oxygen compared with the wild-type bacterium. Only the wild-type organism was capable of microaerobic nitrogenase activity supported by fermentation products. It is proposed that formate dehydrogenase-O may be involved in supplying electrons to a respiratory chain terminated by the -type oxidase, which would remove inhibitory oxygen and supply ATP for nitrogenase activity.


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