THE KLEBSIELLA-PNEUMONIAE CYTOCHROME BD' TERMINAL OXIDASE COMPLEX ANDITS ROLE IN MICROAEROBIC NITROGEN-FIXATION

Cytochrome bd' has been implicated in having an important rare in micr oaerobic nitrogen fixation in the enteric bacterium Klebsiella pneumon iae, where it is expressed under all conditions that permit diazotroph y. In this paper the sequence of the genes encoding this terminal oxid ase (cydAB) of Klebsiella pneumoniae and the characterization of a cyd mutant are reported. The deduced amino acid sequences support the pro posal that His 19, His 186 and Met 393 provide three of the four axial ligands to the re of the three haems in the oxidase complex. The nitr ogen-fixing ability of the mutant was severely impaired in the presenc e of tow concentrations of oxygen compared with the wild-type bacteriu m. Only the wild-type organism was capable of microaerobic nitrogenase activity supported by fermentation products. It is proposed that form ate dehydrogenase-O may be involved in supplying electrons to a respir atory chain terminated by the bd-type oxidase, which would remove inhi bitory oxygen and supply ATP for nitrogenase activity.