GENES FOR A MICROAEROBICALLY INDUCED OXIDASE COMPLEX IN BRADYRHIZOBIUM-JAPONICUM ARE ESSENTIAL FOR A NITROGEN-FIXING ENDOSYMBIOSIS

We report the discovery of a Bradyrhizobium japonicum gene cluster (fi xNOQP) in which mutations resulted in defective soybean root-nodule ba cteroid development and symbiotic nitrogen fixation. The predicted, DN A-derived protein sequences suggested that FixN is a heme b and copper -binding oxidase subunit, FixO a monoheme cytochrome c, FixQ a polypep tide of 54 amino acids, and FixP a diheme cytochrome c and that they a re all membrane-bound. The isolation and analysis of membrane proteins from B. japonicum wild-type and mutant cells revealed two c-type cyto chromes of 28 and 32 kDa as the likely products of the fixO and fixP g enes and showed that both were synthesized only under oxygen-limited g rowth conditions. Furthermore, fixN insertion and fixNO deletion mutan ts grown microaerobically or anaerobically (with nitrate) exhibited a strong decrease in whole-cell oxidase activity as compared with the wi ld type. The data suggest that the fixNOQP gene products are induced a t low oxygen concentrations and constitute a member of the bacterial h eme/copper cytochrome oxidase superfamily. The described features are compatible with the postulate that this oxidase complex is specificall y required to support bacterial respiration in endosymbiosis.