Symbiotic deficiencies associated with a coxWXYZ mutant of Bradyrhizobium japonicum

The terminal oxidase complexes encoded by coxMNOP and coxWXYZ were studied by analysis of mutations in each of the two oxidases, Carbon monoxide diffe rence spectra obtained from membranes of coxMNOP mutant bacteroids were lik e those obtained for the wild type, whereas bacteroid membranes of a coxWXY Z mutant were deficient in GO-reactive cytochrome b. Experiments involving cyanide inhibition of oxidase activity were consistent with the conclusion that the coxX mutant is deficient in a membrane-associated O-2-binding comp onent. The viable cell number (bacteria that could be recultured from crush ed nodules) was 20 to 29% lower for the coxX mutant than for the wild-type or the CoxN(-) strain. In three separate greenhouse studies, nodules of a c oxX mutant had significantly lower (28 to 34% less) acetylene reduction rat es than the wild-type nodules did, and plants inoculated with a double muta nt (coxMNOP coxWZYZ) had rates 30% lower than those of wildtype-inoculated plants.