Genetic Analysis of the Cytochrome C-aa3 Branch of the Bradyrhizobium Japonicum Respiratory Chain

Further genetic evidence is provided here that Bradyrhizobium japonicum possesses a mitochondria-like electron-transport pathway: 2[H]----UQ----bc1----c----aa3----O2. Two Tn5-induced mutants, COX122 and COX132, having cytochrome c oxidase-negative phenotypes, were obtained and characterized. Mutant COX122 was defective in a novel gene, named cycM, which was responsible for the synthesis of a c-type cytochrome with an Mr of 20,000 (20K). This 20K cytochrome c appeared to catalyse electron transport from the cytochrome bc1 complex to the aa3-type terminal oxidase and, unlike mitochondrial cytochrome c, was membrane-bound in B. japonicum. The Tn5 insertion of mutant COX132 was localized in coxA, the structural gene for subunit I of cytochrome aa3. This finding also led to the cloning and sequencing of the corresponding wild-type coxA gene that encoded a 541-amino-acid protein with a predicted Mr of 59,247. The CoxA protein shared about 60% sequence identity with the cytochrome aa3 subunit I of mitochondria. The B. japonicum cycM and coxA mutants were able to fix nitrogen in symbiosis with soybean (Fix+). In contrast, mutants described previously which lacked the bc1 complex did not develop into endosymbiotic bacteroids and were thus Fix-. The data suggest that a symbiosis-specific respiratory chain exists in B. japonicum in which the electrons branch off at the bc1 complex.