The Bradyrhizobium japonicum cox WXYZ gene cluster encodes a bb(3)-type ubiquinol oxidase

Bradyrhizobium japonicum, a symbiotic nitrogen-fixing bacterium, has a complex respiratory electron-transport chain, capable of functioning throughout a wide range of oxygen tensions. It does so by synthesizing a number of terminal oxidases, each appropriate for different environmental conditions. We have previously described the cloning of the large catalytic subunit, coxX, from one of the terminal oxidases from B. japonicum [Surpin, M.A., Moshiri, F., Murphy, A.M. and Maier, R.J. (1994) Genetic evidence for a fourth terminal oxidase from Bradyrhizobum japonicum. Gene 143, 73-77]. In this work, we describe the remaining subunits of this terminal oxidase complex, which is encoded by the coxWXYZ operon. The polypeptide encoded by coxW does not contain any amino acid residues that are known to bind the Cu-A atom of cytochrome c terminal oxidases, but contains residues thought to be involved in ubiquinol binding. Terminal oxidase cyanide inhibition titration pattern comparisons of the wild type with a coxWXYZ insertion mutant indicated the new oxidase is expressed microaerobically. However analysis of hemes extracted from microaerobically incubated cells revealed the absence of heme O in this strain (from both the wild type and the mutant) of B. japonicum. Therefore, coxWXYZ most likely encodes a microaerobically-expressed bb(3)-type ubiquinol oxidase.