Genetic evidence for a fourth terminal oxidase in Bradyrhizobium japonicum

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. Several genes encoding terminal oxidases from B. japonicum have been cloned, but it is unknown what roles these individual oxidases play. In this paper, we describe the cloning and sequencing of the coxX gene encoding the large catalytic subunit for a fourth terminal oxidase from B. japonicum. The coxX gene encodes a 666-amino-acid (aa) protein (M(r) 74,527) that exhibits a high degree of homology to terminal oxidase proteins from a number of prokaryotic and eukaryotic species. This new oxidase exhibits greater homology to the Escherichia coli cytochrome o subunit I than any of the previously reported B. japonicum terminal oxidase genes.