Bradyrhizobium japonicum FixK(2), a Crucial Distributor in the FixLJ-Dependent Regulatory Cascade for Control of Genes Inducible by Low Oxygen Levels

Bradyrhizobium japonicum possesses a second fixK-like gene, fixK(2), in addition to the previously identified fixK(1) gene. The expression of both genes depends in a hierarchical fashion on the low-oxygen-responsive two-component regulatory system FixLJ, whereby FixJ first activates fixK(2), whose product then activates fixK(1). While the target genes for control by FixK(1) are unknown, there is evidence for activation of the fixNOQP, fixGHIS, and rpoN(1) genes and some heme biosynthesis and nitrate respiration genes by FixK(2). FixK(2) also regulates its own structural gene, directly or indirectly, in a negative way.