Complex Regulatory Network for nif and fix Gene Expression in Bradyrhizobium Japonicum

Abstract

In the nitrogen-fixing soybean symbiont, Bradyrhizobium japonicum, the expression of nif and fix genes is regulated predominantly, if not exclusively, in response to the cellular oxygen status. Activation of these genes occurs during symbiosis in root nodules or in free-living cells grown at low oxygen concentrations. We have identified three regulatory elements that are involved in oxygen control at different levels. First, with regard to nif and fix gene expression, the positive regulatory protein NifA plays a crucial role. It was demonstrated that the binding of NifA to its target site in the upstream region of the B japonicum nifD promoter was regulated by oxygen and required metal ions. Second, fixLJ-like genes that had been shown to activate the Rhizobium meliloti nifA gene under low oxygen conditions (1, 2), were identified also in B.japonicum. However, they were probably not involved in the expression of nifA, but in the regulation of other oxygen-controlled genes. Third, we found two functional genes (rpoNl, rpoN2) encoding specific σ factors (σ⁵⁴) and demonstrated that rpoNl expression was oxygen regulated via a mechanism involving fixLJ whereas rpoN2 was subject to negative autoregulation.