Two Oxygen-Responsive Regulatory Cascades Control Nitrogen Fixation Genes in Bradyrhizobium Japonicum

Abstract

The soybean symbiont Bradyrhizobium japonicum is able to activate the genes required for nitrogen fixation under both free-living and symbiotic growth conditions. Conditions of low oxygen tension present in root nodules and in microaerobic or anaerobic bacterial cultures act as the critical signal for the transcriptional activation of different sets of genes. Apart from the nif and fix genes, the cellular oxygen conditions also control the biosynthesis of the constituents of several respiratory chains. Our laboratory is interested in the elucidation of the genetic regulatory circuits involved in the symbiotic nitrogen fixation process. Here we report on a complex regulatory network involved in oxygen control of gene expression in B. japonicum. We focus on recent results that contributed new details to the regulatory model developed over the last years (see recent review by Hennecke et al., 1993). This model involves two largely independent regulatory cascades which control the expression of different subsets of genes in response to the cellular oxygen conditions.KeywordsNitrogen FixationSymbiotic Nitrogen FixationlacZ FusionNitrogen Fixation GeneNifA ProteinThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.