The symbiotic nitrogen fixation genes (nif, fix) of Bradyrhizobium japonicum, the root nodule endosymbiont of soybean, are organized in at least two separate chromosomal gene clusters. These genes code for proteins of the nitrogenase complex, for proteins involved in their assembly with cofactors and for putative electron transport functions. One gene, nif A, codes for a transcriptional regulatory protein that plays a central role in the control of expression of the other genes in response to the cellular oxygen status. Only at low partial pressures of O2 will the target promoters be activated by NifA.
