A Bradyrhizobium japonicum gene essential for nodulation competitiveness is differentially regulated from two promoters.

We report the identification and nucleotide sequence of a new symbiotic gene (nfeC) from the soybean root nodule bacterium, Bradyrhizobium japonicum. A Tn5 insertion (NAD14) in this gene did not affect nitrogen fixation but caused a significant delay in soybean nodulation. In addition, this mutant exhibited a reduction in its competitive ability to nodulate soybean when coinoculated with the wild type. Sequence analysis of the mutated region revealed that the NAD14 Tn5 insertion mapped within an open reading frame of 825 bp. Primer extension using B. japonicum mRNA from three different growth conditions, aerobic, anaerobic, and bacteroids (i.e., symbiotic form) indicated that the upstream region of the gene contained two promoters, which were differentially regulated in response to the growth conditions. One promoter was expressed in bacteroids, but not under aerobic or anaerobic free-living conditions. The other promoter was expressed only under aerobic conditions.