A BRADYRHIZOBIUM-JAPONICUM GENE ESSENTIAL FOR NODULATION COMPETITIVENESS IS DIFFERENTIALLY REGULATED FROM 2 PROMOTERS

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