The pleiotropic nature of symbiotic regulatory mutants: Bradyrhizobium japonicum nifA gene is involved in control of nif gene expression and formation of determinate symbiosis

In the slow‐growing soybean symbiont, Bradyrhizobium japonicum (strain 110), a nifA‐like regulatory gene was located immediately upstream of the previously mapped fixA gene. By interspecies hybridization and partial DNA sequencing the gene was found to be homologous to nifA from Klebsiella pneumoniae and Rhizobium meliloti, and to a lesser extent, also to ntrC from K. pneumoniae. The B. japonicum nifA gene product was shown to activate B. japonicum and K. pneumoniae nif promoters (using nif::lacZ translational fusions) both in Escherichia coli and B. japonicum backgrounds. In the heterologous E. coli system activation was shown to be dependent on the ntrA gene product. Site‐directed insertion and deletion/replacement mutagenesis revealed that nifA is probably the promoter‐distal cistron within an operon. NifA‐ mutants were Fix‐ and pleiotropic: (i) they were defective in the synthesis of several proteins including the nifH gene product (nitrogenase Fe protein); the same proteins had been known to be repressed under aerobic growth of B. japonicum but derepressed at low O2 tension; (ii) the mutants had an altered nodulation phenotype inducing numerous, small, widely distributed soybean nodules in which the bacteroids were subject to severe degradation. These results show that nifA not only controls nitrogenase genes but also one or more genes involved in the establishment of a determinate, nitrogen‐fixing root nodule symbiosis.