Cloning and characterization of nifA and ntrC genes of the stem nodulating bacterium ORS571, the nitrogen fixing symbiont of Sesbania rostrata: Regulation of nitrogen fixation (nif) genes in the free living versus symbiotic state

Summary A cosmid bank of ORS571, a diazotrophic bacterium capable of inducing aerial stem and root nodules on Sesbania rostrata, was constructed in the vector pLAFR1. A DNA probe carrying the Klebsiella pneumoniae nifA gene was used to identify nifA-and ntrC-like regions of ORS571 in the cosmid bank by colony hybridization. Cosmids carrying these regions were mapped by restriction endonuclease analysis, Southern blotting and transposon Tn5 mutagenesis. Selected Tn5 insertion mutations in the nifA/ntrC homologous regions were used for gene-replacement experiments and the resulting ORS571 mutants were examined for Nif, Fix and Ntr phenotypes. Two clearly distinct regulatory loci were thus identified and named nifA and ntrC. Plasmids carrying gene fusions of the ORS571 nifH and nifD genes to lacZ were constructed and the regulation of the ORS571 nifHDK promoter, and of the Rhizobium meliloti nifHDK promoter, was studied under varying physiological conditions in ORS571, ORS571 nifA::Tn5 and ORS571 nitrC::Tn5 strains. A model for the role of nifA and ntrC in the regulation of ORS571 nif and other nitrogen assimilation genes is proposed.