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Genetic analysis of nitrogen fixation in a tropical fast‐growing Rhizobium

The Rhizobium strain ORS571, which is associated with the tropical legume Sesbania rostrata, has the property of growing in the free‐living state at the expense of ammonia or N2 as sole nitrogen source. Five mutants, isolated as unable to form colonies on plates under conditions of nitrogen fixation, were studied. All of them, which appear as Fix‐ in planta, are nif mutants. With mutant 5740, nitrogenase activity of the crude extract was restored by addition of pure Mo‐Fe protein of Klebsiella pneumoniae. A 13‐kb BamHI DNA fragment from the wild‐type strain, which hybridized with a probe carrying the nifHDK genes of K. pneumoniae, was cloned in vector pRK290 to yield plasmid pRS1. The extent of homology between the probe and the BamHI fragment was estimated at 4 kb and hybridization with K. pneumoniae nifH, nifK, and possibly nifD was detected. The pRS1 plasmid was introduced into the sesbania rhizobium nif mutants. Genetic complementation was observed with strain 5740(pRS1) both in the free‐living state and in planta. It thus appears that biochemistry and genetics of nitrogen fixation in this particular Rhizobium strain can be performed with bacteria grown under non‐symbiotic conditions.