UNUSUAL LOCALIZATION OF NOD AND NIF GENES IN RHIZOBIUM-LEGUMINOSARUM BV VICIAE

Genomic DNA from seven strains of Rhizobium leguminosarum by. vicine i solated from nodules of field-grown lentils showed homology to nod and nif gene probes, whereas plasmid DNA did not hybridize with these pro bes. The results suggest that symbiotic genes could be located on the chromosome or perhaps on a very large plasmid that could not be resolv ed in Eckhardt gels. Each strain contained one plasmid that hybridized with a pSym isolated from a R. leguminosarum strain of the same field population. This finding led us to hypothesize that the nod and nif g enes of the seven strains might have originated from a Sym plasmid and have been integrated into another replicon. The ability to nodulate v etch was confirmed for all of the seven strains. Thus, wild strains of R. leguminosarum by. vicine that nodulate vetch carry nod and nif gen es either on the chromosome or on an extrachromosomal replicon of size much larger than the pSyms hitherto described.