Abstract
Symbiotic and auxotrophic mutants of Rhizobium japonicum strain USDA191 were isolated using Tn5 mutagenesis and techniques that cause plasmid deletions and plasmid curing. Characterization of several mutants that are unable to nodulate (Nod-) or unable to fix nitrogen (Fix_) showed that nod and nif genes are located within one regions of a 200 MD plasmid (pSym191). Blot hybridization analysis of plasmids in other fast-growing R. japonicum strains showed that nod as well as nif sequences are located on plasmids in eight strains but are apparently carried in the chromosome in two strains.
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