RESUMO
The presence of extrachromosomal DNA [viz. plasma DNA] in Rhizobium leguminosarum F21 and R. phaseoli L1 was investigated. On the basis of antibiotic sensitivity tests, both Rhizobium strains were found to contain plasmids and carrying streptomycin resistance genes. Plasmids DNA was isolated from other strains and analyzed by agarose gel electrophoresis. The plasmid DNA of all strains was detected as one band on the gel indicating that R. leguminosarum F21 and R. Phaseoli L1 contain just one plasmid of 21,200 kbp. Transferring of R. leguminosarum plasmid into R. Phaseoli resulted in the transfer of faba bean infectivity to R. phaseoli. Moreover, the transformed strain exhibited a high efficiency in nodulating roots of both faba bean as well as in fixing nitrogen