FREE-LIVING AND SYMBIOTIC CHARACTERISTICS OF PLASMID-CURED DERIVATIVES OF RHIZOBIUM-GALEGAE

Rhizobium galegae strain NBIMTC2250 contains two plasmids one of which (the larger one) is the symbiotic plasmid. To determine the participa tion of the smaller plasmid in cellular functions and in the symbiosis , we used a positive selection system for isolation of derivatives cur ed of this plasmid. The derivatives did not show differences from the wild type with respect to the following free-living cellular propertie s: antibiotic-resistance pattern, carbohydrates and organic acid utili zation, growth rate and colony morphology in pure culture, lipopolysac charide (LPS) profile, motility and sensitivity to extreme temperature s, which indicated that these functions were not plasmid encoded. Some of the cured derivatives differed from the wild type in the symbiotic characteristics, and the possible reason for the differences was disc ussed. In these cases reintroduction of the plasmid did not restore th e derivatives to a wild type but the plasmid-free and plasmid-recontai ning derivatives had similar symbiotic phenotype. Hybridization study showed that the smaller plasmid had been lost integrally in all cured derivatives. These results and the fact that almost half of the plasmi d-cured derivatives have the symbiotic characteristics similar to thos e of the parent strain suggested that the smaller plasmid of strain Rh izobium galegae NBIMTC2250 did not participate in the symbiotically im portant functions of this strain. (C) 1997 Elsevier Science Ireland Lt d.