I so I at i on and c h a ra ct e r i za t i o n of Rhizobium meliloti Tn5 mutants showing enhanced symbiotic effectiveness

A series of Tn5-induced mutants which showed enhanced symbiotic effectiveness (Eff ) was isolated from Rhizobiurn rneliloti strains CXMI-105 and CXMI-188. Alfalfa plants inoculated by the Eff ++ mutants had significantly higher shoot dry mass than plants inoculated by the parental strains. In the greenhouse, the most effective mutants increased the shoot dry mass of the host plants by 23-26 O/O and plant total nitrogen by 23-27 %. Interestingly, the frequency of the Eff-* mutants in strain CXMI-188 was higher than in the strain CXMI-105 (1.1 O/O versus 0.4%); this was also the case for the auxotrophic mutants (1.2O/0 in CXMI-188 versus 0.3% in CXMI-105). Genetic analysis of the mutants showed that the enhanced symbiotic effectiveness was cotransducible with Tn5. By the use of Southern hybridization and plasmid transfer, it was found that ten Tn5 insertions were located in the chromosome, five in megaplasmid 1, and six in megaplasmid 2.