Homology of genes for exopolysaccharide synthesis in Rhizobium leguminosarum and effect of cloned exo genes on nodule formation

A 5.4 kb BamHl fragment of R. leguminosarum bv. trifolii TA1 was found to carry genes involved in exopolysaccharide synthesis (caro genes). This fragment was strongly hybridized to the total DNA from R. I. bv. viciae and bv. phaseoli digested with EcoRI. No homology was found with total DNA of R. meliloti and Rhizobium sp. NGR 234. The exo genes from R. I. bv. trifoliiTAl conjugally introduced into R. I. bv. viciae 1302 considerably affected the symbiosis: the nodules induced on vetch were abortive and did not fix nitrogen. On the other hand, Phaseolus beans infected with R. I. bv. phaseoli harbouring R. I. bv. trifolii exo genes formed the nitrogen-fixing nodules. It can be concluded that additional copies of exo genes introduced into wild type Rhizobium leguminosarum strains can disturb the synthesis of acidic exo- polysaccharides and affect symbiosis of the plants forming indeterminate nodules, but do not affect symbiosis of the plants forming the determinate nodules.