Rhizobium lexuminosarum bv. trifolii produces an acidic exopolysac- charide (EPS) which plays an important role in the development of nitrogen-fixing nodules. Tn5 mutant of R. trifolii <>3 defective in EPS production (Exo") forms ineffective (Fix-) nodules on red clover. This Exo mutation is complemented by the pARF1368 and pARF25c<>Miiid.s isolated fnuii bank of Rhizobium trifolii TA1, but the complementation is not correlated with restoration of Fix* phenotypt-. Furthermore, these cosmids introduced to wild-type of R. trifolii 24 repress its ability to form nitrogen- -fixing nodules. These results might suggest that bacteria with cosmids carrying the exo region form FPS of altered structure. It has been shown by 'll-n.m.r. that exopolysaccharides produced by R. trifolii 93pARF- 1368 and 93pARF25 contain less non-carbohydrate residues (acetyl, pyruvyl and 3-hydroxybutanoyl) than the wild type KPS. These data suggest that the biological activity of the exopolysaccharide of R. trifolii depends on the contents of the non-carbohydrate substitutions.
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