All strains of R. leguminosarum bv. trifolii tested (R200, TA1, ANU 843, 0403) aggregated with RapA-coated beads, so did strains of R. leguminosarum bvs. viciae and phaseoli. Other rhizobia, e.g. S. meliloti and S.fredii USDA 257 were negative in PAA, so was R. leguminosarum bv. trifolii strain TA1-133 (pssD::Tn5, EPS"), non-mucoid and defective in EPS biosynthesis. This indicates that RapA binds to an EPS-structure present on the bacterial cell-surface. Purified EPS and CPS from several R. leguminosarum bv. trifolii strains and from R. leguminosarum bv. viciae 300, completely inhibited the aggregation between R. leguminosarum strains and RapA-coated beads. This shows that RapA binds to both cell-associated and culture medium EPS.
PlyA and PlyB (Finnie et al. 1998) and most likely RapB are involved in processing EPS. The Ra-domains in these proteins are probably involved in specific binding to the substrate. The function of the RapA proteins, however, seems to be only binding of EPS/CPS. Bearing in mind that the multicellular behavior of several gram-negative pathogens is dependent on proteinaceous fimbriae and even cellulose, it seems possible that also in rhizobia different extracellular components (proteins, EPS, cellulose) interact with each other to build an organized matrix facilitating communication between individual bacteria and contacts with the surroundings.
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