Genetic analyses of Rhizobium meliloti exopolysaccharides

We have recently obtained strong genetic evidence that the acidic Calcofluor-binding exopolyssaccharide (EPS I) of Rhizobium meliloti Rm1021 is required for nodule invasion and possibly for later events in nodule development. Thirteen loci on the second megaplasmid have been identified that are required for, or affect, the synthesis of EPS I. Mutations in certain of these loci completely abolish the production of EPS I and result in mutants that form empty Fix^- nodules. exoH mutants fail to succinylate their EPS I and form empty Fix^- nodules. We have identified two unlinked regulatory loci, exoR and exoS, whose products play negative roles in the regulation of expression of the exo genes. We have recently discovered that R. meliloti has a latent capacity to synthesize a second exopolysaccharide (EPS II) that can substitute for the role(s) of EPS I in nodulation of alfalfa but not of other hosts. Possible roles for the Rhizobium exopolysaccharides in nodulation are discussed.