Genetic analysis of a cluster of genes required for synthesis of the calcofluor-binding exopolysaccharide of Rhizobium meliloti

Rhizobium meliloti produces an acidic, Calcofluor-binding exopolysaccharide which plays a role in nodulation of alfalfa plants by this bacterium. We constructed and mapped 102 transposon insertions in a 48-kilobase (kb) region previously shown to contain several exo genes. Mutations affecting production of the Calcofluor-binding exopolysaccharide were clustered in a 22-kb region and fell into 12 complementation groups. Strains carrying mutations in seven of the complementation groups (exoA, exoB, exoF, exoL, exoM, exoP, and exoQ) produced no Calcofluor-binding exopolysaccharide and induced non-nitrogen-fixing nodules on alfalfa. Mutants in an eighth complementation group, exoH (Leigh et al., Cell 51:579-587, 1987), produce an altered exopolysaccharide and also induce the formation of non-nitrogen-fixing nodules. Mutants in the remaining four complementation groups produced less Calcofluor-binding material than the wild type. Mutants carrying mutations in two of these complementation groups (exoK and exoN) formed apparently normal, nitrogen-fixing nodules, while mutants in the other two groups (exoG and exoJ) formed normal nodules less efficiently than the wild type.