A sp. () calcofluor dark (Cal) mutant, named Cal10, was obtained following Tn-insertion mutagenesis. It is affected in the synthesis of exopolysaccharide and presents an altered lipopolysaccharide that is not recognized by a polyclonal antibody against the lipopolysaccharide of the parental strain. The residual exopolysaccharide obtained from the mutant lacks galactose and the high-molecular-mass acidic fraction. This mutant was complemented by plasmid pD56 that restores the production of exopolysaccharide, the alteration of lipopolysaccharide and the Cal phenotype. The data presented indicate that the gene in which the mutant is defective is homologous to the gene of and fails to synthesize UDP-glucose 4′-epimerase. The Cal10 mutant was Fix on (sulla) although it develops an indeterminate type of nodule, indicating that exopolysaccharide is not essential for a successful nodulation in this symbiotic association.


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