Symbiotic Mutants of Rhizobium Meliloti Which Produce Non-Succinylated Exopolysaccharide

Abstract

We reported previously (1) that mutants (Exo^-) of Rhizobium meliloti SU47 which failed to secrete the acidic extra-cellular polysaccharide formed abnormal nodules on Medicago sativa that did not fix nitrogen and did not contain bacteroids. This was true of mutants belonging to six different genetic complementation groups, and established a strong correlation between the ability to secrete the polysaccharide and the ability to invade nodules. These mutants were isolated on the basis of their non-fluorescent colony phenotype on agar medium containing the fluorescent stain calcofluor. Three of the loci defined by these mutants (ExoA, ExoB, and ExoF) were shown to reside in a cluster on the megaplasmid pRmeSU47b (2).