Biosynthesis of succinoglycan, a symbiotically important exopolysaccharide of Rhizobium meliloti.

The exo genes of Rhizobium meliloti are needed for the synthesis of an acidic exopolysaccharide, succinoglycan. We have assigned biosynthetic roles to the products of the exo genes by characterizing succinoglycan biosynthetic intermediates from exo mutant strains. We propose a model of succinoglycan biosynthesis in which the products of the exoY and exoF genes function in the addition of the first sugar, galactose, to the lipid carrier; the products of the exoA, exoL, exoM, exoO, exoU, and exoW genes function in subsequent sugar additions; and the product of the exoV gene functions in the addition of pyruvate. The products of the exoP, exoQ, and exoT genes are required for polymerization of the octasaccharide subunits or transport of the completed polymer.