GENES NEEDED FOR THE MODIFICATION, POLYMERIZATION, EXPORT, AND PROCESSING OF SUCCINOGLYCAN BY RHIZOBIUM-MELILOTI - A MODEL FOR SUCCINOGLYCAN BIOSYNTHESIS

The major acidic exopolysaccharide of Rhizobium meliloti, termed succi noglycan, is required for nodule invasion and possibly nodule developm ent. Succinoglycan is a polymer of octasaccharide subunits composed of one galactose residue, seven glucose residues, and acetyl, succinyl, and pyruvyl modifications, which is synthesized on an isoprenoid lipid carrier. A cluster of exo genes in R. meliloti are required for succi noglycan production, and the biosynthetic roles of their gene products have recently been determined (T. L. Reuber and G. C. Walker, Cell 74 :269-280, 1993). Our sequencing of 16 kb of this cluster of exo genes and further genetic analysis of this region resulted in the discovery of several new exo genes and has allowed a correlation of the genetic map with the DNA sequence. In this paper we present the sequences of g enes that are required for the addition of the succinyl and pyruvyl mo difications to the lipid-linked intermediate and genes required for th e polymerization of the octasaccharide subunits or the export of succi noglycan. In addition, on the basis of homologies to known proteins, w e suggest that ExoN is a uridine diphosphoglucose pyrophosphorylase an d that ExoK is a beta(1,3)-beta(1,4)-glucanase. We propose a model for succinoglycan biosynthesis and processing which assigns roles to the products of nineteen exo genes.