The presence of a novel type of surface polysaccharide in Rhizobium meliloti requires a new fatty acid synthase‐like gene cluster involved in symbiotic nodule development

Bacterial exopolysaccharide (EPS) and lipopolysaccharide (LPS) molecules have been shown to play important roles in plant‐bacterium interactions. Here we have demonstrated that the fix‐23 loci, which compensate for exo mutations during symbiotic nodule development, are involved in the production of a novel polysaccharide that is rich in 3‐deoxy‐D manno‐2‐octulosonic acid (Kdo) but is not the classical LPS. This molecule is likely to be a surface antigen since antiserum to whole Rhizobium meliloti cells reacts strongly with it, and since mutations in fix‐23 result in an inability to produce this polysaccharide and to bind bacteriophage 16‐3. It is likely that this Kdo‐rich polysaccharide is analogous to certain Escherichia coli K‐antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one gene cluster of this region revealed that the predicted protein products of six genes exhibit a high degree of homology and similar organization to those of the rat fatty acid synthase multifunctional enzyme domains.