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) molecul es have been shown to play important roles in plant-bacterium interact ions. Here we have demonstrated that the fix-23 loci, which compensate for exo mutations during symbiotic nodule development, are involved i n the production of a novel polysaccharide that is rich in 3-deoXy-D-m anno-2-octulosonic acid (Kdo) but is not the classical LPS. This molec ule is likely to be a surface antigen since antiserum to whole Rhizobi um meliloti cells reacts strongly with it, and since mutations in fix- 23 result in an inability to produce this polysaccharide and to bind b acteriophage 16-3. It is likely that this Kdo-rich polysaccharide is a nalogous to certain Escherichia coli K-antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one g ene cluster of this region revealed that the predicted protein product s of six genes exhibit a high degree of homology and similar organizat ion to those of the rat fatty acid synthase multifunctional enzyme dom ains.