Biosynthesis of the Escherichia coli K5 polysaccharide, a representative of group II capsular polysaccharides: Polymerization in vitro and characterization of the product

Biosynthesis of the capsular K5 polysaccharide of Escherichia coli, which has the structure 4)-0GlcA-1,4-aGlcNAc-(1, was studied with membrane preparations from an E. coli K5 wild-type strain and from a recombinant K-12 strain expressing the KS capsule. Polymerization occurs at the inner face of the cytoplasmic membrane without the participation of lipid-linked oligosaccharides. The serological K5 specificity of the in vitro product was determined with a K5-specific monoclonal antibody in an antigen-binding assay. The KS polysaccharide, as obtained from the membranes after an in vitro incubation, has 2-keto-3-deoxyoctulosonic acid as the reducing sugar, which indicates that the polysaccharide grows by chain elongation at the nonreducing end. Escherichia coli capsules consist of acidic polysaccharides (K antigens), which are divided into two groups, as charac-terized by chemical, biochemical, and genetic parameters (13). The expression of group II capsular polysaccharides, which are characteristic of invasive E. coli, is temperature regulated. These polysaccharides are expressed at 37°C but not at 20°C and below (25). Their expression is determine