FIGURE 1.

Organization of the O-antigen biosynthesis gene cluster (wb*) from K. pneumoniae O12 and R. terrigena ATCC 33257 and the corresponding O-antigenic polysaccharide biosynthesis product. WecA is encoded at a separate site within the chromosome. The rmlBADC genes encode enzymes for the production of dTDP-l-Rha precursor and wzm-wzt encode the ABC transporter TMD and NBDs, respectively. The system requires two proteins with glycosyltransferase activities. WbbL is a monofunctional rhamnosyltransferase, whereas WbbB contains three glycosyltransferase domains; two are required for O12 polymerization and one for addition of the terminal β-Kdo residue. The component sugars are 2-acetylamino-2-deoxyglucose (N-acetylglucosamine; GlcNAc), 6-deoxymannose (rhamnose; Rha), and 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo).