Figure 14. Biosynthesis and assembly of O polysaccharides in an ABC-transporter-dependent pathway.

Panel A shows the predicted structures of undecaprenyl-linked intermediates in the biosynthesis of the O9a antigen of E. coli and the D-galactan I polymer found in several serotypes of K. pneumoniae. The glycosyltransferase enzymes involved in each step are indicated below the structures. Biosynthesis is broken down into the formation of a common primer (shown in red), addition of an adaptor region, chain extension of the repeat unit, and addition of a chain terminator (for details see the text). In the O9a structure, the chain is terminated by addition of 3-O-methylmannose but the enzyme responsible has not been identified. In D-galactan I, the structure of the chain terminator has not been established. Chain elongation occurs by processive glycosyl transfer to the non-reducing terminus. Several enzymes in these pathways are bifunctional. In the case of the galactosyltransferase WbbO, both of its activities are required for adaptor synthesis, but only one (WbbO²) participates in chain extension. Panel B provides a model for the trans-membrane assembly system. The glycosyltransferases are shown in green. The ABC-transporter formed by Wzm and Wzt is required for transfer of the undecaprenyl-linked polymer to the periplasmic face of the membrane, where it is ligated to lipid A-core and translocated to the outer membrane. It is presumed that the polymer remains attached to the undecaprenyl carrier throughout the export process. While chain extension and export are separable by mutations in the ABC-transporter, it is conceivable that the two processes are temporally coupled in vivo. Within the nascent polymer, the primer/adaptor is identified by the black-filled hexagon, the residues of the repeating-unit domain by gray-filled circles, and the chain terminator by the open hexagon.