Assembly of the R1-type core oligosaccharide of Escherichia coli lipopolysaccharide

There are 5 known core oligosaccharide (core OS) structures in the lipopoly saccharides of Escherichia coli. The different structures reflect diversity in the chromosomal waa locus, primarily in the central waaQ operon encodin g enzymes involved in the assembly of the core OS. The R1 core type is most prevalent among clinical isolates and provides our prototype for functiona l studies of core OS assembly. To establish the core OS assembly pathway, n on-polar insertions were used to mutate each of 9 genes in the major operon of the R1 waa locus. Core OS structures were then determined for each muta nt to assign functions to the relevant gene products. From currently availa ble sequence data, five genes (designated waaA, waaC, waaQ, waaP, and waaY) are highly conserved in ail of the core types; their products are responsi ble for assembly and phosphorylation of the inner-core region. Also conserv ed is waaG, whose product is an alpha-glucosyrtransferase that adds the fir st residue (HexI) of the outer core. A family of related HexII and HexIII a lpha-glucosyltransferase extend the outer core OS backbones in all of the c ore OS types. The waaO and waaT gene products fulfil these roles in the R1 core OS type. A related glycosyltransferase (WaaW) adds the alpha-galactosy l substituent on HexIII. The last step in assembly of the core OS carbohydr ate backbone involves substitution of HexII by a beta-linked glucosyl resid ue. This residue distinguishes the R1 core OS and it provides the attachmen t site for ligation of O antigen.