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

Abstract

There are 5 known core oligosaccharide (core OS) structures in the lipopolysaccharides of Escherichia coli. The different structures reflect diversity in the chromosomal waa locus, primarily in the central waaQ operon encoding enzymes involved in the assembly of the core OS. The R1 core type is most prevalent among clinical isolates and provides our prototype for functional studies of core OS assembly. To establish the core OS assembly pathway, non-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 mutant to assign functions to the relevant gene products. From currently available sequence data, five genes (designated waaA, waaC, waaQ, waaP, and waaY) are highly conserved in all of the core types; their products are responsible for assembly and phosphorylation of the inner-core region. Also conserved is waaG, whose product is an α-glucosyltransferase that adds the first residue (HexI) of the outer core. A family of related HexII and HexIII αglycosyltransferases extend the outer core OS backbones in all of the core OS types. The waaO and waaT gene products fulfil these roles in the R1 core OS type. A related glycosyltransferase (WaaW) adds the α-galactosyl substituent on HexIII. The last step in assembly of the core OS carbohydrate backbone involves substitution of HexII by a β-linked glucosyl residue. This residue distinguishes the R1 core OS and it provides the attachment site for ligation of O antigen.