THE ASSEMBLY SYSTEM FOR THE LIPOPOLYSACCHARIDE R2 CORE-TYPE OF ESCHERICHIA-COLI IS A HYBRID OF THOSE FOUND IN ESCHERICHIA-COLI K-12 AND SALMONELLA-ENTERICA - STRUCTURE AND FUNCTION OF THE R2 WAAK AND WAAL HOMOLOGS

In Escherichia coli F632, the 14-kilobase pair chromosomal region loca ted between waaC (formerly rfaC) and waaA (kdtA) contains genes encodi ng enzymes required for the synthesis of the type R2 core oligosacchar ide portion of lipopolysaccharide. Ten of the 13 open reading frames e ncode predicted products sharing greater than 90% total similarity wit h homologs in E. coli K-12, However, the products of waaK (rfaK) and w aaL (rfaL) each resemble homologs in Salmonella enterica serovar Typhi murium but share little similarity with E. coli K-12. The F632 WaaK an d WaaL proteins therefore define differences between the type R2 and K -12 outer core oligosaccharides of E. coli lipopolysaccharides. Based on the chemical structure of the core oligosaccharide of an E. coli F6 32 waaK::aacC1 mutant and in vitro glycosyltransferase analyses, waaK encodes UDP-N-acetyl-glucosamine:(glucose) lipopolysaccharide alpha 1, 2-N-acetyl-glucosaminyltransferase. The WaaK enzyme adds a terminal Gl cNAc side branch substituent that is crucial for the recognition of co re oligosaccharide acceptor by the O-polysaccharide ligase, WaaL, Resu lts of complementation analyses of E. coli K-12 and F632 waaL mutants suggest that structural differences between the WaaL proteins play a r ole in recognition of, and interaction with, terminal lipopolysacchari de core moieties.