Construction of recombinant aroA salmonellae stably producing the Shigella dysenteriae serotype 1 O-antigen and structural characterization of the Salmonella/Shigella hybrid LPS

The TN501 mercury resistant transposon containing the rfp and rfb loci encoding biosynthesis of the O-antigen of Shigella dysenteriae serotype 1 lipopolysaccharide (LPS) was constructed and introduced into aroA mutants of Salmonella typhimurium and Salmonella dublin. In five recombinant strains, both homologous LPS and hybrid LPS, consisting of Salmonella lipid A-core and Shigella O-antigen, were produced. All derivatives but one (SL3235) stably inherited the new trait. Immunofluorescence microscopy, using mixtures of differentially-labelled antibodies specific for either the Salmonella or the Shigella O-antigen, demonstrated that individual bacteria produced both types of LPS. Qualitative and quantitative analysis of polysaccharides obtained by mild hydrolysis of purified LPS was carried out by methylation analysis and NMR spectroscopy, and revealed that the ratio of Salmonella to Shigella O-antigen repeating units in the high molecular weight fraction of isolated polysaccharides varied from 1.3: 1 to 8.4:1 as based on the relative proportions of 1,4,5-tri-O-acetyl-2,3-di-O-methyl-L- rhamnitol (Salmonella repeating unit) and 1,3,5-tri-O-acetyl-2,4-di-O-methyl-L-rhamnitol (Shigella repeating unit). The attachment site of the Shigella O-antigen to the Salmonella core was investigated by construction of a mutant rfp-rfb gene cluster encoding the synthesis of only one repeat unit of the Shigella dysenteriae type 1 O-antigen, and its introduction into a rough Salmonella strain. This hybrid organism produced a polysaccharide with the following structure, [formula: see text] demonstrating that the Shigella dysenteriae type 1 O-antigen is linked at position O-4 of the subterminal D-glucose unit in the Salmonella core.