GENETIC-ANALYSIS OF THE O-SPECIFIC LIPOPOLYSACCHARIDE BIOSYNTHESIS REGION (RFB) OF ESCHERICHIA-COLI-K-12-W3110 - IDENTIFICATION OF GENES THAT CONFER GROUP-6 SPECIFICITY TO SHIGELLA-FLEXNERI SEROTYPE-Y AND SEROTYPE-4A

We recently reported a novel genetic locus located in the sbcB-his reg ion of the chromosomal map of Escherichia coli K-12 which directs the expression of group 6-positive phenotype in Shigella flexneri lipopoly saccharide, presumably due to the transfer of O-acetyl groups onto rha mnose residues of the S. flexneri O-specific polysaccharide (Z. Yao, H . Liu, and M. A. Valvano, J. Bacteriol. 174:7500-7508, 1992). In this study, we identified the genetic region encoding group 6 specificity a s part of the rfb gene cluster of E. coli K-12 strain W3110 and establ ished the DNA sequence of most of this cluster. The rfbBDACX block of genes, located in the upstream region of the rfb cluster, was found to be strongly conserved in comparison with the corresponding region in Shigella dysenteriae type 1 and Salmonella enterica. Six other genes,f our of which were shown to be essential for the expression of group 6 reactivity in S. flexneri serotypes Y and 4a, were identified downstre am of rfbX One of the remaining two genes showed similarities with lif e (O-antigen polymerase) of S. enterica serovar typhimurium, whereas t he other, located in the downstream end of the cluster next to and (gl uconate 6-phosphate dehydrogenase), had an IS5 insertion. Recently, it has been reported that the IS5 insertion mutation (rfb-50) can be com plemented, resulting in the formation of O16-specific polysaccharide b y E. coli K-12 (D. Liu and P. R. Reeves, Microbiology 140:49-57, 1994) . We present immunochemical evidence suggesting that S. flexneri rfb g enes also complement the rfb-50 mutation; in the presence of rfb genes of E. coli K-12, S. flexneri isolates express O16-specific polysaccha ride which is also acetylated in its rhamnose residues; thereby elicit ing group 6 specificity.