| Shigella strains are important human pathogens, which were emerged with the development of human beings, and are major causes of diarrhea. Escherichia coli strains causing diarrhea in human beings express different virulence factors and share specific O-antigens. Some serotypes of E. coli were found to be associated with outbreaks and the life threatening haemolytic uraemic syndrome (HUS). Shigella and E. coli strains are generally thought to be sufficiently similar to be placed in the same species. The O-antigen contributes major antigenic variability to the cell surface under the selection pressure from host. There are 166 O-antigens in the E. coli scheme, and 33 O-antigens in the Shigella scheme. 13 O-antigens are overlap between E. coli and Shigella. The genes for synthesis of O-antigen is one of the best materials to study the molecular evolution in bacteria.In this study, the O-antigen loci in four Shigella strains and four E. coli strains were sequenced and analyzed. The original O-antigen locus of the E. coli O57 genome DNA was also sequenced and analyzed. Glycosyltransferase genes and O-antigen processing genes were studied, and the sequences upstream and downstream O-antigen gene clusters including houskeeping genes and non-encoding regions were compared. Three pairs of E. coli and Shigella sharing identical O-antigens were also compared by adjacent-gene PCR assays.It is shown that DNA lateral transfers of interspecies and intraspecies are the major way to form the O-antigen polymorphyism. The O-antigen gene cluster of Shigella dysenteriae O7 is the homologue of a surface polysaccharide gene cluster in Shewanella oneidensis MR-1. It is proposed that two gene clusters were evolved from the common ancestor. This is a typical evidence of interspecies DNA lateral transfer, and is the first time to find a polysaccharide gene cluster lateral transfer between two distantly related species. O-antigen gene clusters of E. coli O105 and S. boydii Ol 1 have the same genes and organization. The O-antigen gene clusters of S. boydii Ol 1 and E. coli O105 both show one atypical feature, reverse orientation of wzx gene, suggesting that it may have been recently assembled in one of the two strains then spread to the other. This is a typical evidence of intraspecies DNA lateral transfer. Insertion sequences (IS) are also important for the O-antigen polymorphyism. It is shown that the O-antigen gene clusters of E. coli O3, O15 and O57 are formed by the insertion sequences. It is also shown that three major groups of O-antigen genes have different origin and evolutionary history. The recombination and lateral transfer of O-antigen gene clusters were mediated by housekeeping genes next to O-antigen gene clusters, conserved regions in the non encoding sequences next to O-antigen gene clusters, and the insertion sequences.In this study, we also showed:1. No O-antigen gene cluster was found between galF and hisl genes in E. coli O57. But the remnant sequence of rmlB gene, the presence of O-antigen promoter sequence and JUMPStart sequence, and the presence of IS1 and H-repeat insertion sequences show that the O-antigen gene cluster was located between galF and gnd in the ancestor of E. coli O57. The original O-antigen gene cluster has been lost together with the insertion of IS and the noval O-antigen gene cluster may be located on the plasmid or other location in E. coli O57.2. A novel Hidden Markov Model for O-antigen polymerase and O-antigen flippase. sequences, which can be used to identify the function of putative O-antigen processing genes, were constructed. The conserved regions were proposed for the glycosyltransferase family 1 and 2. The relationship between glycosyltransferase families and superfamilies were discussed. Functions of a lot of glycosyltransferase genes were proposed by bioinformatics method.3. Genes specific to O-antigens of four Shigella serogroups and seven E. coli serogroups were identified. By screening clinical isolates and pork, the specificity and sensitivity of O-antigen-specifi...
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