The Vi antigen is a capsular polysaccharide expressed by , the agent of human typhoid fever. Expression of this antigen is controlled by the and chromosomal loci. The locus is composed of 11 genes designated - (typhi Vi), - (Vi antigen export) and ORF11. We constructed Ty2 strains carrying non-polar mutations in ten genes located at the locus and examined the individual contribution of each gene to Vi phenotype. Phenotypes of the mutants and complementation experiments suggested that synthesis of Vi antigen monomer was catalysed by the TviB and TviC polypeptides. Subsequent polymerization of the polysaccharide might be catalysed by the TviE protein, but required functional TviD product. Proteins encoded by , and directed transport of the polymer to the bacterial cell surface. Anchoring of the Vi antigen at the bacterial cell surface was dependent of the VexE protein. The TviA protein was not essential for Vi polymer synthesis. However, disruption of the gene on Ty2 chromosome strongly decreased expression of Vi antigen. This defect was fully complemented by providing on a recombinant plasmid. By using transcriptional fusions, it was shown that the TviA product positively regulated co-transcription of the and genes from a promoter located upstream of . Moreover, we showed that a fusion was not expressed in a () mutant of . However, expression of the - fusion was restored in this mutant either by the gene of , or by the gene of when present in high copy number. This suggested that the and products could be involved in the same regulatory pathway modulating Vi antigen expression in . Together these results demonstrated that proteins encoded by the locus are not only involved in Vi polymer synthesis and translocation of the polysaccharide to the bacterial cell surface, but also in regulation of Vi antigen expression in .


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