Overview of the <i>viaB</i> operon and Vi capsular polysaccharide biosynthesis pathway.

<p>(<b>A</b>) Schematic diagram of the <i>S</i>. Typhi <i>viaB</i> operon. (<b>B</b>) The Vi antigen is a linear polymer of α-1,4-linked <i>N</i>-acetylgalatosaminuronate, nonstoichiometrically esterified with acetyl groups at the C-3. A TviB-catalyzed oxidation of UDP-<i>N</i>-acetylglucosamine (UDP-GlcNAc) followed by the TviC-catalyzed epimerization at the C-4 of UDP-<i>N</i>-acetylglucosaminuronate (UDP-GlcNAcA) results in the formation of UDP-<i>N</i>-acetylgalactosaminuronate (UDP-GalNAcA), the building block for Vi polymer formation. The Vi polymer is synthesized in the cytoplasm and assembly is dependent on TviD and TviE. The enzyme catalyzing the O-acetylation of the capsular polysaccharide has not yet been identified. Subsequent translocation of the polysaccharide to the cell surface follows an ATP-binding cassette (ABC) transporter-dependent process. The transporter consists of VexA, VexB, VexC and VexD. The precise function of VexE is equivocal although it might be involved in anchoring the Vi to the cell surface.