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The Wzx/Wzy-dependent pathway is the predominant pathway for O-antigen production in Gram-negative bacteria. The O-antigen repeat unit (O unit) is an oligosaccharide that is assembled at the cytoplasmic face of the membrane on undecaprenyl pyrophosphate. Wzx then flips it to the periplasmic face for polymerization by Wzy, which adds an O unit to the reducing end of a growing O-unit polymer in each round of polymerization. Wzx and Wzy both exhibit enormous sequence diversity. It has recently been shown that, contrary to earlier reports, the efficiency of diverse Wzx forms can be significantly reduced by minor structural variations to their native O-unit substrate. However, details of Wzy substrate specificity remain unexplored. The closely related galactose-initiated Salmonella O antigens present a rare opportunity to address these matters. The D1 and D2 O units differ only in an internal mannose-rhamnose linkage, and D3 expresses both in the same chain. D1 and D2 polymerases were shown to be specific for O units with their respective α or β configuration for the internal mannose-rhamnose linkage. The Wzy encoded by D3 gene cluster polymerizes only D1 O units, and deleting the gene does not eliminate polymeric O antigen, both observations indicating the presence of an additional wzy gene. The levels of Wzx and Wzy substrate specificity will affect the ease with which new O units can evolve, and also our ability to modify O antigens, capsules or secreted polysaccharides by glyco-engineering, to generate novel polysaccharides, as the Wzx/Wzy-dependent pathway is responsible for much of the diversity.

Three Wzy polymerases are specific for particular forms of an internal linkage in otherwise identical O units