Wzx flippases exhibiting complex O‐unit preferences require a new model for Wzx–substrate interactions

The Wzx flippase is a critical component of the O‐antigen biosynthesis pathway, being responsible for the translocation of oligosaccharide O units across the inner membrane in Gram‐negative bacteria. Recent studies have shown that Wzx has a strong preference for its cognate O unit, but the types of O‐unit structural variance that a given Wzx can accommodate are poorly understood. In this study, we identified two Yersinia pseudotuberculosis Wzx that can distinguish between different terminal dideoxyhexose sugars on a common O‐unit main‐chain, despite both being able to translocate several other structurally‐divergent O units. We also identified other Y. pseudotuberculosis Wzx that can translocate a structurally divergent foreign O unit with high efficiency, and thus exhibit an apparently relaxed substrate preference. It now appears that Wzx substrate preference is more complex than previously suggested, and that not all O‐unit residues are equally important determinants of translocation efficiency. We propose a new “Structure‐Specific Triggering” model in which Wzx translocation proceeds at a low level for a wide variety of substrates, with high‐frequency translocation only being triggered by Wzx interacting with one or more preferred O‐unit structural elements found on its cognate O unit(s).