Hong Yaoqin, Morcilla Vincent A, Liu Michael A, Russell Elsa L M, Reeves Peter R
School of Molecular Bioscience, Building D17, University of Sydney, NSW 2006, Australia.
Microbiology (Reading). 2015 Aug;161(8):1639-1647. doi: 10.1099/mic.0.000113. Epub 2015 May 18.
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.
Wzx/Wzy 依赖途径是革兰氏阴性菌中 O 抗原产生的主要途径。O 抗原重复单元（O 单元）是一种寡糖，在十一异戊烯焦磷酸上于细胞膜的细胞质面组装而成。然后 Wzx 将其翻转至周质面，由 Wzy 进行聚合，Wzy 在每一轮聚合反应中都会将一个 O 单元添加到正在生长的 O 单元聚合物的还原端。Wzx 和 Wzy 都表现出巨大的序列多样性。最近研究表明，与早期报道相反，不同形式的 Wzx 的效率会因对其天然 O 单元底物的微小结构变化而显著降低。然而，Wzy 底物特异性的细节仍未得到探索。密切相关的以半乳糖起始的沙门氏菌 O 抗原为解决这些问题提供了难得的机会。D1 和 D2 O 单元仅在内部甘露糖 - 鼠李糖连接上有所不同，并且 D3 在同一链中同时表达两者。已表明 D1 和 D2 聚合酶对具有各自内部甘露糖 - 鼠李糖连接的α或β构型的 O 单元具有特异性。由 D3 基因簇编码的 Wzy 仅聚合 D1 O 单元，并且删除该基因并不会消除聚合的 O 抗原，这两个观察结果都表明存在另一个 wzy 基因。Wzx 和 Wzy 底物特异性的水平将影响新 O 单元进化的难易程度，也会影响我们通过糖工程修饰 O 抗原、荚膜或分泌多糖以生成新型多糖的能力，因为 Wzx/Wzy 依赖途径是造成大部分多样性的原因。
