Clostridioides difficile is a common cause of nosocomial diarrhoea. Toxins TcdA and TcdB are considered to be the main virulence factors and are encoded by the PaLoc region, while the binary toxin encoded in the CdtLoc region also contributes to pathogenicity. Variant toxinotypes reflect the genetic diversity of a key toxin-encoding 19 kb genetic element (the PaLoc). Here, we present analysis of a comprehensive collection of all known major C. difficile toxinotypes to address the evolutionary relationships of the toxin gene variants, the mechanisms underlying the origin and development of variability in toxin genes and the PaLoc, and the relationship between structure and function in TcdB variants. The structure of both toxin genes is modular, composed of interspersed blocks of sequences corresponding to functional domains and having different evolutionary histories, as shown by the distribution of mutations along the toxin genes and by incongruences of domain phylogenies compared to overall C. difficile cluster organization. In TcdB protein, four mutation patterns could be differentiated, which correlated very well with the type of TcdB cytopathic effect (CPE) on cultured cells. Mapping these mutations to the three-dimensional structure of the TcdB showed that the majority of the variation occurs in surface residues and that point mutation at residue 449 in alpha helix 16 differentiated strains with different types of CPE. In contrast to the PaLoc, phylogenetic trees of the CdtLoc were more consistent with the core genome phylogenies, but there were clues that CdtLoc can also be exchanged between strains.

中文翻译：

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艰难梭菌毒素型的比较基因组学鉴定基于模块的毒素基因进化

艰难梭菌 是医院内腹泻的常见原因。毒素 TcdA 和 TcdB 被认为是主要的毒力因子，由 PaLoc 区域编码，而在 CdtLoc 区域编码的二元毒素也有助于致病性。变异的毒素型反映了一个关键的毒素编码 19 kb 遗传元件 (PaLoc) 的遗传多样性。在这里，我们对所有已知主要艰难梭菌的综合收集进行了分析 毒素基因变异的进化关系，毒素基因和 PaLoc 变异的起源和发展机制，以及 TcdB 变异的结构和功能之间的关系。两种毒素基因的结构都是模块化的，由散布的序列块组成，对应于功能域并具有不同的进化历史，如沿毒素基因的突变分布和域系统发育与整体艰难梭菌相比的不一致所示 集群组织。在 TcdB 蛋白中，可以区分四种突变模式，这与 TcdB 细胞病变效应 (CPE) 对培养细胞的类型非常相关。将这些突变映射到 TcdB 的三维结构表明大多数变异发生在表面残基中，而 α 螺旋 16 中残基 449 处的点突变具有不同类型的 CPE 分化菌株。与 PaLoc 相比，CdtLoc 的系统发育树与核心基因组系统发育更一致，但有线索表明 CdtLoc 也可以在菌株之间进行交换。