The process by which bacterial cells build their intricate flagellar motility apparatuses has long fascinated scientists. Our understanding of this process comes mainly from studies of purified flagella from two species, Escherichia coli and Salmonella enterica. Here, we used electron cryo-tomography (cryo-ET) to image the assembly of the flagellar motor in situ in diverse Proteobacteria: Hylemonella gracilis, Helicobacter pylori, Campylobacter jejuni, Pseudomonas aeruginosa, Pseudomonas fluorescens, and Shewanella oneidensis. Our results reveal the in situ structures of flagellar intermediates, beginning with the earliest flagellar type III secretion system core complex (fT3SScc) and MS-ring. In high-torque motors of Beta-, Gamma-, and Epsilon-proteobacteria, we discovered novel cytoplasmic rings that interact with the cytoplasmic torque ring formed by FliG. These rings, associated with the MS-ring, assemble very early and persist until the stators are recruited into their periplasmic ring; in their absence the stator ring does not assemble. By imaging mutants in Helicobacter pylori, we found that the fT3SScc proteins FliO and FliQ are required for the assembly of these novel cytoplasmic rings. Our results show that rather than a simple accretion of components, flagellar motor assembly is a dynamic process in which accessory components interact transiently to assist in building the complex nanomachine.

中文翻译：

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新型瞬时细胞质环可稳定细菌鞭毛马达的组装。

细菌细胞构建复杂的鞭毛运动装置的过程长期以来一直令科学家们着迷。我们对这一过程的理解主要来自对大肠杆菌和沙门氏菌这两个物种的纯化鞭毛的研究。在这里，我们使用电子冷冻断层扫描（cryo-ET）对不同变形菌中鞭毛运动的原位组装进行成像：纤细小纤维菌、幽门螺杆菌、空肠弯曲菌、铜绿假单胞菌、荧光假单胞菌和奥奈德希瓦氏菌。我们的结果揭示了鞭毛中间体的原位结构，从最早的鞭毛 III 型分泌系统核心复合体 (fT3SScc) 和 MS 环开始。在 Beta、Gamma 和 Epsilon 变形菌的高扭矩电机中，我们发现了与 FliG 形成的细胞质扭矩环相互作用的新型细胞质环。这些与 MS 环相关的环很早就组装并持续存在，直到定子被招募到它们的周质环中；如果没有它们，定子环就无法组装。通过对幽门螺杆菌突变体进行成像，我们发现 fT3SScc 蛋白 FliO 和 FliQ 是组装这些新型细胞质环所必需的。我们的结果表明，鞭毛马达组装不是简单的组件堆积，而是一个动态过程，其中辅助组件瞬时相互作用以协助构建复杂的纳米机器。