Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.
mBio. 2013 Sep 3;4(5):e00432-13. doi: 10.1128/mBio.00432-13.
Many polarly flagellated bacteria require similar two-component regulatory systems (TCSs) and σ(54) to activate transcription of genes essential for flagellar motility. Herein, we discovered that in addition to the flagellar type III secretion system (T3SS), the Campylobacter jejuni flagellar MS ring and rotor are required to activate the FlgSR TCS. Mutants lacking the FliF MS ring and FliG C ring rotor proteins were as defective as T3SS mutants in FlgSR- and σ(54)-dependent flagellar gene expression. Also, FliF and FliG required each other for stability, which is mediated by atypical extensions to the proteins. A FliF mutant that presumably does not interact with the T3SS protein FlhA did not support flagellar gene transcription, suggesting that FliF-T3SS interactions are essential to generate a signal sensed by the cytoplasmic FlgS histidine kinase. Furthermore, the flagellar T3SS was required for FlgS to immunoprecipitate with FliF and FliG. We propose a model whereby the flagellar T3SS facilitates FliF and FliG multimerization into the MS ring and rotor. As a result, these flagellar structures form a cytoplasmic complex that interacts with and is sensed by FlgS. The synthesis of these structures appears to be a regulatory checkpoint in flagellar biogenesis that the FlgS kinase monitors to initiate signal transduction that activates σ(54) and expression of genes required for the next stage of flagellation. Given that other polar flagellates have flagellar transcriptional hierarchies that are organized similarly as in C. jejuni, this regulatory checkpoint may exist in a broad range of bacteria to influence similar TCSs and flagellar gene transcription.
Despite the presence of numerous two-component regulatory systems (TCSs) in bacteria, direct signals sensed by TCSs to activate signal transduction are known for only a minority. Polar flagellates, including Pseudomonas, Vibrio, Helicobacter, and Campylobacter species, require a similar TCS and σ(54) for flagellar gene transcription, but the activating signals for these TCSs are unknown. We explored signals that activate the Campylobacter jejuni FlgSR TCS to initiate σ(54)-dependent flagellar gene transcription. Our discoveries suggest that the FlgS histidine kinase monitors the formation of the flagellar type III secretion system and the surrounding MS and C rings. The synthesis of these structures creates a regulatory checkpoint in flagellar biogenesis that is sensed by FlgS to ensure proper transcription of the next set of genes for subsequent steps in flagellation. Given the conservation of flagellar-associated TCSs and transcriptional cascades in polar flagellates, this regulatory checkpoint in flagellar biogenesis likely impacts flagellation in a broad range of bacteria.
许多极性鞭毛细菌需要类似的双组分调节系统（TCS）和 σ（54）来激活鞭毛运动所必需基因的转录。在此，我们发现，除了鞭毛 III 型分泌系统（T3SS）之外，空肠弯曲菌鞭毛 MS 环和转子也需要激活 FlgSR TCS。缺乏 FliF MS 环和 FliG C 环转子蛋白的突变体在 FlgSR 和 σ（54）依赖性鞭毛基因表达方面与 T3SS 突变体一样有缺陷。此外，FliF 和 FliG 彼此需要稳定，这是由蛋白质的非典型延伸介导的。一种推测不会与 T3SS 蛋白 FlhA 相互作用的 FliF 突变体不能支持鞭毛基因转录，这表明 FliF-T3SS 相互作用对于产生由细胞质 FlgS 组氨酸激酶感知的信号是必需的。此外，鞭毛 T3SS 对于 FlgS 与 FliF 和 FliG 的免疫沉淀是必需的。我们提出了一个模型，即鞭毛 T3SS 促进 FliF 和 FliG 多聚化为 MS 环和转子。结果，这些鞭毛结构形成了一个细胞质复合物，与 FlgS 相互作用并被其感知。这些结构的合成似乎是鞭毛发生中的一个调节检查点，FlgS 激酶监测该检查点以启动信号转导，从而激活 σ（54）并表达鞭毛发生下一阶段所需的基因。鉴于其他极性鞭毛生物具有类似于空肠弯曲菌的组织方式的鞭毛转录层次结构，该调节检查点可能存在于广泛的细菌中，以影响类似的 TCS 和鞭毛基因转录。
尽管细菌中存在许多双组分调节系统（TCS），但仅少数 TCS 知道直接感知的信号来激活信号转导。包括假单胞菌、弧菌、幽门螺杆菌和弯曲杆菌属在内的极性鞭毛生物都需要类似的 TCS 和 σ（54）来进行鞭毛基因转录，但这些 TCS 的激活信号尚不清楚。我们探索了激活空肠弯曲菌 FlgSR TCS 以启动 σ（54）依赖性鞭毛基因转录的信号。我们的发现表明，FlgS 组氨酸激酶监测着鞭毛 III 型分泌系统和周围的 MS 和 C 环的形成。这些结构的合成在鞭毛发生中创建了一个调节检查点，FlgS 通过该检查点感知到，以确保随后鞭毛发生阶段的下一组基因的正确转录。鉴于极性鞭毛生物中与鞭毛相关的 TCS 和转录级联的保守性，这种鞭毛发生中的调节检查点可能会影响广泛范围内的细菌的鞭毛发生。
