Genetic and molecular analyzes of bacterial flagellum which acts as a transcription-controlling apparatus

在鼠伤寒沙门氏菌鞭毛形态发生过程中，参与纤丝组装的基因只有在钩子组装完成后才能完全表达。这种基因表达与形态发生的耦合是通过成熟的钩基体结构将鞭毛特异性的抗Sigma因子FlgM输出到细胞外来实现的。因此，鞭毛特定的输出设备必须能够感知鞭毛结构的组装状态，并在钩子组装的特定阶段打开FlgM输出。在本研究中，我们在分子水平上对这种出口开关机制进行了剖析。主要结论如下。(1)在钩子组装完成之前，FlgM的出口门被FlhB和RflH蛋白双重锁定。(2)这些锁在突变体中被打开，从而产生没有钩的异常拉长的杆。这表明钩子长度监测系统测量的是杆子加钩子的长度。(3)FlgM门还参与晚期鞭毛蛋白FlgK、FlgL、Flid和FliC的出口调控，FlhB还负责在钩子组装完成后切断钩蛋白的出口过程。(4)分离到RflH锁被结构性打开的突变体。对这些突变体的遗传分析表明，至少有两种蛋白质参与了RflH锁的打开过程。(5)FlhB锁在吊钩组装初期打开，而RflH锁在吊钩组装完成后打开。

During flagellar morphogenesis in Salmonella typhirnurium, the genes involved in filament assembly are expressed fully only after completion of hook assembly. This coupling of gene expression to morphogenesis is achieved by exporting the flagellum-specific anti-sigma factor, FlgM, out of the cell through the mature hook-basal body structure. Therefore, the flagellum-specific export apparatus must be able to sense the assembly state of the flagellar structure and to turn on FlgM export at the specific stage of hook assembly. In this study, this export switching machinery was dissected at a molecular level. The main conclusions are as follows. (1) The export gate for FlgM is double locked by FlhB and RflH proteins before completion of hook assembly. (2) These locks are opened in the mutants which produce abnormally elongated rods lacking hooks. This indicates that the monitor system of hook length measures the length of the rod plus the hook. (3) The FlgM gate is also involved in the export control of the late flagellar proteins, FlgK, FlgL, FliD, and FliC.FlhB is also responsible for shutting-off process of the export of hook protein after completion of hook assembly. (4) Mutants in which the RflH lock is opened constitutively were isolated. Genetic analysis of these mutants suggested that at least two proteins are involved in the opening process of the RflH lock. (5) The FlhB lock is opened at the early stage of hook assembly, whereas the RflH lock is opened upon completion of hook assembly.