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

在伤寒沙门氏菌鞭毛形态发生过程中，参与鞭毛组装的基因只有在完成钩状组装后才能完全表达。这种基因表达与形态发生的耦合是通过将鞭毛特异性抗σ因子FlgM通过成熟的钩-基体结构输出到细胞外来实现的。因此，鞭毛特异性输出装置必须能够感知鞭毛结构的组装状态，并在钩组装的特定阶段开启FlgM输出。在这项研究中，这种出口转换机制在分子水平上进行了剖析。主要结论如下。(1)FlgM的输出门在完成钩组装之前被FlhB和RflH蛋白双重锁定。(2)这些锁在突变体中打开，产生异常伸长的杆，缺乏钩。这表明吊钩长度监测系统测量的是杆加上吊钩的长度。(3)FlgM门还参与晚期鞭毛蛋白FlgK、FlgL、FliD和FliC的输出控制。FlhB还负责在完成钩组装后关闭钩蛋白输出的过程。(4)分离其中RflH锁组成型打开的突变体。这些突变体的遗传分析表明，至少有两种蛋白质参与RflH锁的打开过程。(5)FlhB锁在钩组装的早期阶段打开，而RflH锁在钩组装完成后打开。

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.