HOOK-LENGTH CONTROL OF THE EXPORT-SWITCHING MACHINERY INVOLVES A DOUBLE-LOCKED GATE IN SALMONELLA-TYPHIMURIUM FLAGELLAR MORPHOGENESIS

During flagellar morphogenesis in Salmonella typhimurium, the genes in volved in filament assembly are expressed fully only after completion of hook-basal body assembly. This coupling of gene expression to morph ogenesis is achieved by exporting the flagellum-specific anti-sigma fa ctor, FlgM, out of the cell through the mature hook-basal body structu re. Therefore, the flagellum-specific export apparatus must be able to sense the assembly state of the flagellar structure and to turn on Fl gM export at a specific stage of hook assembly, It has been suggested that FlhB may act as the molecular switch which mediates this ordered export. Here, I report genetic evidence that in addition to FlhB, the product of a newly identified gene, rflH, is involved in the negative regulation of FlgM export. FlgM is released through the basal body str ucture lacking the hook and the filament only when the flhB and rflH g enes are both defective, Therefore, the export gate for FlgM should be double locked by FlhB and RflH. The rflH gene is located at around 52 min, where no flagellum-related gene has been found. I propose a revi sed model of the export-switching machinery which consists of two syst ems, the hook-length signal transduction pathway and the double-locked gate for FlgM export.