EXCRETION OF THE ANTI-SIGMA FACTOR THROUGH A FLAGELLAR SUBSTRUCTURE COUPLES FLAGELLAR GENE-EXPRESSION WITH FLAGELLAR ASSEMBLY IN SALMONELLA-TYPHIMURIUM

More than 50 genes are required for flagellar formation and function i n Salmonella typhimurium. According to the cascade model of the flagel lar regulon, the flagellar operons are divided into three classes, 1, 2, and 3, with reference to their relative positions in the transcript ional hierarchy. This sequential transcription is coupled to the assem bly process of the flagellar structure, that is, genes involved in for mation of the hook-basal body complex belong to the class-2 operons, w hereas those involved in formation of filament belong to the class-3 o perons. The fliA gene encodes an alternative sigma factor specific for transcription of the class-3 operons. A negative regulatory gene, flg M, which is responsible for the coupling of expression of class-3 oper ons to flagellar assembly, encodes an anti-sigma factor that binds to FliA and prevents its association with RNA polymerase core enzyme. In the present study, we showed that the flgM gene is transcribed from tw o different promoters: one is its own class-3 promoter and the other i s the class-2 promoter for the upstream gene, flgA. Furthermore, we sh owed that FlgM is excreted into culture medium from cells of the wild- type strain and of class-3 mutants that can produce complete hook-basa l body structures. On the other hand, FlgM is not excreted from mutant s defective in the hook-basal body genes. These results indicate that FlgM is excreted from the cells through the flagellar substructures th at are formed by the function of the hook-basal body genes. We believe that this is the mechanism by which FlgM can monitor the assembly sta te of flagellar structure and couple flagellar gene expression to flag ellar assembly.