Molecular dissection of the flagellum-specific anti-sigma factor, FlgM, of Salmonella typhimurium

In the flagellar regulon of Salmonella typhimurium, the flagellar operons are divided into three classes, 1, 2 and 3, with respect to transcriptional hierarchy. Class 3 operons are controlled positively by FliA, a flagellum-specific sigma factor, and negatively by FlgM, an anti-sigma factor which binds to FliA and inhibits its activity. The sequential expression of flagellar operons is coupled to the assembly process of flagellar structures. This coupling is achieved by the fact that FlgM is exported out of the cell through the flagellar structures that are formed by the functions of the class 1 and 2 genes. Therefore, FlgM has a dual function: it can bind to FhA and is capable of being exported through the flagellar structure. In this study, using a set of deletion mutants of flgM in high-expression plasmids, we demonstrated that polypeptides containing the C-terminal portion of FlgM could inhibit the FliA-dependent transcription of the class 3 genes. Loss of amino acids near the N-terminus eliminated the export of the protein, while loss of C-terminal amino acids did not affect this function. These results indicate that the domain essential for export lies in the N-terminal region and that for FliA-binding in the C-terminal region.