MOLECULAR DISSECTION OF THE FLAGELLUM SPECIFIC ANTI-SIGMA FACTOR, FLGM, OF SALMONELLA-TYPHIMURIUM

In the flagellar regulon of Salmonella typhimurium, the flagellar oper ons are divided into three classes, 1, 2 and 3, with respect to transc riptional hierarchy. Class 3 operons are controlled positively by FliA , a flagellum-specific sigma factor, and negatively by FlgM, an anti-s igma factor which binds to FliA and inhibits its activity. The sequent ial expression of flagellar operons is coupled to the assembly process of flagellar structures. This coupling is achieved by the fact that F lgM is exported out of the cell through the flagellar structures that are formed by the functions of the class 1 and 2 genes. Therefore, Flg M has a dual function: it can bind to FliA and is capable of being exp orted through the flagellar structure. In this study, using a set of d eletion mutants of flgM in high-expression plasmids, we demonstrated t hat polypeptides containing the C-terminal portion of FlgM could inhib it the FliA-dependent transcription of the class 3 genes. Loss of amin o acids near the N-terminus eliminated the export of the protein, whil e loss of C-terminal amino acids did not affect this function. These r esults indicate that the domain essential for export lies in the N-ter minal region and that for FliA-binding in the C-terminal region.