ROLE OF THE FLIA-FLGM REGULATORY SYSTEM ON THE TRANSCRIPTIONAL CONTROL OF THE FLAGELLAR REGULON AND FLAGELLAR FORMATION IN 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 trans criptional hierarchy. The class 2 operons are controlled positively by the class 1 genes, flhD and flhC. The class 3 operons are controlled positively by fliA and negatively by flgM. It has been shown that FliA is a sigma factor specific for class 3, whereas FlgM is an anti-sigma factor which binds FliA to prevent its association with RNA polymeras e core enzyme. Therefore, the FliA-FlgM regulatory system has been bel ieved to control specifically the class 3 operons. In the present stud y, we showed that the flgM mutation enhanced the expression of class 2 by more than fivefold. When a fliA mutation was present simultaneousl y, this enhancement was not observed. These results indicate that the FliA-FlgM regulatory system is involved not only in the expression of class 3 but also in that of class 2. However, though neither flhD nor flhC mutants could express the class 2 operons, the fliA mutants permi tted the basal-level expression of those operons. Therefore, FlhD and FlhC are indispensable for the expression of class 2, whereas FIM is r equired only for its enhancement in the FlgM depletion condition. Furt hermore, we shelved that the flgM mutation resulted in a two- to three fold increase in flagellar number. On the basis of these results, we p ropose that the relative concentration of FliA and FlgM may play an im portant role in the determination of flagellar numbers produced by a s ingle cell.