Role of the FliA-FlgM regulatory system on the transcriptional control of the flagellar regulon and flagellar formation in Salmonella typhimurium

Abstract

In the flagellar regulon of Salnonella typhimurium, the flagellar operons are divided into three classes, 1, 2, and 3, with respect to transcriptional hierarchy. The class 2 operons are controlled positively by the class 1 genes,flhD andflhC. The class 3 operons are controlled positively byfliA and negatively byflgM. 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 polymerase core enzyme. Therefore, the FliA-FlgM regulatory system has been believed to control specifically the class 3 operons. In the present study, we showed that theflgM mutation enhanced the expression of class 2 by more than fivefold. When afliAL mutation was present simultaneously, 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 neitherflhD norflhC mutants could express the class 2 operons, thefli4 mutants permitted the basal-level expression of those operons. Therefore, FlhD and FlhC are indispensable for the expression of class 2, whereas FliA is required only for its enhancement in the FlgM-depletion condition. Furthermore, we showed that theflgM mutation resulted in a two-to threefold increase in flagellar number. On the basis of these results, we propose that the relative concentration of FliA and FlgM may play an important role in the determination of flagellar numbers produced by a single cell.Salmonella typhimurium and Escherichia coli cells have 5 to 10 flagella which are responsible for motility and chemotaxis. An individual flagellum is composed of three structural parts, a basal body, a hook, and a filament. More than 50 genes are required for formation of functional flagellum (21). These flagellar genes constitute at least 15 different operons in the chromosome (Fig. 1), and each flagellar operon is named after the gene which is transcribed first in that operon (14,18). Examination of transcriptional interaction of the flagellar operons revealed that they are organized into a regulon (13,17). According to the flagellar regulon model of S. typhimurium, the flagellar operons are divided into three classes, 1, 2, and 3, with respect to transcriptional hierarchy (17). Class 1 contains only the flhD operon, which has been called the master operon and is required for expression of the class 2 operons. Class 2 contains seven operons, fig4, flgB, flhB, fliA, fliE,fliF, and fliL, which are under the positive control of class 1 and required for expression of class 3. Class 3 contains at least five operons, flgK, tar, motA, fliC, and fliD. The striking feature of the flagellar regulon is the coupling of the sequential expression of the flagellar operons with the assembly process of flagellar structures; that is, all the genes involved in hook-basal body assembly belong to class 2, and those involved in filament assembly belong to class 3. Mutations in any one of the hook-basal body genes inhibit ...

Two novel regulatory genes, fliT and fliZ, in the flagellar regulon of Salmonella.

“…DNase I footprinting analysis revealed the FlhD/FlhC-binding sites 30 to 80 bp upstream of the transcriptional start sites (Liu and Matsumura, 1994). In all class 2 operons of Salmonella, FlhD/FlhC-dependent transcription was shown in vivo to be enhanced by the flgM mutation in the presence of a functional fliA operon (Kutsukake and Iino, 1994). Though some of the class 2 operons have been shown to contain σ 28 -dependent promoters in addition to class 2 promoters (Liu and Matsumura, 1996), most of the class 2 operons lack σ 28 -dependent promoters (Ikebe et al, 1999b).…”

“…We previously showed that enhancement of class 2 Ikebe et al (1999a) expression by the flgM mutation is cancelled by the fliA::Tn10 mutation (Kutsukake and Iino, 1994). However, because the fliA operon was later found to contain the fliZ gene (Fig.1), the fliA strain we had used should not express the fliZ gene efficiently owing to a polar effect of the Tn10 insertion.…”

“…The fliT and fliZ genes on these plasmids are expressed from nonflagellar promoters on the vector sequences. These plasmids were introduced by transformation into KK1107 (flhD-lac) and KK1108 (fliA-lac) (Table 1), and ß-galactosidase activity was assayed with the resulting transformants grown in minimal medium containing 0.3% Casamino acids and 0.2% glycerol according to the method described previously (Kutsukake and Iino, 1994). The multicopy fliT and fliZ genes had no significant effect on expression of the flhD-lac fusion gene (Table 2).…”

Mathematical model of flagella gene expression dynamics in Salmonella enterica serovar typhimurium

“…In addition, simulations of our model in a DflgM mutant leads to elimination of the lag in expression of class 3 genes. In a HBB mutant, since no secretion is possible, class 3 expression is eliminated (Kutsukake and Iino 1994). However, there is no significant impact on class 2 gene expression.…”

Involvement of Lipoprotein NlpI in the Virulence of Adherent Invasive Escherichia coli Strain LF82 Isolated from a Patient with Crohn's Disease

“…The flagellar operons are organized as a hierarchy within the regulon, with the expression of operons at a given level affecting the expression of operons at lower levels (20,24). The flhDC operon lies on the top of the hierarchy as the sole class I operon, with both of its gene products being absolutely required for the expression of all other genes of the flagellar regulon (15), i.e., those belonging to the class II and III operons.…”