Operon structure of flagellar genes in Salmonella typhimurium

Building a complete image of genome regulation in the model organism Escherichia coli

The model organism, Escherichia coli, contains a total of more than 4,500 genes, but the total number of RNA polymerase (RNAP) core enzyme or the transcriptase is only about 2,000 molecules per genome. The regulatory targets of RNAP are, however, modulated by changing its promoter selectivity through two-steps of protein-protein interplay with 7 species of the sigma factor in the first step, and then 300 species of the transcription factor (TF) in the second step. Scientists working in the field of prokaryotic transcription in Japan have made considerable contributions to the elucidation of genetic frameworks and regulatory modes of the genome transcription in E. coli K-12. This review summarizes the findings by this group, first focusing on three sigma factors, the stationary-phase sigma RpoS, the heat-shock sigma RpoH, and the flagellar-chemotaxis sigma RpoF, as examples. It also presents an overview of the current state of the systematic research being carried out to identify the regulatory functions of all TFs from a single and the same bacterium E. coli K-12, using the genomic SELEX and PS-TF screening systems. All these studies have been undertaken with the aim of understanding the genome regulation in E. coli K-12 as a whole.

Functional and expressional analyses of the anti-FlhD4C2 factor gene ydiV in Escherichia coli

Although Escherichia coli and Salmonella enterica serovar Typhimurium have a similar flagellar regulatory system, the response of flagellar synthesis to nutrient conditions is quite different between the two: that is, in low-nutrient conditions, flagellar synthesis is inhibited in Salmonella and enhanced in E. coli. In Salmonella, this inhibition is mediated by an anti-FlhD(4)C(2) factor, YdiV, which is expressed in low-nutrient conditions and binds to FlhD(4)C(2) to inhibit the expression of the class 2 flagellar genes. The fliZ gene encodes a repressor of the ydiV gene, and thus is required for efficient flagellar gene expression in low-nutrient conditions in Salmonella. In this study, we showed that the E. coli ydiV gene encodes a protein which inhibits motility and flagellar production when expressed from a multicopy plasmid. We showed further that E. coli YdiV binds to FlhD(4)C(2) and inhibits its binding to the class 2 flagellar promoter. These results indicate that E. coli YdiV can also act as an anti-FlhD(4)C(2) factor. However, although the ydiV gene was transcribed efficiently in E. coli cells, the intracellular level of the YdiV protein was extremely low due to its inefficient translation. Consistent with this, E. coli cells did not require FliZ for efficient motility development. This indicates that, unlike in Salmonella, the FliZ-YdiV regulatory system does not work in the nutritional control of flagellar gene expression in E. coli.

The transcript from the σ(28)-dependent promoter is translationally inert in the expression of the σ(28)-encoding gene fliA in the fliAZ operon of Salmonella enterica serovar Typhimurium

There are three classes of promoters for flagellar operons in Salmonella. Class 2 promoters are transcribed by σ(70) RNA polymerase in the presence of an essential activator, FlhD(4)C(2), and activated by an auxiliary regulator, FliZ. Class 3 promoters are transcribed by σ(28) RNA polymerase and repressed by an anti-σ(28) factor, FlgM. σ(28) (FliA) and FliZ are encoded by the fliA and fliZ genes, respectively, which together constitute an operon transcribed in this order. This operon is transcribed from both class 2 and class 3 promoters, suggesting that it should be activated by its own product, σ(28), even in the absence of FlhD(4)C(2). However, σ(28)-dependent transcription occurs in vivo only in the presence of FlhD(4)C(2), indicating that transcription from the class 2 promoter is a prerequisite to that from the class 3 promoter. In this study, we examined the effects of variously modified versions of the fliA regulatory region on transcription and translation of the fliA gene. We showed that FliA is not significantly translated from the class 3 transcript. In contrast, the 5'-terminal AU-rich sequence found in the class 2 transcript confers efficient fliA translation. Replacement of the Shine-Dalgarno sequence of the fliA gene with a better one improved fliA translation from the class 3 transcript. These results suggest that the 5'-terminal AU-rich sequence of the class 2 transcript may assist ribosome binding. FliZ was shown to be expressed from both the class 2 and class 3 transcripts.

FliZ acts as a repressor of the ydiV gene, which encodes an anti-FlhD4C2 factor of the flagellar regulon in Salmonella enterica serovar typhimurium

YdiV acts as an anti-FlhD4C2 factor, which negatively regulates the class 2 flagellar operons in poor medium in Salmonella enterica serovar Typhimurium. On the other hand, one of the class 2 flagellar genes, fliZ, encodes a positive regulator of the class 2 operons. In this study, we found that the FliZ-dependent activation of class 2 operon expression was more profound in poor medium than in rich medium and not observed in the ydiV mutant background. Transcription of the ydiV gene was shown to increase in the fliZ mutant. Purified FliZ protein was shown in vitro to bind to the promoter region of the nlpC gene, which is located just upstream of the ydiV gene, and to repress its transcription. These results indicate that FliZ is a repressor of the nlpC-ydiV operon and activates the class 2 operons by repressing ydiV expression. Therefore, the fliZ and ydiV genes form a regulatory loop.

EAL domain protein YdiV acts as an anti-FlhD4C2 factor responsible for nutritional control of the flagellar regulon in Salmonella enterica Serovar Typhimurium

Flagellar operons are divided into three classes with respect to their transcriptional hierarchy in Salmonella enterica serovar Typhimurium. The class 1 gene products FlhD and FlhC act together in an FlhD(4)C(2) heterohexamer, which binds upstream of the class 2 promoters to facilitate binding of RNA polymerase. In this study, we showed that flagellar expression was much reduced in the cells grown in poor medium compared to those grown in rich medium. This nutritional control was shown to be executed at a step after class 1 transcription. We isolated five Tn5 insertion mutants in which the class 2 expression was derepressed in poor medium. These insertions were located in the ydiV (cdgR) gene or a gene just upstream of ydiV. The ydiV gene is known to encode an EAL domain protein and to act as a negative regulator of flagellar expression. Gene disruption and complementation analyses revealed that the ydiV gene is responsible for nutritional control. Expression analysis of the ydiV gene showed that its translation, but not transcription, was enhanced by growth in poor medium. The ydiV mutation did not have a significant effect on either the steady-state level of flhDC mRNA or that of FlhC protein. Purified YdiV protein was shown in vitro to bind to FlhD(4)C(2) through interaction with FlhD subunit and to inhibit its binding to the class 2 promoter, resulting in inhibition of FlhD(4)C(2)-dependent transcription. Taking these data together, we conclude that YdiV is a novel anti-FlhD(4)C(2) factor responsible for nutritional control of the flagellar regulon.

Flagella facilitate escape of Salmonella from oncotic macrophages

The intracellular parasite Salmonella enterica serovar Typhimurium causes a typhoid-like systemic disease in mice. Whereas the survival of Salmonella in phagocytes is well understood, little has been documented about the exit of intracellular Salmonella from host cells. Here we report that in a population of infected macrophages Salmonella induces "oncosis," an irreversible progression to eukaryotic cell death characterized by swelling of the entire cell body. Oncotic macrophages (OnMphis) are terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling negative and lack actin filaments (F-actin). The plasma membrane of OnMphis filled with bacilli remains impermeable, and intracellular Salmonella bacilli move vigorously using flagella. Eventually, intracellular Salmonella bacilli intermittently exit host cells in a flagellum-dependent manner. These results suggest that induction of macrophage oncosis and intracellular accumulation of flagellated bacilli constitute a strategy whereby Salmonella escapes from host macrophages.

Detection of pathogenic intestinal bacteria by Toll-like receptor 5 on intestinal CD11c+ lamina propria cells

Toll-like receptors (TLRs) recognize distinct microbial components and induce innate immune responses. TLR5 is triggered by bacterial flagellin. Here we generated Tlr5-/- mice and assessed TLR5 function in vivo. Unlike other TLRs, TLR5 was not expressed on conventional dendritic cells or macrophages. In contrast, TLR5 was expressed mainly on intestinal CD11c+ lamina propria cells (LPCs). CD11c+ LPCs detected pathogenic bacteria and secreted proinflammatory cytokines in a TLR5-dependent way. However, CD11c+ LPCs do not express TLR4 and did not secrete proinflammatory cytokines after exposure to a commensal bacterium. Notably, transport of pathogenic Salmonella typhimurium from the intestinal tract to mesenteric lymph nodes was impaired in Tlr5-/- mice. These data suggest that CD11c+ LPCs, via TLR5, detect and are used by pathogenic bacteria in the intestinal lumen.

A mechanical role for the chemotaxis system in swarming motility

The chemotaxis system, but not chemotaxis, is essential for swarming motility in Salmonella enterica serovar Typhimurium. Mutants in the chemotaxis pathway exhibit fewer and shorter flagella, downregulate class 3 or 'late' motility genes, and appear to be less hydrated when propagated on a surface. We show here that the output of the chemotaxis system, CheY approximately P, modulates motor bias during swarming as it does during chemotaxis, but for a distinctly different end. A constitutively active form of CheY was found to promote swarming in the absence of several upstream chemotaxis components. Two point mutations that suppressed the swarming defect of a cheY null mutation mapped to FliM, a protein in the motor switch complex with which CheY approximately P interacts. A common property of these suppressors was their increased frequency of motor reversal. These and other data suggest that the ability to switch motor direction is important for promoting optimal surface wetness. If the surface is sufficiently wet, exclusively clockwise or counterclockwise directions of motor rotation will support swarming, suggesting also that the bacteria can move on a surface with flagellar bundles of either handedness.

FljA-mediated posttranscriptional control of phase 1 flagellin expression in flagellar phase variation of Salmonella enterica serovar Typhimurium

Flagellar phase variation of Salmonella is a phenomenon where two flagellin genes, fliC (phase 1) and fljB (phase 2), are expressed alternately. This is controlled by the inversion of a DNA segment containing the promoter for the fljB gene. The fljB gene constitutes an operon with the fljA gene, which encodes a negative regulator for fliC expression. Previous biochemical analysis suggested that phase variation might depend on alternative synthesis of phase-specific flagellin mRNA (H. Suzuki and T. Iino, J. Mol. Biol. 81:57-70, 1973). However, recently reported results suggested that FljA-dependent inhibition might be mediated by a posttranscriptional control mechanism (H. R. Bonifield and K. T. Hughes, J. Bacteriol. 185:3567-3574, 2003). In this study, we reexamined the mechanism of FljA-mediated inhibition of fliC expression more carefully. Northern blotting analysis revealed that no fliC mRNA was detected in phase 2 cells. However, only a moderate decrease in beta-galactosidase activity was observed from the fliC-lacZ transcriptional fusion gene in phase 2 cells compared with that in phase 1 cells. In contrast, the expression of the fliC-lacZ translational fusion gene was severely impaired in phase 2 cells. The half-life of fliC mRNA was shown to be much shorter in phase 2 cells than in phase 1 cells. Purified His-tagged FljA protein was shown to bind specifically to fliC mRNA and inhibit the translation from fliC mRNA in vitro. On the basis of these results, we propose that in phase 2 cells, FljA binds to fliC mRNA and inhibits its translation, which in turn facilitates its degradation.

Azithromycin inhibits the formation of flagellar filaments without suppressing flagellin synthesis in Salmonella enterica serovar typhimurium

The present study shows that a sub-MIC of the macrolide antibiotic azithromycin (AZM) diminishes the virulence function of Salmonella enterica serovar Typhimurium. We first constructed an AZM-resistant strain (MS248) by introducing ermBC, an erythromycin ribosome methylase gene, into serovar Typhimurium. The MIC of AZM for MS248 exceeded 100 microg/ml. Second, we managed to determine the efficacy with which a sub-MIC of AZM reduced the virulence of MS248 in vitro. On the one hand, AZM (10 microg/ml) in the culture medium was unable to inhibit the total protein synthesis, growth rate, or survival within macrophages of MS248. On the other hand, AZM (10 microg/ml) reduced MS248's swarming and swimming motilities in addition to its invasive activity in Henle-407 cells. Electron micrographs revealed no flagellar filaments on the surface of MS248 after overnight growth in L broth supplemented with AZM (10 microg/ml). However, immunoblotting analysis showed that flagellin (FliC) was fully synthesized within the bacterial cells in the presence of AZM (10 microg/ml). In contrast, the same concentration of AZM reduced the export of FliC to the culture medium. These results indicate that a sub-MIC of AZM was able to affect the formation of flagellar filaments, specifically by reducing the amount of flagellin exported from bacterial cells, but it was not involved in suppressing the synthesis of flagellin. Unfortunately, AZM treatment was ineffective against murine salmonellosis caused by MS248.

Characterization of six flagellin genes in the H3, H53 and H54 standard strains of Escherichia coli

Six flagellin genes in three H standard Escherichia coli strains for H3, H53 and H54 were characterized. Each strain has two flagellin genes, one of which is expressed as its standard H antigen. A pair of flagellin genes flkA3 (encoding for H3 antigen) and fliC16 (H16) was cloned from Bi7327-41, flkA53 (H53) and fliC-53 from E480-68, and flmA54 (H54) and fliC-54 from E223-69. Two fliC genes, fliC-53 and fliC-54, are nonfunctional owing to the insertions of IS1 and IS1222, respectively. The flkA and flmA regions are located in the 3' end of the rnpB gene and near the nlpA gene, respectively. Each of them is followed by a gene homologous to fljA, which is known to repress the expression of fliC(i) in Salmonella enterica serovar Typhimurium. These results suggest that they are derived from the same origin of the fljBA operon. However, these regions contain neither the hin gene nor the invertible H segment. The four flagellin genes, fliC16, flkA3, flkA53 and flmA54, share high homology in nucleotide and amino-acid sequences with one another and with the S. enterica serovar Typhimurium flagellin genes. The promoter sequence of fliC16 is homologous to that of fliC(i), whereas the promoter sequences of flkA and flmA are homologous to that of fljB. The terminator sequences of the fliC16, fliC-53 and fliC-54 genes are conserved among themselves and identical with that of the E. coli fliC48 gene. Three FljA repressors, FljA3, FljA53 and FljA54, are homologous highly with one another and moderately with FljA of Salmonella. These results indicate that six flagellin genes analyzed are markedly similar to the Salmonella flagellin genes, suggesting their lateral transfer from Salmonella.

Chemotaxis-guided movements in bacteria

Motile bacteria often use sophisticated chemotaxis signaling systems to direct their movements. In general, bacterial chemotactic signal transduction pathways have three basic elements: (1) signal reception by bacterial chemoreceptors located on the membrane; (2) signal transduction to relay the signals from membrane receptors to the motor; and (3) signal adaptation to desensitize the initial signal input. The chemotaxis proteins involved in these signal transduction pathways have been identified and extensively studied, especially in the enterobacteria Escherichia coli and Salmonella enterica serovar typhimurium. Chemotaxis-guided bacterial movements enable bacteria to adapt better to their natural habitats via moving toward favorable conditions and away from hostile surroundings. A variety of oral microbes exhibits motility and chemotaxis, behaviors that may play important roles in bacterial survival and pathogenesis in the oral cavity.

Salmonella flagellin is not a dominant protective antigen in oral immunization with attenuated live vaccine strains

We found that oral immunization with flagellum-defective mutant strains of Salmonella enterica serovar Typhimurium with the ClpXP-deficient background protected mice against oral challenge with the virulent strain. These data indicate that Salmonella flagellin is not a dominant protective antigen in oral immunization with attenuated live vaccine strains.

Pathways leading from BarA/SirA to motility and virulence gene expression in Salmonella

The barA and sirA genes of Salmonella enterica serovar Typhimurium encode a two-component sensor kinase and a response regulator, respectively. This system increases the expression of virulence genes and decreases the expression of motility genes. In this study, we examined the pathways by which SirA affects these genes. We found that the master regulator of flagellar genes, flhDC, had a positive regulatory effect on the primary regulator of intestinal virulence determinants, hilA, but that hilA had no effect on flhDC. SirA was able to repress flhDC in a hilA mutant and activate hilA in an flhDC mutant. Therefore, although the flhDC and hilA regulatory cascades interact, sirA affects each of them independently. A form of BarA lacking the two N-terminal membrane-spanning domains, BarA198, autophosphorylates in the presence of ATP and transfers the phosphate to purified SirA. Phosphorylated SirA was found to directly bind the hilA and hilC promoters in gel mobility shift assays but not the flhD, fliA, hilD, and invF promoters. Given that the CsrA/csrB system is known to directly affect flagellar gene expression, we tested the hypothesis that SirA affects flagellar gene expression indirectly by regulating csrA or csrB. The sirA gene did not regulate csrA but did activate csrB expression. Consistent with these results, phosphorylated SirA was found to directly bind the csrB promoter but not the csrA promoter. We propose a model in which SirA directly activates virulence expression via hilA and hilC while repressing the flagellar regulon indirectly via csrB.

Regulation cascade of flagellar expression in Gram-negative bacteria

Flagellar motility helps bacteria to reach the most favourable environments and to successfully compete with other micro-organisms. These complex organelles also play an important role in adhesion to substrates, biofilm formation and virulence process. In addition, because their synthesis and functioning are very expensive for the cell (about 2% of biosynthetic energy expenditure in Escherichia coli) and may induce a strong immune response in the host organism, the expression of flagellar genes is highly regulated by environmental conditions. In the past few years, many data have been published about the regulation of motility in polarly and laterally flagellated bacteria. However, the mechanism of motility control by environmental factors and by some regulatory proteins remains largely unknown. In this respect, recent experimental data suggest that the master regulatory protein-encoding genes at the first level of the cascade are the main target for many environmental factors. This mechanism might require DNA topology alterations of their regulatory regions. Finally, despite some differences the polar and lateral flagellar cascades share many functional similarities, including a similar hierarchical organisation of flagellar systems. The remarkable parallelism in the functional organisation of flagellar systems suggests an evolutionary conservation of regulatory mechanisms in Gram-negative bacteria.

The ATPase FliI can interact with the type III flagellar protein export apparatus in the absence of its regulator, FliH

Salmonella FliI is the ATPase that drives flagellar protein export. It normally exists as a complex together with the regulatory protein FliH. A fliH null mutant was slightly motile, with overproduction of FliI resulting in substantial improvement of its motility. Mutations in the cytoplasmic domains of FlhA and FlhB, which are integral membrane components of the type III flagellar export apparatus, also resulted in substantially improved motility, even at normal FliI levels. Thus, FliH, though undoubtedly important, is not essential.

Turnover of FlhD and FlhC, master regulator proteins for Salmonella flagellum biogenesis, by the ATP-dependent ClpXP protease

In enterobacteria such as Salmonella, flagellar biogenesis is dependent upon the master operon flhDC at the apex of the flagellar gene hierarchy, which is divided into three classes 1, 2 and 3. Previously we reported that depletion of the ClpXP ATP-dependent protease results in dramatic enhancement of class 2 and class 3 gene transcription, whereas the transcription level of the flhDC operon remains normal in Salmonella enterica serovar Typhimurium. This suggests that the ClpXP protease may be responsible for the turnover of the FlhD and FlhC master regulators (Tomoyasu, T., Ohkishi, T., Ukyo, Y., Tokumitsu, A., Takaya, A., Suzuki, M. et al., 2002, J Bacteriol 184:645-653). In this study, to establish the role of the ClpXP protease in the turnover of FlhD and FlhC proteins, we analysed levels of the FlhD and FlhC proteins in wild-type and ClpXP mutant cells using anti-FlhD and anti-FlhC antibodies. The results show that both FlhD and FlhC proteins are markedly accumulated in ClpXP mutant cells and the half-life of FlhC is approximately fivefold longer in the ClpXP mutant, suggesting that the ClpXP protease is responsible for the degradation of FlhD and FlhC. The results also show that the ClpXP protease degrades both proteins in FlhD2FlhC2 complex but does not seem to recognize the respective subunits synthesized individually. Taken together, it is suggested that the cellular concentration of the FlhD2FlhC2 master regulator is tightly controlled at the post-translational level by the ClpXP protease. We also examined the role of other members of the ATP-dependent protease family in the regulation of flagellar biogenesis and concluded that only ClpXP in this family functions as a negative regulator for flagellar biogenesis in Salmonella.

Oral immunization with ATP-dependent protease-deficient mutants protects mice against subsequent oral challenge with virulent Salmonella enterica serovar typhimurium

We evaluated the efficacy of mutants with a deletion of the stress response protease gene as candidates for live oral vaccine strains against Salmonella infection through infection studies with mice by using a Salmonella enterica serovar Typhimurium mutant with a disruption of the ClpXP or Lon protease. In vitro, the ClpXP protease regulates flagellum synthesis and the ClpXP-deficient mutant strain exhibits hyperflagellated bacterial cells (T. Tomoyasu et al., J. Bacteriol. 184:645-653, 2002). On the other hand, the Lon protease negatively regulates the efficacy of invading epithelial cells and the expression of invasion genes (A. Takaya et al., J. Bacteriol. 184:224-232, 2002). When 5-week-old BALB/c mice were orally administered 5 x 10(8) CFU of the ClpXP- or Lon-deficient strain, bacteria were detected with 10(3) to 10(4) CFU in the spleen, mesenteric lymph nodes, Peyer's patches, and cecum 1 week after inoculation and the bacteria then decreased gradually in each tissue. Significant increases of lipopolysaccharide-specific immunoglobulin G (IgG) and secretory IgA were detected at week 4 and maintained until at least week 12 after inoculation in serum and bile, respectively. Immunization with the ClpXP- or Lon-deficient strain protected mice against oral challenge with the serovar Typhimurium virulent strain. Both the challenged virulent and immunized avirulent salmonellae were completely cleared from the spleen, mesenteric lymph nodes, Peyer's patches, and even cecum 5 days after the challenge. These data indicate that Salmonella with a disruption of the ATP-dependent protease ClpXP or Lon can be useful in developing a live vaccine strain.

The ClpXP ATP-dependent protease regulates flagellum synthesis in Salmonella enterica serovar typhimurium

The ClpXP protease is a member of the ATP-dependent protease family and plays a dynamic role in the control of availability of regulatory proteins and the breakdown of abnormal and misfolded proteins. The proteolytic activity is rendered by the ClpP component, while the substrate specificity is determined by the ClpX component that has ATPase activity. We describe here a new role of the ClpXP protease in Salmonella enterica serovar Typhimurium in which ClpXP is involved in the regulation of flagellum synthesis. Cells deleted for ClpXP show "hyperflagellate phenotype," exhibit overproduction of the flagellar protein, and show a fourfold increase in the rate of transcription of the fliC encoding flagellar filament. The assay for promoter activity of the genes responsible for expression of the fliC showed that the depletion of ClpXP results in dramatic enhancement of the expression of the fliA encoding sigma factor final sigma(28), leaving the expression level of the flhD master operon lying at the top of the transcription hierarchy of flagellar regulon almost normal. These results suggest that the ClpXP may be responsible for repressing the expression of flagellar regulon through the control of the FlhD/FlhC master regulators at the posttranscriptional and/or posttranslational levels. Proteome analysis of proteins secreted from the mutant cells deficient for flhDC and clpXP genes demonstrated that the DeltaflhD mutation abolished the enhanced effect by DeltaclpXP mutation on the production of flagellar proteins, suggesting that the ClpXP possibly defines a regulatory pathway affecting the expression of flagellar regulon that is dependent on FlhD/FlhC master regulators.

Intergenic suppression between the flagellar MS ring protein FliF of Salmonella and FlhA, a membrane component of its export apparatus

The MS ring of the flagellar basal body of Salmonella is an integral membrane structure consisting of about 26 subunits of a 61-kDa protein, FliF. Out of many nonflagellate fliF mutants tested, three gave rise to intergenic suppressors in flagellar region II. The pseudorevertants swarmed, though poorly; this partial recovery of motile function was shown to be due to partial recovery of export function and flagellar assembly. The three parental mutants were all found to carry the same mutation, a six-base deletion corresponding to loss of Ala-174 and Ser-175 in the predicted periplasmic domain of the FliF protein. The 19 intergenic suppressors identified all lay in flhA, and they consisted of 10 independent examples at the nucleotide level or 9 at the amino acid level. Since two of the nine corresponded to different substitutions at the same amino acid position, only eight positions in the FlhA protein have given rise to suppressors. Thus, FliF-FlhA intergenic suppression is a fairly rare event. FlhA is a component of the flagellar protein export apparatus, with an integral membrane domain encompassing the N-terminal half of the sequence and a cytoplasmic C-terminal domain. All of the suppressing mutations lay within the integral membrane domain. These mutations, when placed in a wild-type fliF background, had no mutant phenotype. In the fliF mutant background, mutant FlhA was dominant, yielding a pseudorevertant phenotype. Wild-type FlhA did not exert significant negative dominance in the pseudorevertant background, indicating that it does not compete effectively with mutant FlhA for interaction with mutant FliF. Mutant FliF was partially dominant over wild-type FliF in both the wild-type and second-site FlhA backgrounds. Membrane fractionation experiments indicated that the fliF mutation, though preventing export, was mild enough to permit assembly of the MS ring itself, and also assembly of the cytoplasmic C ring onto the MS ring. The data from this study provide genetic support for a model in which at least the FlhA component of the export apparatus physically interacts with the MS ring within which it is housed.

A flagellar gene fliZ regulates the expression of invasion genes and virulence phenotype in Salmonella enterica serovar Typhimurium

We previously reported that the fliZ gene encodes a positive regulatory factor for the class 2 flagellar operons in Salmonella enterica serovar Typhimurium. In this study, we found that the fliZ mutation reduced not only the amounts of excreted flagellar proteins, but also those of several secreted invasion proteins encoded by the genes within Salmonella pathogenicity island 1. Using the lacZ gene fused to a subset of virulence-associated genes, we show that this downregulation was caused by a decreased transcription of the hilA gene, which encodes a positive regulator for the invasion genes. We further show that the fliZ mutation reduced invasion ability of S. enterica serovar Typhimurium to HEp-2 cells. Consistent with these results, orally challenged cells of the fliZ mutant show an attenuated virulence phenotype in a mouse typhoid model. These results indicate that the fliZ gene product positively regulates the invasion genes and is necessary for expression of full virulence.

Identification of two distinct types of flagellar cap proteins, FliD, in Pseudomonas aeruginosa

Binding of Pseudomonas aeruginosa strain PAK to mucin has been shown to be mediated by the flagellar cap protein, product of the fliD gene. Since the flagellar cap is very likely an exposed structure, the FliD polypeptide should be recognized by the host immune system, analogous to the recognition of dominant epitopes located in the exposed parts of the flagellin polypeptide within the assembled flagellum. In P. aeruginosa, a number of distinct flagellin variants are made, and these variable sequences presumably allow the newly infected P. aeruginosa to escape recognition by the antibody induced during a previous infection. Since similar mechanisms may direct the selection of FliD variants, we examined the extent of sequence heterogeneity among various FliD sequences among a selected group of P. aeruginosa. The results of PCR and nucleotide sequencing of the fliD region of eight different P. aeruginosa strains (laboratory strains PAK, PAO1, and PA103; clinical strains 1244, CS2, and CS32; cystic fibrosis strains CS29 and MDR) suggested that there were two distinct types of FliD in P. aeruginosa, which we named A type and B type. The results of Western blotting using the polyclonal antibodies raised against the purified FliD of A type (PAK) or B type (PAO1) further confirmed the existence of two distinct antigenic types of FliD proteins, with no cross-reactivity between the two serotypes. Further Western immunoblot analysis of the same strains using polyclonal FliC antibody showed that the strains with A-type FliD possessed a-type FliC and those with B-type FliD had b-type FliC. Similar Western blot analyses of 50 more P. aeruginosa strains obtained from varied sources revealed that all strains contained either A-type or B-type FliD, suggesting the existence of only two types of FliD in P. aeruginosa and indicating that fliC and fliD were coinherited. This limited diversity of FliC and FliD serotypes seems to be a unique feature of flagellar proteins. A chromosomal mutant having an insertion in the fliD gene of P. aeruginosa PAO1 was constructed. The motility defect of this mutant and a previously constructed PAK fliD mutant was better complemented with the fliD gene of the homologous types.

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

The flagellar operons of Salmonella are divided into three classes with respect to their transcriptional hierarchy. Expression of the class 2 operons requires the class 1 gene products, FlhD and FlhC, and is increased by mutation in the flgM gene, which encodes a class 3-specific anti-sigma factor. Here we report the identification of two novel regulatory genes for class 2 transcription. Presence of the fliZ and fliT genes on multicopy plasmids enhanced and inhibited, respectively, transcription from a chromosomal class 2 promoter. Disruption of the fliZ and fliT genes on the chromosome decreased and increased, respectively, class 2 expression. These results suggest that the fliZ and fliT genes may encode positive and negative regulatory factors, respectively, for class 2 expression. Enhancement of class 2 expression by the flgM mutation was cancelled by the coexisting fliZ mutation, indicating that FliZ is essential for this enhancement.

Structure and transcriptional control of the flagellar master operon of Salmonella typhimurium

The flhD and flhC genes constitute the flagellar master operon whose products are required for expression of all the remaining flagellar operons in Salmonella typhimurium. Here we report the molecular structure and in vivo and in vitro expression of the flhD operon. Nucleotide sequence analysis revealed that the upstream region of this operon contains the consensus sequence for the cAMP-CRP binding site. Primer extension analysis demonstrated six possible transcription start sites for this operon. They include CRP-dependent and CRP-repressible transcription start sites. The CRP-dependent transcription start site is located 203 bp upstream of the initiation codon of the flhD gene and preceded by the consensus sequences of the -10 and -35 regions of the sigma 70-dependent promoter. The putative cAMP-CRP binding site is located centered 70 bp upstream of this start site. The CRP-repressible transcription start site is located within this putative cAMP-CRP binding site. These two start sites were confirmed by in vitro transcription experiments using sigma 70-RNA polymerase with or without cAMP-CRP.

Peptidoglycan-hydrolyzing activity of the FlgJ protein, essential for flagellar rod formation in Salmonella typhimurium

Because the rod structure of the flagellar basal body crosses the inner membrane, the periplasmic space, and the outer membrane, its formation must involve hydrolysis of the peptidoglycan layer. So far, more than 10 genes have been shown to be required for rod formation in Salmonella typhimurium. Some of them encode the component proteins of the rod structure, and most of the remaining genes are believed to encode proteins involved in the export process of the component proteins. Although FlgJ has also been known to be involved in rod formation, its exact role has not been understood. Recently, it was suggested that the C-terminal half of the FlgJ protein has homology to the active center of some muramidase enzymes from gram-positive bacteria. In this study, we showed that the purified FlgJ protein from S. typhimurium has a peptidoglycan-hydrolyzing activity and that this activity is localized in its C-terminal half. Through oligonucleotide-directed mutagenesis, we constructed flgJ mutants with amino acid substitutions in the putative active center of the muramidase. The resulting mutants produced FlgJ proteins with reduced enzymatic activity and showed poor motility. These results indicate that the muramidase activity of FlgJ is essential for flagellar formation. Immunoblotting analysis with the fractionated cell extracts revealed that FlgJ is exported to the periplasmic space, where the peptidoglycan layer is localized. On the basis of these results, we conclude that FlgJ is the flagellum-specific muramidase which hydrolyzes the peptidoglycan layer to assemble the rod structure in the periplasmic space.

Dual promoters are responsible for transcription initiation of the fla/che operon in Bacillus subtilis

The fla/che region contains more than 30 genes required for flagellar synthesis and chemotaxis in Bacillus subtilis, including the gene for the flagellum-specific sigmaD factor, sigD. Sequence and primer extension data demonstrate that a PA promoter immediately upstream of flgB, henceforth referred to as the fla/che PA, and the PD-3 promoter are active in vivo. Transcription from the PD-3 element is dependent on sigmaD activity and is regulated by the flagellum-specific negative regulator, FlgM. In a strain containing a deletion of fla/che PA (PADelta), sigmaD protein was not detected, demonstrating that the fla/che PA is necessary for wild-type expression of the sigD gene. Thus, sigD is part of the >26-kb fla/che operon. Consistent with a lack of detectable sigmaD protein, the PADelta strain grows as long filaments and does not express a sigmaD-dependent hag::lacZ reporter construct. These phenotypes are indicative of a lack of sigD expression or complete inhibition of sigmaD activity by FlgM. However, sigmaD activity is found in a double mutant containing the PADelta and a null mutation in flgM. The double mutant no longer grows as long filaments, and expression of hag::lacZ is partially restored. These data demonstrate that a low level of sigmaD activity does exist in the PADelta mutant but can be detected only in the presence of a null mutation in flgM. Therefore, normal expression of sigD may also involve another promoter(s) within the fla/che operon.

Flagellar filament elongation can be impaired by mutations in the hook protein FlgE of Salmonella typhimurium: a possible role of the hook as a passage for the anti-sigma factor FlgM

Among motile revertants isolated from flagellar hook-deficient (flgE) mutants of Salmonella typhimurium, one produced only short flagellar filaments in L broth, despite the fact that flagellin itself has the ability to polymerize into long filaments in vitro. This pseudorevertant has an intragenic suppressor, resulting in a two-amino-acid substitution (Asp-Gln-->Ala-Arg) in the C-terminal region of the hook protein, FlgE. The flagellation of the pseudorevertant was greatly affected by the concentration of NaCl in the culture media: we observed no filaments in the absence of NaCl, short filaments in 1% NaCl and full-length filaments in 2% NaCl. Electron microscopy of osmotically shocked cells showed that the number of hook-basal bodies on cells was constant under various NaCl conditions. Furthermore, we found that the mutant hook was straight rather than curved. We monitored the cellular flagellin level of this pseudorevertant under various NaCl concentrations by immunoblotting. It was revealed that little flagellin was present under NaCl-free conditions in contrast with the ordinary amounts of flagellin present in 2% NaCl. As the expression of flagellin is regulated by competitive interaction of a sigma factor, FliA, and a corresponding anti-sigma factor, FlgM, we also observed the effect of NaCl on the secretion of FlgM. FlgM was secreted into the media in more than 1% NaCl but accumulated inside the cells in the absence of NaCl, indicating that the failure of secretion of FlgM in the absence of salt was the cause of the impaired elongation of filaments.

The Pseudomonas aeruginosa flagellar cap protein, FliD, is responsible for mucin adhesion

Mucin-specific adhesion of Pseudomonas aeruginosa plays an important role in the initial colonization of this organism in the airways of cystic fibrosis patients. We report here that the flagellar cap protein, FliD, participates in this adhesion process. A polar chromosomal insertional mutation in the P. aeruginosa fliD gene made this organism nonadhesive to mucin in an in vitro mucin adhesion assay. The adhesive phenotype was restored by providing the fliD gene alone on a multicopy plasmid, suggesting involvement of this gene in mucin adhesion of P. aeruginosa. Further supporting this observation, the in vitro competition experiments demonstrated that purified FliD protein inhibited the mucin adhesion of nonpiliated P. aeruginosa PAK-NP, while the same concentrations of PilA and FlaG proteins of P. aeruginosa were ineffective in this function. The regulation of the fliD gene was studied and was found to be unique in that the transcription of the fliD gene was independent of the flagellar sigma factor sigma28. Consistent with this finding, no sigma28 binding sequence could be identified in the fliD promoter region. The results of the beta-galactosidase assays suggest that the fliD gene in P. aeruginosa is regulated by the newly described transcriptional regulator FleQ and the alternate sigma factor sigma54 (RpoN).

Identification of a novel Escherichia coli gene whose expression is dependent on the flagellum-specific sigma factor, FliA, but dispensable for motility development

FliA is an alternative sigma factor specific for class 3 flagellar operons. Using a promoter-probe vector, we randomly cloned Escherichia coli DNA fragments, which showed FliA-dependent promoter activities. Among the DNA fragments cloned, one was found to be derived from a non-flagellar region. Hybridization analysis with the Kohara E. coli library indicated that this DNA fragment is located at around 35.4 min on the E. coli chromosome where no flagellar gene has been reported yet. DNA sequence analysis revealed that it contains an FliA-dependent promoter-like sequence followed by an open reading frame (ORF) that can encode a 110-amino-acid protein. A rho-independent terminator-like sequence follows this ORF. This putative gene was named flxA. A gene disruptant was constructed by inserting the kan gene cassette into the flxA gene on the chromosome. This mutant was found to be actively motile, suggesting that this gene is unlikely to be involved in the motility phenotype of E. coli.

The FliO, FliP, FliQ, and FliR proteins of Salmonella typhimurium: putative components for flagellar assembly

The flagellar genes fliO, fliP, fliQ, and fliR of Salmonella typhimurium are contiguous within the fliLMNOPQR operon. They are needed for flagellation but do not encode any known structural or regulatory components. They may be involved in flagellar protein export, which proceeds by a type III export pathway. The genes have been cloned and sequenced. The sequences predict proteins with molecular masses of 13,068, 26,755, 9,592, and 28,933 Da, respectively. All four gene products were identified experimentally; consistent with their high hydrophobic residue content, they segregated with the membrane fraction. From N-terminal amino acid sequence analysis, we conclude that fliO starts immediately after fliN rather than at a previously proposed site downstream. FliP existed in two forms, a 25-kDa form and a 23-kDa form. N-terminal amino acid analysis of the 23-kDa form demonstrated that it had undergone cleavage of a signal peptide--a rare process for prokaryotic cytoplasmic membrane proteins. Site-directed mutation at the cleavage site resulted in impaired processing, which reduced, but did not eliminate, complementation of a fliP mutant in swarm plate assays. A cloned fragment encoding the mature form of the protein could also complement the fliP mutant but did so even more poorly. Finally, when the first transmembrane span of MotA (a cytoplasmic membrane protein that does not undergo signal peptide cleavage) was fused to the mature form of FliP, the fusion protein complemented very weakly. Higher levels of synthesis of the mutant proteins greatly improved function. We conclude that, for insertion of FliP into the membrane, cleavage is important kinetically but not absolutely required.

Hook-length control of the export-switching machinery involves a double-locked gate in Salmonella typhimurium flagellar morphogenesis

During flagellar morphogenesis in Salmonella typhimurium, the genes involved in filament assembly are expressed fully only after completion of hook-basal body assembly. This coupling of gene expression to morphogenesis is achieved by exporting the flagellum-specific anti-sigma factor, FlgM, out of the cell through the mature hook-basal body structure. Therefore, the flagellum-specific export apparatus must be able to sense the assembly state of the flagellar structure and to turn on FlgM export at a specific stage of hook assembly. It has been suggested that FlhB may act as the molecular switch which mediates this ordered export. Here, I report genetic evidence that in addition to FlhB, the product of a newly identified gene, rflH, is involved in the negative regulation of FlgM export. FlgM is released through the basal body structure lacking the hook and the filament only when the flhB and rflH genes are both defective. Therefore, the export gate for FlgM should be double locked by FlhB and RflH. The rflH gene is located at around 52 min, where no flagellum-related gene has been found. I propose a revised model of the export-switching machinery which consists of two systems, the hook-length signal transduction pathway and the double-locked gate for FlgM export.

Induction of cytokine granulocyte-macrophage colony-stimulating factor and chemokine macrophage inflammatory protein 2 mRNAs in macrophages by Legionella pneumophila or Salmonella typhimurium attachment requires different ligand-receptor systems

The attachment of bacteria to macrophages is mediated by different ligands and receptors and induces various intracellular molecular responses. In the present study, induction of cytokines and chemokines, especially granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage inflammatory protein 2 (MIP-2), was examined, following bacterial attachment, with regard to the ligand-receptor systems involved. Attachment of Legionella pneumophila or Salmonella typhimurium to cultured mouse peritoneal macrophages increased the steady-state levels of cellular mRNAs for the cytokines interleukin 1beta (IL-1beta), IL-6, and GM-CSF as well as the chemokines MIP-1beta, MIP-2, and KC. However, when macrophages were treated with alpha-methyl-D-mannoside (alphaMM), a competitor of glycopeptide ligands, induction of cytokine mRNAs was inhibited, but the levels of chemokine mRNAs were not. Pretreatment of the bacteria with fresh mouse serum enhanced the level of GM-CSF mRNA but not the level of MIP-2 mRNA. In addition, serum treatment reduced the inhibitory effect of alphaMM on GM-CSF mRNA. These results indicate that bacterial attachment increases the steady-state levels of the cytokine and chemokine mRNAs tested by at least two distinct receptor-ligand systems, namely, one linked to cytokine induction and involving mannose or other sugar residues and the other linked to chemokine induction and relatively alphaMM insensitive. Furthermore, opsonization with serum engages other pathways in the cytokine response which are relatively independent of the alphaMM-sensitive system. Regarding bacterial surface ligands involved in cytokine mRNA induction, evidence is presented that the flagellum may be important in stimulating cytokine GM-CSF message but not chemokine MIP-2 message. Analysis of cytokine GM-CSF and chemokine MIP-2 signaling pathways with protein kinase inhibitors revealed the involvement of calmodulin and myosin light-chain kinase in GM-CSF but not MIP-2 mRNA induction, adding further evidence that several distinct receptor systems are engaged during the process of bacterial attachment and induction of cytokines and chemokines, such as GM-CSF and MIP-2, respectively.

Negative regulation by fliD, fliS, and fliT of the export of the flagellum-specific anti-sigma factor, FlgM, in Salmonella typhimurium

The fliD operon of Salmonella typhimurium consists of three flagellar genes, fliD, fliS, and fliT, and is transcribed in this order. It has been shown that an fliD::Tn10 mutation causes an excess export of the flagellum-specific anti-sigma factor, FlgM, resulting in an overexpression of flagellar class 3 operons. In this study, using gene-disruption mutants in the individual genes in the fliD operon, we showed that mutations in any one of the genes in the operon enhanced both FlgM export and the expression of flagellar regulon. This indicates that all three genes in the operon are involved in the negative regulation of FlgM export.

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 FliA 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.

Genetic map of Salmonella typhimurium, edition VIII

We present edition VIII of the genetic map of Salmonella typhimurium LT2. We list a total of 1,159 genes, 1,080 of which have been located on the circular chromosome and 29 of which are on pSLT, the 90-kb plasmid usually found in LT2 lines. The remaining 50 genes are not yet mapped. The coordinate system used in this edition is neither minutes of transfer time in conjugation crosses nor units representing "phage lengths" of DNA of the transducing phage P22, as used in earlier editions, but centisomes and kilobases based on physical analysis of the lengths of DNA segments between genes. Some of these lengths have been determined by digestion of DNA by rare-cutting endonucleases and separation of fragments by pulsed-field gel electrophoresis. Other lengths have been determined by analysis of DNA sequences in GenBank. We have constructed StySeq1, which incorporates all Salmonella DNA sequence data known to us. StySeq1 comprises over 548 kb of nonredundant chromosomal genomic sequences, representing 11.4% of the chromosome, which is estimated to be just over 4,800 kb in length. Most of these sequences were assigned locations on the chromosome, in some cases by analogy with mapped Escherichia coli sequences.

Transcriptional analysis of the flgK and fliD operons of Salmonella typhimurium which encode flagellar hook-associated proteins

In Salmonella typhimurium, three hook-associated proteins, HAP1, HAP2 and HAP3, are known to be essential for formation of flagellar filament. HAP1 and HAP2 are encoded by the flgK and flgL genes, respectively, which together constitute an operon, called the flgK operon. HAP3 is encoded by the fliD gene which forms part of the fliD operon together with the fliS and fliT genes. In the flagellar regulon, the operons are divided into three classes, 1, 2 and 3, based on their positions within a transcriptional hierarchy. Transcriptional analysis suggested that the flgK and fliD operons should belong to class 3, whose expression is dependent on the flagellum-specific sigma factor FliA. However, biochemical data indicated that these HAP proteins are detectable even in the hook-basal body structures produced by the fliA mutant. This work was carried out to resolve this discrepancy. More careful examination of transcription revealed that the fliA mutation reduces but does not eliminate the expression of these operons, whereas a mutation in the flhD operon, which encodes activator proteins for the class 2 operons, eliminates their expression. This suggests that the flgK and fliD operons may be transcribed from both class 2 and class 3 promoters. Primer extension analysis indicated that the promoter region of fliD contains both class 2 and class 3 promoters, while that of flgK contains only a class 3 promoter. Transposon insertion into the flgB operon, which belongs to class 2 and lies upstream of the flgK operon, was found to decrease the expression of the flgK operon to the basal level.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of the Salmonella typhimurium flhB operon and its protein products

The flhB and flhA genes constitute an operon called flhB operon on the Salmonella typhimurium chromosome. Their gene products are required for formation of the rod structure of flagellar apparatus. Furthermore, several lines of evidence suggest that they, together with FliI and FliH, may constitute the export apparatus of flagellin, the component protein of flagellar filament. In this study, we determined the nucleotide sequence of the entire flhB operon from S. typhimurium. It was shown that the flhB and flhA genes encode highly hydrophobic polypeptides with calculated molecular masses of 42,322 and 74,848 Da, respectively. Both proteins have several potential membrane-spanning segments, suggesting that they may be integral membrane proteins. The flhB operon was found to contain an additional open reading frame capable of encoding a polypeptide with a calculated molecular mass of 14,073 Da. We designated this open reading frame flhE. The N-terminal 16 amino acids of FlhE displays a feature of a typical signal sequence. A maxicell labeling experiment enabled us to identify the precursor and mature forms of the flhE gene products. Insertion of a kanamycin-resistant gene cartridge into the chromosomal flhE gene did not affect the motility of the cells, indicating that the flhE gene is not essential for flagellar formation and function. We have overproduced and purified N-terminally truncated FlhB and FlhA proteins and raised antibodies against them. By use of these antibodies, localization of the FlhB and FlhA proteins was analyzed by Western blotting (immunoblotting) with the fractionated cell extracts. The results obtained indicated that both proteins are localized in the cytoplasmic membrane.

Isolation and characterization of FliK-independent flagellation mutants from Salmonella typhimurium

A flagellum of Salmonella typhimurium and Escherichia coli consists of three structural parts, a basal body, a hook, and a filament. Because the fliK mutants produce elongated hooks, called polyhooks, lacking filament portions, the fliK gene product has been believed to be involved in both the determination of hook length and the initiation of the filament assembly. In the present study, we isolated two mutants from S. typhimurium which can form flagella even in the absence of the fliK gene product. Flagellar structures were fractionated from these suppressor mutants and inspected by electron microscopy. The suppressor mutants produced polyhook-filament complexes in the fliK mutant background, while they formed flagellar structures apparently indistinguishable from those of the wild-type strain in the fliK+ background. Genetic and sequence analyses of the suppressor mutations revealed that they are located near the 3'-end of the flhB gene, which has been believed to be involved in the early process of the basal body assembly. On the basis of these results, we discuss the mechanism of suppression of the fliK defects by the flhB mutations and propose a hypothesis on the export switching machinery of the flagellar proteins.

Nucleotide sequence of the flgD gene of Salmonella typhimurium which is essential for flagellar hook formation

We determined the complete nucleotide sequence of the flgD gene of Salmonella typhimurium which is essential for flagellar hook formation. The sequence predicts a protein of 232 amino acids and a calculated molecular mass of 23,987 Da. However, the N-terminal 86 amino acids of FlgD were found sufficient to complement all the flgD mutations examined. The predicted secondary structure suggested that FlgD has a high content of beta structure.

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 in Salmonella typhimurium. According to the cascade model of the flagellar regulon, the flagellar operons are divided into three classes, 1, 2, and 3, with reference to their relative positions in the transcriptional hierarchy. This sequential transcription is coupled to the assembly process of the flagellar structure, that is, genes involved in formation of the hook-basal body complex belong to the class-2 operons, whereas those involved in formation of filament belong to the class-3 operons. The fliA gene encodes an alternative sigma factor specific for transcription of the class-3 operons. A negative regulatory gene, flgM, which is responsible for the coupling of expression of class-3 operons 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 two different promoters: one is its own class-3 promoter and the other is the class-2 promoter for the upstream gene, flgA. Furthermore, we showed that FlgM is excreted into culture medium from cells of the wild-type strain and of class-3 mutants that can produce complete hook-basal body structures. On the other hand, FlgM is not excreted from mutants defective in the hook-basal body genes. These results indicate that FlgM is excreted from the cells through the flagellar substructures that are formed by the function of the hook-basal body genes.(ABSTRACT TRUNCATED AT 250 WORDS)

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 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 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 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 the flgM mutation enhanced the expression of class 2 by more than fivefold. When a fliA 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 neither flhD nor flhC mutants could express the class 2 operons, the fliA 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 the flgM 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.

Cloning and sequencing of two new fli genes, the products of which are essential for Salmonella flagellar biosynthesis

We have identified two new open reading frames downstream from fliC, which appear to encode 19- and 20-kDa proteins that are essential for flagellar formation. The nucleotide and the predicted amino acid sequences of the genes are presented. Both genes, termed fliU and fliV, are preceded by the flagellar-specific -10 consensus promoter DNA sequence, suggesting their expression in the earlier stages of flagellar-component biosynthesis.

A novel transcriptional regulation mechanism in the flagellar regulon of Salmonella typhimurium: an antisigma factor inhibits the activity of the flagellum-specific sigma factor, sigma F

We have studied the molecular mechanism of the negative regulation by flgM of the late operons of the flagellar regulon of Salmonella typhimurium. A 7.8 kDa protein that was identified as the flgM gene product was purified to homogeneity; its amino-terminal sequence was identical to the deduced sequence except for the lack of the initiating methionine. The purified FlgM repressed transcription from the fliC promoter, one that is activated by the sigma factor, FliA (sigma F). No DNA-binding activity was detected in FlgM. Chemical cross-linking experiments showed that the purified FlgM bound to sigma F and disturbed its ability to form a complex with RNA polymerase core enzyme. These results indicate that FlgM is a novel type of negative regulator that probably inactivates the flagellum-specific sigma factor through direct interaction, i.e. it is an anti-sigma factor.

Invasion by Salmonella typhimurium is affected by the direction of flagellar rotation

When grown aerobically, Salmonella typhimurium exhibits a low level of entry into tissue culture cells. We have isolated an S. typhimurium Tn10 mutant which, when grown under aerobic conditions, efficiently invades HEp-2 cells. Sequencing of S. typhimurium DNA adjacent to the site of the Tn10 element showed that the insertion disrupted transcription of the aspartate receptor gene, tar. Polar effects of the transposon on downstream genes also eliminated chemotaxis. Isogenic nonchemotactic (Che-), as well as nonmotile (Mot-) and nonflagellated (Fla-), S. typhimurium strains were examined for their ability to invade HEp-2 cells. "Smooth" swimming Che- mutants (cheA, cheW, cheR, and cheY) were found to possess increased invasiveness for cultured mammalian cells. In contrast, a "tumbly" cheB mutant and Mot- (flagellated) strain were found to have decreased levels of tissue culture invasiveness. A Fla- strain was found to be as invasive as the wild-type strain if centrifugation was used to facilitate contact with the monolayer surface. In addition, the observed hyperinvasiveness of the smooth swimming tar::Tn10 mutant was suppressed when the strain was paralyzed by the introduction of a mot or fla mutation. A murine infection model was used to demonstrate that the mutant invasive phenotypes were also observed in vivo. These data are most consistent with the idea that the rotation and physical orientation of flagella around the bacteria affect the ability of salmonellae to enter host cells.

Genetic analysis of the bacterial flagellum

Escherichia coli and Salmonella typhimurium invest considerable resources in making flagella, motor organelles that function much like the propellers on a ship. Both classical and molecular genetic studies have begun to reveal how flagellar genes are regulated and how their products build and operate these remarkable devices.

Salmonella typhimurium mutants defective in flagellar filament regrowth and sequence similarity of FliI to F0F1, vacuolar, and archaebacterial ATPase subunits

Many flagellar proteins are exported by a flagellum-specific export pathway. In an initial attempt to characterize the apparatus responsible for the process, we designed a simple assay to screen for mutants with export defects. Temperature-sensitive flagellar mutants of Salmonella typhimurium were grown at the permissive temperature (30 degrees C), shifted to the restrictive temperature (42 degrees C), and inspected in a light microscope. With the exception of switch mutants, they were fully motile. Next, cells grown at the permissive temperature had their flagellar filaments removed by shearing before the cells were shifted to the restrictive temperature. Most mutants were able to regrow filaments. However, flhA, fliH, fliI, and fliN mutants showed no or greatly reduced regrowth, suggesting that the corresponding gene products are involved in the process of flagellum-specific export. We describe here the sequences of fliH, fliI, and the adjacent gene, fliJ; they encode proteins with deduced molecular masses of 25,782, 49,208, and 17,302 Da, respectively. The deduced sequence of FliI shows significant similarity to the catalytic beta subunit of the bacterial F0F1 ATPase and to the catalytic subunits of vacuolar and archaebacterial ATPases; except for limited similarity in the motifs that constitute the nucleotide-binding or catalytic site, it appears unrelated to the E1E2 class of ATPases, to other proteins that mediate protein export, or to a variety of other ATP-utilizing enzymes. We hypothesize that FliI is either the catalytic subunit of a protein translocase for flagellum-specific export or a proton translocase involved in local circuits at the flagellum.

Negative regulatory loci coupling flagellin synthesis to flagellar assembly in Salmonella typhimurium

The complex regulation of flagellin gene expression in Salmonella typhimurium was characterized in vivo by using lac transcriptional fusions to the two flagellin structural genes (fliC [H1] and fljB [H2]). Phase variation was measured as the rate of switching of flagellin gene expression. Switching frequencies varied from 1/500 per cell per generation to 1/10,000 per cell per generation depending on the particular insertion and the direction of switching. There is a 4- to 20-fold bias in favor of switching from the fljB(On) to the fljB(Off) orientation. Random Tn10dTc insertions were isolated which failed to express flagellin. While most of these insertions mapped to loci known to be required for flagellin expression, several new loci were identified. The presence of functional copies of all of the genes responsible for complete flagellar assembly, except the hook-associated proteins (flgK, flgL, and fliD gene products), were required for expression of the fliC or fljB flagellin genes. Two novel loci involved in negative regulation of fliC and fljB in fla mutant backgrounds were identified. One of these loci, designated the flgR locus, mapped to the flg operon at 23 min on the Salmonella linkage map. An flgR insertion mutation resulted in relief of repression of the fliC and fljB genes in all fla mutant backgrounds except for mutants in the positive regulatory loci (flhC, flhD, and fliA genes).

The bacterial flagellum and flagellar motor: structure, assembly and function

The bacterial flagellum is a complex multicomponent structure which serves as the propulsive organelle for many species of bacteria. Rotation of the helical flagellar filament, driven by a proton-powered motor embedded in the cell wall, enables the flagellum to function as a screw propeller. It seems likely that almost all of the genes required for flagellar formation and function have been identified. Continuing analysis of the portions of the genome containing these genes may reveal the existence of a few more. Transcription of the flagellar genes is under the control of the products of a single operon, and so these genes constitute a regulon. Other controls, both transcriptional and post-transcriptional, have been identified. Many of these genes have been sequenced, and the information obtained will aid in the design of experiments to clarify the various regulatory mechanisms of the flagellar regulon. The flagellum is composed of several substructures. The long helical filament is connected via the flexible hook to the complex basal body which is located in the cell wall. The filament is composed of many copies of a single protein, and can adopt a number of distinct helical forms. Structural analyses of the filament are adding to our understanding of this dynamic polymer. The component proteins of the hook and filament have all been identified. Continuing studies on the structure of the basal body have revealed the presence of several hitherto unknown basal-body proteins, whose identities and functions have yet to be elucidated. The proteins essential for energizing the motor, the Mot and switch proteins, are thought to exist as multisubunit complexes peripheral to the basal body. These complexes have yet to be identified biochemically or morphologically. Not surprisingly, flagellar assembly is a complex process, occurring in several stages. Assembly occurs in a proximal-to-distal fashion; the basal body is assembled before the hook, and the hook before the filament. This pattern is also maintained within the filament, with monomers added at the distal end of the polymer; the same is presumably true of the other axial components. An exception to this general pattern is assembly of the Mot proteins into the motor, which appears to be possible at any time during flagellar assembly. With the identification of the genes encoding many of the flagellar proteins, the roles of these proteins in assembly is understood, but the function of a number of gene products in flagellar formation remains unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Flagellar hook and hook-associated proteins of Salmonella typhimurium and their relationship to other axial components of the flagellum

Within the bacterial flagellum the basal-body rod, the hook, the hook-associated proteins (HAPs), and the helical filament constitute an axial substructure whose elements share structural features and a common export pathway. We present here the amino acid sequences of the hook protein and the three HAPs of Salmonella typhimurium, as deduced from the DNA sequences of their structural genes (flgE, flgK, flgL and fliD, respectively). We compared these sequences with each other and with those for the filament protein (flagellin) and four rod proteins, which have been described previously (Joys, 1985; Homma et al., 1990; Smith & Selander, 1990). Hook protein most strongly resembled the distal rod protein (FlgG) and the proximal HAP (HAP1), which are thought to be attached to the proximal and distal ends of the hook, respectively; the similarities were most pronounced near the N and C termini. Hook protein and flagellin, which occupy virtually identical helical lattices, did not resemble each other strongly but showed some limited similarities near their termini. HAP3 and HAP2, which form the proximal and distal boundaries of the filament, showed few similarities to flagellin, each other, or the other axial proteins. With the exceptions of the N-terminal region of HAP2, and the C-terminal region of flagellin, proline residues were absent from the terminal regions of the axial proteins. Moreover, with the exception of the N-terminal region of HAP2, the terminal regions contained hydrophobic residues at intervals of seven residues. Together, these observations suggest that the axial proteins may have amphipathic alpha-helical structure at their N and C termini. In the case of the filament and the hook, the terminal regions are believed to be responsible for the quaternary interactions between subunits. We suggest that this is likely to be true of the other axial structures as well, and specifically that interaction between N-terminal and C-terminal alpha-helices may be important in the formation of the axial structures of the flagellum. Although consensus sequences were noted among some of the proteins, such as the rod, hook and HAP1, no consensus extended to the entire set of axial proteins. Thus the basis for recognition of a protein for export by the flagellum-specific pathway remains to be identified.

Gene fliA encodes an alternative sigma factor specific for flagellar operons in Salmonella typhimurium

Through genetic studies, the fliA gene product has been shown to regulate positively gene expression in late operons of the flagellar regulon in Salmonella typhimurium. In the present study, the fliA gene was cloned and sequenced. The fliA coding region consisted of 717 nucleotides beginning with the GTG initiation codon and the conserved sequence specific to promoters for flagellar operons was found to exist upstream of the coding region. The fliA gene product deduced from the nucleotide sequence was a protein with 239 amino acid residues and the calculated molecular mass was 27,470 dalton. The deduced amino acid sequence was homologous with that of sigma 28, a flagellar specific sigma factor of Bacillus subtilis. The fliA gene product was identified as a protein of molecular mass 29 kDa in the in vitro transcription-translation system, while three proteins of 29 kDa, 31 kDa and 32 kDa were found in the products programmed by the fliA gene in minicells and in maxicells. The 29 kDa FliA protein was purified from the FliA overproducing strain which carried the ptac-fliA fusion. This protein activated the in vitro synthesis of flagellin, the fliC gene product. RNA polymerase containing the purified FliA protein was shown to transcribe the fliC gene. These results indicate that FliA protein functions as an alternative sigma factor specific for S. typhimurium flagellar operons.