Regulation of FlbD activity by flagellum assembly is accomplished through direct interaction with the trans-acting factor, FliX

Dual Control of Flagellar Synthesis and Exopolysaccharide Production by FlbD-FliX Class II Regulatory Proteins in Bradyrhizobium diazoefficiens

Bradyrhizobium diazoefficiens, the N₂-fixing symbiont of soybean, has two independent flagellar systems: a single subpolar flagellum and several lateral flagella. Each flagellum is a very complex organelle composed of 30 to 40 different proteins located inside and outside the cell whereby flagellar gene expression must be tightly controlled. Such control is achieved by a hierarchy of regulators that ensure the timing of synthesis and the allocation of the different flagellar substructures. Previously, we analyzed the gene organization, expression, and function of the lateral flagellar system. Here, we studied the role of the response regulator FlbD and its trans-acting regulator FliX in the regulation of subpolar flagellar genes. We found that the LP-ring, distal rod, and hook of the subpolar flagellum were tightly controlled by FlbD and FliX. Furthermore, we obtained evidence for the existence of cross-regulation between these gene products and the expression of LafR, the master regulator of lateral flagella. In addition, we observed that extracellular polysaccharide production and biofilm formation also responded to these flagellar regulators. In this regard, FlbD might contribute to the switch between the planktonic and sessile states.IMPORTANCE Most environmental bacteria switch between two free-living states: planktonic, in which individual cells swim propelled by flagella, and sessile, in which bacteria form biofilms. Apart from being essential for locomotion, the flagellum has accessory functions during biofilm formation. The synthesis of flagella is a highly regulated process, and coordination with accessory functions requires the interconnection of various regulatory networks. Here, we show the role of class II regulators involved in the synthesis of the B. diazoefficiens subpolar flagellum and their possible participation in cross-regulation with the lateral flagellar system and exopolysaccharide production. These findings highlight the coordination of the synthetic processes of external structures, such as subpolar and lateral flagella, with exopolysaccharides, which are the main component of the biofilm matrix.

An organelle-tethering mechanism couples flagellation to cell division in bacteria

In some free-living and pathogenic bacteria, problems in the synthesis and assembly of early flagellar components can cause cell-division defects. However, the mechanism that couples cell division with the flagellar biogenesis has remained elusive. Herein, we discover the regulator MadA that controls transcription of flagellar and cell-division genes in Caulobacter crescentus. We demonstrate that MadA, a small soluble protein, binds the type III export component FlhA to promote activation of FliX, which in turn is required to license the conserved σ⁵⁴-dependent transcriptional activator FlbD. While in the absence of MadA, FliX and FlbD activation is crippled, bypass mutations in FlhA restore flagellar biogenesis and cell division. Furthermore, we demonstrate that MadA safeguards the divisome stoichiometry to license cell division. We propose that MadA has a sentinel-type function that senses an early flagellar biogenesis event and, through cell-division control, ensures that a flagellated offspring emerges.

A Polar Flagellar Transcriptional Program Mediated by Diverse Two-Component Signal Transduction Systems and Basal Flagellar Proteins Is Broadly Conserved in Polar Flagellates

Relative to peritrichous bacteria, polar flagellates possess regulatory systems that order flagellar gene transcription differently and produce flagella in specific numbers only at poles. How transcriptional and flagellar biogenesis regulatory systems are interlinked to promote the correct synthesis of polar flagella in diverse species has largely been unexplored. We found evidence for many Gram-negative polar flagellates encoding two-component signal transduction systems with activity linked to the formation of flagellar type III secretion systems to enable production of flagellar rod and hook proteins at a discrete, subsequent stage during flagellar assembly. This polar flagellar transcriptional program assists, in some manner, the FlhF/FlhG flagellar biogenesis regulatory system, which forms specific flagellation patterns in polar flagellates in maintaining flagellation and motility when activity of FlhF or FlhG might be altered. Our work provides insight into the multiple regulatory processes required for polar flagellation.

Bacterial flagella are rotating nanomachines required for motility. Flagellar gene expression and protein secretion are coordinated for efficient flagellar biogenesis. Polar flagellates, unlike peritrichous bacteria, commonly order flagellar rod and hook gene transcription as a separate step after production of the MS ring, C ring, and flagellar type III secretion system (fT3SS) core proteins that form a competent fT3SS. Conserved regulatory mechanisms in diverse polar flagellates to create this polar flagellar transcriptional program have not been thoroughly assimilated. Using in silico and genetic analyses and our previous findings in Campylobacter jejuni as a foundation, we observed a large subset of Gram-negative bacteria with the FlhF/FlhG regulatory system for polar flagellation to possess flagellum-associated two-component signal transduction systems (TCSs). We present data supporting a general theme in polar flagellates whereby MS ring, rotor, and fT3SS proteins contribute to a regulatory checkpoint during polar flagellar biogenesis. We demonstrate that Vibrio cholerae and Pseudomonas aeruginosa require the formation of this regulatory checkpoint for the TCSs to directly activate subsequent rod and hook gene transcription, which are hallmarks of the polar flagellar transcriptional program. By reprogramming transcription in V. cholerae to more closely follow the peritrichous flagellar transcriptional program, we discovered a link between the polar flagellar transcription program and the activity of FlhF/FlhG flagellar biogenesis regulators in which the transcriptional program allows polar flagellates to continue to produce flagella for motility when FlhF or FlhG activity may be altered. Our findings integrate flagellar transcriptional and biogenesis regulatory processes involved in polar flagellation in many species.

Feedback regulation of Caulobacter crescentus holdfast synthesis by flagellum assembly via the holdfast inhibitor HfiA

To permanently attach to surfaces, Caulobacter crescentusproduces a strong adhesive, the holdfast. The timing of holdfast synthesis is developmentally regulated by cell cycle cues. When C. crescentusis grown in a complex medium, holdfast synthesis can also be stimulated by surface sensing, in which swarmer cells rapidly synthesize holdfast in direct response to surface contact. In contrast to growth in complex medium, here we show that when cells are grown in a defined medium, surface contact does not trigger holdfast synthesis. Moreover, we show that in a defined medium, flagellum synthesis and regulation of holdfast production are linked. In these conditions, mutants lacking a flagellum attach to surfaces over time more efficiently than either wild-type strains or strains harboring a paralyzed flagellum. Enhanced adhesion in mutants lacking flagellar components is due to premature holdfast synthesis during the cell cycle and is regulated by the holdfast synthesis inhibitor HfiA. hfiA transcription is reduced in flagellar mutants and this reduction is modulated by the diguanylate cyclase developmental regulator PleD. We also show that, in contrast to previous predictions, flagella are not necessarily required for C. crescentus surface sensing in the absence of flow, and that arrest of flagellar rotation does not stimulate holdfast synthesis. Rather, our data support a model in which flagellum assembly feeds back to control holdfast synthesis via HfiA expression in a c-di-GMP-dependent manner under defined nutrient conditions.

Interplay between flagellation and cell cycle control in Caulobacter

The assembly of the flagellum, a sophisticated nanomachine powering bacterial locomotion in liquids and across surfaces, is highly regulated. In the synchronizable α-Proteobacterium Caulobacter crescentus, the flagellum is built at a pre-selected cell pole and flagellar transcript abundance oscillates during the cell cycle. Conserved regulators not only dictate when the transcripts encoding flagellar structural proteins peak, but also those encoding polarization factors. Additionally, post-transcriptional cell cycle cues facilitate flagellar (dis-)assembly at the new cell pole. Because of this regulatory complexity and the power of bacterial genetics, motility is a suitable and simple proxy for dissecting how bacteria implement cell cycle progression and polarity, while also providing clues on how bacteria might decide when and where to display other surface structures.

The REC domain mediated dimerization is critical for FleQ from Pseudomonas aeruginosa to function as a c-di-GMP receptor and flagella gene regulator

FleQ is an AAA+ ATPase enhancer-binding protein that regulates both flagella and biofilm formation in the opportunistic pathogen Pseudomonas aeruginosa. FleQ belongs to the NtrC subfamily of response regulators, but lacks the corresponding aspartic acid for phosphorylation in the REC domain (FleQ(R), also named FleQ domain). Here, we show that the atypical REC domain of FleQ is essential for the function of FleQ. Crystal structure of FleQ(R) at 2.3Å reveals that the structure of FleQ(R) is significantly different from the REC domain of NtrC1 which regulates gene expression in a phosphorylation dependent manner. FleQ(R) forms a novel active dimer (transverse dimer), and mediates the dimerization of full-length FleQ in an unusual manner. Point mutations that affect the dimerization of FleQ lead to loss of function of the protein. Moreover, a c-di-GMP binding site deviating from the previous reported one is identified through structure analysis and point mutations.

A complex LuxR-LuxI type quorum sensing network in a roseobacterial marine sponge symbiont activates flagellar motility and inhibits biofilm formation

Bacteria isolated from marine sponges, including the Silicibacter-Ruegeria (SR) subgroup of the Roseobacter clade, produce N-acylhomoserine lactone (AHL) quorum sensing signal molecules. This study is the first detailed analysis of AHL quorum sensing in sponge-associated bacteria, specifically Ruegeria sp. KLH11, from the sponge Mycale laxissima. Two pairs of luxR and luxI homologues and one solo luxI homologue were identified and designated ssaRI, ssbRI and sscI (sponge-associated symbiont locus A, B and C, luxR or luxI homologue). SsaI produced predominantly long-chain 3-oxo-AHLs and both SsbI and SscI specified 3-OH-AHLs. Addition of exogenous AHLs to KLH11 increased the expression of ssaI but not ssaR, ssbI or ssbR, and genetic analyses revealed a complex interconnected arrangement between SsaRI and SsbRI systems. Interestingly, flagellar motility was abolished in the ssaI and ssaR mutants, with the flagellar biosynthesis genes under strict SsaRI control, and active motility only at high culture density. Conversely, ssaI and ssaR mutants formed more robust biofilms than wild-type KLH11. AHLs and the ssaI transcript were detected in M. laxissima extracts, suggesting that AHL signalling contributes to the decision between motility and sessility and that it may also facilitate acclimation to different environments that include the sponge host.

Direct interaction of FliX and FlbD is required for their regulatory activity in Caulobacter crescentus

BACKGROUND

The temporal and spatial expression of late flagellar genes in Caulobacter crescentus is activated by the transcription factor FlbD and its partner trans-acting factor FliX. The physical interaction of these two proteins represents an alternative mechanism for regulating the activity of σ54 transcription factors. This study is to characterize the interaction of the two proteins and the consequences of the interaction on their regulatory activity.

RESULTS

FliX and FlbD form stable complexes, which can stand the interference of 2.65 M NaCl. The stability of FliX and FlbD was affected by the co-existence of each other. Five FliX mutants (R71A, L85K, Δ117-118, T130L, and L136K) were created by site-directed mutagenesis in conserved regions of the protein. All mutants were successfully expressed in both wild-type and ΔfliX Caulobacter strains. All but FliXL85K could rescue the motility and cell division defects of a ΔfliX mutant strain. The ability of FliX to regulate the transcription of class II and class III/IV flagellar promoters was fully diminished due to the L85K mutation. Co-immunoprecipitation experiment revealed that FliXL85K was unable to physically interact with FlbD.

CONCLUSIONS

FliX interacts with FlbD and thereby directly regulates the activity of FlbD in response to flagellar assembly. Mutations in highly conserved regions of FliX could severely affect the recognition between FliX and FlbD and hence interrupt the normal progression of flagellar synthesis and other developmental events in Caulobacter.

Probing flagellar promoter occupancy in wild-type and mutant Caulobacter crescentus by chromatin immunoprecipitation

In the asymmetric predivisional cell of Caulobacter crescentus, TipF and TipN mark the cellular pole for future flagellar development. TipF is essential for motility and contains a cyclic-di-GMP phosphodiesterase-like (EAL) domain that is necessary for proper function. TipN is localized to the flagellar pole before TipF and is essential for the proper placement of the flagellum in C. crescentus. Using β-galactosidase promoter-probe assays and quantitative chromatin immunoprecipitation, we investigated the influence of the C. crescentus flagellar assembly regulator TipF on flagellar gene transcription. We compared the transcriptional activity of class II-fliF-lacZ, class III-flgE-lacZ, and class IV-fljL-lacZ fusions in a ΔtipF mutant with that of other flagellar mutants and the wild-type strain. We subsequently verified the in vivo occupancy of the fliF, flgE, and fljL flagellar promoters by the flagellar regulators CtrA, FlbD, and FliX in addition to RNA polymerase. We deduce that TipF contributes to proper expression of flagellar genes in C. crescentus by acting both within and outside of the canonical flagellar gene expression hierarchy.

Getting in the loop: regulation of development in Caulobacter crescentus

Caulobacter crescentus is an aquatic Gram-negative alphaproteobacterium that undergoes multiple changes in cell shape, organelle production, subcellular distribution of proteins, and intracellular signaling throughout its life cycle. Over 40 years of research has been dedicated to this organism and its developmental life cycles. Here we review a portion of many developmental processes, with particular emphasis on how multiple processes are integrated and coordinated both spatially and temporally. While much has been discovered about Caulobacter crescentus development, areas of potential future research are also highlighted.

Sense and sensibility: flagellum-mediated gene regulation

The flagellum, a rotary engine required for motility in many bacteria, plays key roles in gene expression. It has been known for some time that flagellar substructures serve as checkpoints that coordinate flagellar gene expression with assembly. Less well understood, however, are other more global effects on gene expression. For instance, the flagellum acts as a 'wetness' sensor in Salmonella typhimurium, and as a mechanosensor in other bacteria. Additionally, it has been implicated in a variety of bacterial processes, including biofilm formation, pathogenesis and symbiosis. Although for many of these processes it might be simply that motility is required, in other cases it seems that the flagellum plays an underappreciated role in regulating gene expression.

Deciphering bacterial flagellar gene regulatory networks in the genomic era

Synthesis of the bacterial flagellum is a complex process involving dozens of structural and regulatory genes. Assembly of the flagellum is a highly-ordered process, and in most flagellated bacteria the structural genes are expressed in a transcriptional hierarchy that results in the products of these genes being made as they are needed for assembly. Temporal regulation of the flagellar genes is achieved through sophisticated regulatory networks that utilize checkpoints in the flagellar assembly pathway to coordinate expression of flagellar genes. Traditionally, flagellar transcriptional hierarchies are divided into various classes. Class I genes, which are the first genes expressed, encode a master regulator that initiates the transcriptional hierarchy. The master regulator activates transcription a set of structural and regulatory genes referred to as class II genes, which in turn affect expression of subsequent classes of flagellar genes. We review here the literature on the expression and activity of several known master regulators, including FlhDC, CtrA, VisNR, FleQ, FlrA, FlaK, LafK, SwrA, and MogR. We also examine the Department of Energy Joint Genomes Institute database to make predictions about the distribution of these regulators. Many bacteria employ the alternative sigma factors sigma(54) and/or sigma(28) to regulate transcription of later classes of flagellar genes. Transcription by sigma(54)-RNA polymerase holoenzyme requires an activator, and we review the literature on the sigma(54)-dependent activators that control flagellar gene expression in several bacterial systems, as well as make predictions about other systems that may utilize sigma(54) for flagellar gene regulation. Finally, we review the prominent systems that utilize sigma(28) and its antagonist, the anti-sigma(28) factor FlgM, along with some systems that utilize alternative mechanisms for regulating flagellar gene expression.

Flagellar biogenesis of Xanthomonas campestris requires the alternative sigma factors RpoN2 and FliA and is temporally regulated by FlhA, FlhB, and FlgM

In prokaryotes, flagellar biogenesis is a complicated process involving over 40 genes. The phytopathogen Xanthomonas campestris pv. campestris possesses a single polar flagellum, which is essential for the swimming motility. A sigma54 activator, FleQ, has been shown to be required for the transcriptional activation of the flagellar type III secretion system (F-T3SS), rod, and hook proteins. One of the two rpoN genes, rpoN2, encoding sigma54, is essential for flagellation. RpoN2 and FleQ direct the expression of a second alternative sigma FliA (sigma28) that is essential for the expression of the flagellin FliC. FlgM interacts with FliA and represses the FliA regulons. An flgM mutant overexpressing FliC generates a deformed flagellum and displays an abnormal motility. Mutation in the two structural genes of F-T3SS, flhA and flhB, suppresses the production of FliC. Furthermore, FliA protein levels are decreased in an flhB mutant. A mutant defective in flhA, but not flhB, exhibits a decreased infection rate. In conclusion, the flagellar biogenesis of Xanthomonas campestris requires alternative sigma factors RpoN2 and FliA and is temporally regulated by FlhA, FlhB, and FlgM.

Complex regulatory pathways coordinate cell-cycle progression and development in Caulobacter crescentus

Caulobacter crescentus has become the predominant bacterial model system to study the regulation of cell-cycle progression. Stage-specific processes such as chromosome replication and segregation, and cell division are coordinated with the development of four polar structures: the flagellum, pili, stalk, and holdfast. The production, activation, localization, and proteolysis of specific regulatory proteins at precise times during the cell cycle culminate in the ability of the cell to produce two physiologically distinct daughter cells. We examine the recent advances that have enhanced our understanding of the mechanisms of temporal and spatial regulation that occur during cell-cycle progression.

FtcR is a new master regulator of the flagellar system of Brucella melitensis 16M with homologs in Rhizobiaceae

The flagellar regulon of Brucella melitensis 16M contains 31 genes clustered in three loci on the small chromosome. These genes encode a polar sheathed flagellum that is transiently expressed during vegetative growth and required for persistent infection in a mouse model. By following the expression of three flagellar genes (fliF, flgE, and fliC, corresponding to the MS ring, hook, and filament monomer, respectively), we identified a new regulator gene, ftcR (flagellar two-component regulator). Inactivation of ftcR led to a decrease in flagellar gene expression and to impaired Brucella virulence. FtcR has a two-component response regulator domain as well a DNA binding domain and is encoded in the first flagellar locus of B. melitensis. Both the ftcR sequence and its genomic context are conserved in other related alpha-proteobacteria. During vegetative growth in rich medium, ftcR expression showed a peak during the early exponential phase that paralleled fliF gene expression. VjbR, a quorum-sensing regulator of the LuxR family, was previously found to control fliF and flgE gene expression. Here, we provide some new elements suggesting that the effect of VjbR on these flagellar genes is mediated by FtcR. We found that ftcR expression is partially under the control of VjbR and that the expression in trans of ftcR in a vjbR mutant restored the production of the hook protein (FlgE). Finally, FtcR binds directly to the upstream region of the fliF gene. Therefore, our data support the role of FtcR as a flagellar master regulator in B. melitensis and perhaps in other related alpha-proteobacteria.

Comparative genomic evidence for a close relationship between the dimorphic prosthecate bacteria Hyphomonas neptunium and Caulobacter crescentus

The dimorphic prosthecate bacteria (DPB) are alpha-proteobacteria that reproduce in an asymmetric manner rather than by binary fission and are of interest as simple models of development. Prior to this work, the only member of this group for which genome sequence was available was the model freshwater organism Caulobacter crescentus. Here we describe the genome sequence of Hyphomonas neptunium, a marine member of the DPB that differs from C. crescentus in that H. neptunium uses its stalk as a reproductive structure. Genome analysis indicates that this organism shares more genes with C. crescentus than it does with Silicibacter pomeroyi (a closer relative according to 16S rRNA phylogeny), that it relies upon a heterotrophic strategy utilizing a wide range of substrates, that its cell cycle is likely to be regulated in a similar manner to that of C. crescentus, and that the outer membrane complements of H. neptunium and C. crescentus are remarkably similar. H. neptunium swarmer cells are highly motile via a single polar flagellum. With the exception of cheY and cheR, genes required for chemotaxis were absent in the H. neptunium genome. Consistent with this observation, H. neptunium swarmer cells did not respond to any chemotactic stimuli that were tested, which suggests that H. neptunium motility is a random dispersal mechanism for swarmer cells rather than a stimulus-controlled navigation system for locating specific environments. In addition to providing insights into bacterial development, the H. neptunium genome will provide an important resource for the study of other interesting biological processes including chromosome segregation, polar growth, and cell aging.

The conserved flaF gene has a critical role in coupling flagellin translation and assembly in Caulobacter crescentus

The expression of the flagellin proteins in Caulobacter crescentus is regulated by the progression of flagellar assembly both at the transcriptional and post-transcriptional levels. An early basal body structure is required for the transcription of flagellin genes, whereas the ensuing assembly of a hook structure is required for flagellin protein synthesis. Previous experiments have shown that the negative regulatory protein, FlbT, operates this second post-transcriptional checkpoint by associating with the 5' untranslated region (UTR) of the fljK flagellin transcript, inhibiting translation and destabilizing the mRNA. In this paper we examine the role of flaF in flagellar biogenesis. The flaF gene, which is conserved in several speices of flagellated alpha-proteobacteria, is required for motility and flagellin protein synthesis. A deletion of flbT in a DeltaflaF strain restored flagellin protein expression, but not motility, indicating that FlaF functions in filament assembly. Mutant strains with a deletion in flaF had no detectable fljK mRNA, the levels of which were restored by an additional mutation in flbT. Assay of fljK gene expression using transcription and translation reporter fusions indicated that FlaF was essential for the translation of fljK mRNA. FlaF protein levels were under cell cycle control, peaking during the period of flagellin expression and filament assembly, whereas FlbT was present throughout the cell cycle. These results suggest that FlbT and FlaF activities oppose one another in the regulation of flagellin expression in response to both the progression of flagellar assembly and the cell cycle.

Roles of the regulatory proteins FlhF and FlhG in the Vibrio cholerae flagellar transcription hierarchy

Vibrio cholerae, the causative agent of the human diarrheal disease cholera, is a motile bacterium with a single polar flagellum, and motility has been inferred to be an important aspect of virulence. The V. cholerae flagellar hierarchy is organized into four classes of genes. The expression of each class of genes within a flagellar hierarchy is generally tightly regulated in other bacteria by both positive and negative regulatory elements. To further elucidate flagellar biogenesis in V. cholerae, we characterized the roles of the three putative regulatory genes, flhF, flhG, and VC2061. V. cholerae flhF and flhG mutants appeared nonmotile in a soft agar assay. Electron microscopy revealed that the flhF mutant lacked a polar flagellum, while interestingly, the flhG mutant possessed multiple (8 to 10) polar flagella per cell. The transcriptional activity of class III and class IV gene promoters in the flhF mutant was decreased, suggesting that FlhF acts as a positive regulator of class III gene transcription. The transcription of all four classes of flagellar promoters was increased in the flhG mutant, suggesting that FlhG acts as a negative regulator of class I gene transcription. Additionally, the ability to colonize the infant mouse intestine was reduced for the flhG mutant (approximately 10-fold), indicating that the negative regulation of class I flagellar genes enhances virulence. The V. cholerae VC2061 mutant was motile and produced a polar flagellum indistinguishable from that of the wild type, and the transcriptional activities of the four classes of flagellar promoters were similar to that of the wild type. Our results indicate that FlhG and FlhF regulate class I and class III flagellar transcription, respectively, while VC2061 plays no detectable role in V. cholerae flagellar biogenesis.

The trans-acting flagellar regulatory proteins, FliX and FlbD, play a central role in linking flagellar biogenesis and cytokinesis in Caulobacter crescentus

The FliX/FlbD-dependent temporal transcription of late flagellar genes inCaulobacter crescentusrequires the assembly of an early, class II-encoded flagellar structure. Class II flagellar-mutant strains exhibit a delay in the completion of cell division, with the accumulation of filamentous cells in culture. It is shown here that this cell-division defect is attributable to an arrest in the final stages of cell separation. Normal cell morphology could be restored in class II mutants by gain-of-function alleles of FliX or FlbD, suggesting that the timely completion of cell division requires thesetrans-acting factors. In synchronized cultures, inhibition of cell division by depleting FtsZ resulted in normal initial expression of the late, FlbD-dependentfliKgene; however, the cell cycle-regulated cessation of transcription was delayed, indicating that cell division may be required to negatively regulate FlbD activity. Interestingly, prolonged depletion of FtsZ resulted in an eventual loss of FlbD activity that could be bypassed by a constitutive mutant of FlbD, but not of FliX, suggesting the possible existence of a second cell cycle-dependent pathway for FlbD activation.

Linking structural assembly to gene expression: a novel mechanism for regulating the activity of a σ54transcription factor

In Caulobacter crescentus, the temporal and spatial expression of late flagellar genes is regulated by the sigma54 transcriptional activator, FlbD. Genetic experiments have indicated that the trans-acting factor FliX regulates FlbD in response to the progression of flagellar assembly, repressing FlbD activity until an early flagellar basal body structure is assembled. Following assembly of this structure, FliX is thought to function as an activator of FlbD. Here we have investigated the mechanism of FliX-mediated regulation of FlbD activity. In vitro transcription experiments showed that purified FliX could function as a repressor of FlbD-activated transcription. Transcription activated by a gain-of-function mutant of FlbD (FlbD-1204) that is active in vivo in the absence of an early flagellar structure, was resistant to the repressive effects of FliX. DNA binding studies showed that FliX inhibited the interaction of wild-type FlbD with enhancer DNA but did not effect FlbD-catalysed ATPase activity. DNA binding activity of FlbD-1204 was relatively unaffected by FliX indicating that this mutant protein bypasses the transcriptional requirement for early flagellar assembly by escaping FliX-mediated negative regulation. Gel filtration and co-immunoprecipitation experiments indicated that FliX formed a stable complex with FlbD. These experiments demonstrate that regulation of FlbD activity is unusual among the well-studied sigma54 transcriptional activators, apparently combining a two-component receiver domain with additional control imposed via interaction with a partner protein, FliX.

Role of integration host factor in the transcriptional activation of flagellar gene expression in Caulobacter crescentus

In the Caulobacter crescentus predivisional cell, class III and IV flagellar genes, encoding the extracytoplasmic components of the flagellum, are transcribed in the nascent swarmer compartment. This asymmetric expression pattern is attributable to the compartmentalized activity of the sigma54-dependent transcriptional activator FlbD. Additionally, these temporally transcribed flagellar promoters possess a consensus sequence for the DNA-binding protein integration host factor (IHF), located between the upstream FlbD binding site and the promoter sequences. Here, we deleted the C. crescentus gene encoding the beta-subunit of the IHF, ihfB (himD), and examined the effect on flagellar gene expression. The DeltaihfB strain exhibited a mild defect in cell morphology and impaired motility. Using flagellar promoter reporter fusions, we observed that expression levels of a subset of class III flagellar promoters were decreased by the loss of IHF. However, one of these promoters, fliK-lacZ, exhibited a wild-type cell cycle-regulated pattern of expression in the absence of IHF. Thus, IHF is required for maximal transcription of several late flagellar genes. The DeltaihfB strain was found to express significantly reduced amounts of the class IV flagellin, FljL, as a consequence of reduced transcriptional activity. Our results indicate that the motility defect exhibited by the DeltaihfB strain is most likely attributable to its failure to accumulate the class IV-encoded 27-kDa flagellin subunit, FljL.