Characterization of the Caulobacter crescentus flbF promoter and identification of the inferred FlbF product as a homolog of the LcrD protein from a Yersinia enterocolitica virulence plasmid

A rationale for repression and/or loss of motility by pathogenic Yersinia in the mammalian host

Pathogenic yersiniae either repress flagella expression under host conditions (Yersinia enterocolitica and Yersinia pseudotuberculosis) or have permanently lost this capability by mutation (Yersinia pestis). The block in flagella synthesis for the enteropathogenic Yersinia centers on fliA (sigmaF) repression. This repression ensures the downstream repression of flagellin structural genes which can be cross-recognized and secreted by virulence type III secretion systems. Y. pestis carries several flagellar mutations including a frame shift mutation in flhD, part of the flagellar master control operon. Repression of flagellins in the host environment may be critical because they are potent inducers of innate immunity. Artificial expression of flagellin in Y. enterocolitica completely attenuates virulence, supporting the hypothesis that motility is a liability in the mammalian host.

Mutational analysis of the promoter recognized by Chlamydia and Escherichia coli sigma(28) RNA polymerase

sigma(28) RNA polymerase is an alternative RNA polymerase that has been postulated to have a role in developmental gene regulation in Chlamydia. Although a consensus bacterial sigma(28) promoter sequence has been proposed, it is based on a relatively small number of defined promoters, and the promoter structure has not been systematically analyzed. To evaluate the sequence of the sigma(28)-dependent promoter, we performed a comprehensive mutational analysis of the Chlamydia trachomatis hctB promoter, testing the effect of point substitutions on promoter activity. We defined a -35 element recognized by chlamydial sigma(28) RNA polymerase that resembles the consensus -35 sequence. Within the -10 element, however, chlamydial sigma(28) RNA polymerase showed a striking preference for a CGA sequence at positions -12 to -10 rather than the longer consensus -10 sequence. We also observed a strong preference for this CGA sequence by Escherichia coli sigma(28) RNA polymerase, suggesting that this previously unrecognized motif is the critical component of the -10 promoter element recognized by sigma(28) RNA polymerase. Although the consensus spacer length is 11 nucleotides (nt), we found that sigma(28) RNA polymerase from both Chlamydia and E. coli transcribed a promoter with either an 11- or 12-nt spacer equally well. Altogether, we found very similar results for sigma(28) RNA polymerase from C. trachomatis and E. coli, suggesting that promoter recognition by this alternative RNA polymerase is well conserved among bacteria. The preferred sigma(28) promoter that we defined in the context of the hctB promoter is TAAAGwwy-n(11/12)-ryCGAwrn, where w is A or T, r is a purine, y is a pyrimidine, n is any nucleotide, and n(11/12) is a spacer of 11 or 12 nt.

New members of the ctrA regulon: the major chemotaxis operon in Caulobacter is CtrA dependent

The Caulobacter crescentus che promoter region consists of two divergent promoters, directing expression of the major chemotaxis operon and a novel gene cagA (chemotaxis associated gene A). Analyses of start sites by primer extension and alignment of the divergent promoters revealed significant similarities between them at the -35 promoter region. Both mcpA and cagA are differentially expressed in the cell cycle, with maximal activation of transcription in predivisional cells. The main difference between the mcpA and cagA promoters is that, in common with the fljK flagellin, cagA is expressed in swarmer cells. A cagA--lacZ promoter fusion that contains 36 bases of untranslated mRNA has sufficient information to segregate the lacZ transcript to swarmer cells. Expression of mcpA and cagA was dependent on DNA replication. Transcriptional epistasis experiments were performed to identify potential regulators in the flagellar hierarchy. The sigma factor RpoN, which is required for flagellar biogenesis, is not required for mcpA and cagA expression. Mutations in the genes for the MS-ring and the switch complex (flagellar class II mutants) do not affect expression of mcpA and cagA. However, CtrA, an essential response regulator of flagellar gene transcription, is required.

Temporal regulation of genes encoding the flagellar proximal rod in Caulobacter crescentus

The gram-negative bacterium Caulobacter crescentus has a life cycle that includes two distinct and separable developmental stages, a motile swarmer phase and a sessile stalked phase. The cell cycle-controlled biogenesis of the single polar flagellum of the swarmer cell is the best-studied aspect of this developmental program. The flagellar regulon is arranged into a rigid trans-acting hierarchy of gene expression in which successful expression of early genes is required for the expression of genes that are later in the hierarchy and in which the order of gene expression mirrors the order of assembly of gene products into the completed flagellum. The flgBC-fliE genes were identified as a result of the C. crescentus genome sequencing project and encode the homologues of two flagellar proximal rod proteins, FlgB and FlgC, and one conserved protein, FliE, that is of unknown function. Footprint assays on a DNA fragment containing the operon promoter as well as in vivo mutant suppressor analysis of promoter mutations indicate that this operon is controlled by the cell cycle response regulator CtrA, which with sigma(70) is responsible for regulating transcription of other early flagellar genes in C. crescentus. Promoter analysis, timing of expression, and epistasis experiments place these genes outside of the flagellar regulatory hierarchy; they are expressed in class II mutants, and flgB deletions do not prevent class III gene expression. This operon is also unusual in that it is expressed from a promoter that is divergent from the class II operon containing fliP, which encodes a member of the flagellum-specific protein export apparatus.

On the presence and organization of open reading frames of the nonmotile pathogen Brucella abortus similar to class II, III, and IV flagellar genes and to LcrD virulence superfamily

Brucellae are pathogenic, nonmotile bacteria that are facultative intracellular parasites. Little is known about the genetics of these bacteria. Open reading frames from Brucella abortus with similarity to the flagellin, M-ring, and hook of related bacteria were discovered. The open reading frames encode proteins of three of the four flagellum gene classes, namely II, III, and IV. A homolog of the LcrD virulence superfamily was also found. This superfamily is involved in type III protein secretion. B. abortus has the potential for motility and type III secretion.

FlbT couples flagellum assembly to gene expression in Caulobacter crescentus

The biogenesis of the polar flagellum of Caulobacter crescentus is regulated by the cell cycle as well as by a trans-acting regulatory hierarchy that functions to couple flagellum assembly to gene expression. The assembly of early flagellar structures (MS ring, switch, and flagellum-specific secretory system) is required for the transcription of class III genes, which encode the remainder of the basal body and the external hook structure. Similarly, the assembly of class III gene-encoded structures is required for the expression of the class IV flagellins, which are incorporated into the flagellar filament. Here, we demonstrate that mutations in flbT, a flagellar gene of unknown function, can restore flagellin protein synthesis and the expression of fljK::lacZ (25-kDa flagellin) protein fusions in class III flagellar mutants. These results suggest that FlbT functions to negatively regulate flagellin expression in the absence of flagellum assembly. Deletion analysis shows that sequences within the 5' untranslated region of the fljK transcript are sufficient for FlbT regulation. To determine the mechanism of FlbT-mediated regulation, we assayed the stability of fljK mRNA. The half-life (t(1/2)) of fljK mRNA in wild-type cells was approximately 11 min and was reduced to less than 1.5 min in a flgE (hook) mutant. A flgE flbT double mutant exhibited an mRNA t(1/2) of greater than 30 min. This suggests that the primary effect of FlbT regulation is an increased turnover of flagellin mRNA. The increased t(1/2) of fljK mRNA in a flbT mutant has consequences for the temporal expression of fljK. In contrast to the case for wild-type cells, fljK::lacZ protein fusions in the mutant are expressed almost continuously throughout the C. crescentus cell cycle, suggesting that coupling of flagellin gene expression to assembly has a critical influence on regulating cell cycle expression.

Isolation of Helicobacter pylori genes that modulate urease activity

Helicobacter pylori urease, a nickel-requiring metalloenzyme, hydrolyzes urea to NH3 and CO2. We sought to identify H. pylori genes that modulate urease activity by constructing pHP8080, a plasmid which encodes both H. pylori urease and the NixA nickel transporter. Escherichia coli SE5000 and DH5alpha transformed with pHP8080 resulted in a high-level urease producer and a low-level urease producer, respectively. An H. pylori DNA library was cotransformed into SE5000 (pHP8080) and DH5alpha (pHP8080) and was screened for cotransformants expressing either lowered or heightened urease activity, respectively. Among the clones carrying urease-enhancing factors, 21 of 23 contained hp0548, a gene that potentially encodes a DNA helicase found within the cag pathogenicity island, and hp0511, a gene that potentially encodes a lipoprotein. Each of these genes, when subcloned, conferred a urease-enhancing activity in E. coli (pHP8080) compared with the vector control. Among clones carrying urease-decreasing factors, 11 of 13 clones contained the flbA (also known as flhA) flagellar biosynthesis/regulatory gene (hp1041), an lcrD homolog. The LcrD protein family is involved in type III secretion and flagellar secretion in pathogenic bacteria. Almost no urease activity was detected in E. coli (pHP8080) containing the subcloned flbA gene. Furthermore, there was significantly reduced synthesis of the urease structural subunits in E. coli (pHP8080) containing the flbA gene, as determined by Western blot analysis with UreA and UreB antiserum. Thus, flagellar biosynthesis and urease activity may be linked in H. pylori. These results suggest that H. pylori genes may modulate urease activity.

The virulence plasmid of Yersinia, an antihost genome

The 70-kb virulence plasmid enables Yersinia spp. (Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It encodes the Yop virulon, an integrated system allowing extracellular bacteria to disarm the cells involved in the immune response, to disrupt their communications, or even to induce their apoptosis by the injection of bacterial effector proteins. This system consists of the Yop proteins and their dedicated type III secretion apparatus, called Ysc. The Ysc apparatus is composed of some 25 proteins including a secretin. Most of the Yops fall into two groups. Some of them are the intracellular effectors (YopE, YopH, YpkA/YopO, YopP/YopJ, YopM, and YopT), while the others (YopB, YopD, and LcrV) form the translocation apparatus that is deployed at the bacterial surface to deliver the effectors into the eukaryotic cells, across their plasma membrane. Yop secretion is triggered by contact with eukaryotic cells and controlled by proteins of the virulon including YopN, TyeA, and LcrG, which are thought to form a plug complex closing the bacterial secretion channel. The proper operation of the system also requires small individual chaperones, called the Syc proteins, in the bacterial cytosol. Transcription of the genes is controlled both by temperature and by the activity of the secretion apparatus. The virulence plasmid of Y. enterocolitica and Y. pseudotuberculosis also encodes the adhesin YadA. The virulence plasmid contains some evolutionary remnants including, in Y. enterocolitica, an operon encoding resistance to arsenic compounds.

Identification of the fliI and fliJ components of the Caulobacter flagellar type III protein secretion system

Caulobacter crescentus is motile by virtue of a polar flagellum assembled during the predivisional stage of the cell cycle. Three mutant strains in which flagellar assembly was blocked at an early stage were isolated. The mutations in these strains mapped to an operon of two genes, fliI and fliJ, both of which are necessary for motility. fliI encodes a 50-kDa polypeptide whose sequence is closely related to that of the Salmonella typhimurium FliI protein, an ATPase thought to energize the export of flagellar subunits across the cytoplasmic membrane through a type III protein secretion system. fliJ encodes a 16-kDa hydrophilic protein of unknown function. Epistasis experiments demonstrated that the fliIJ operon is located in class II of the C. crescentus flagellar regulatory hierarchy, suggesting that the gene products act at an early stage in flagellar assembly. The expression of fliIJ is induced midway through the cell cycle, coincident with other class II operons, but the FliI protein remains present throughout the cell cycle. Subcellular fractionation showed that FliI is present both in the cytoplasm and in association with the membrane. Mutational analysis of FliI showed that two highly conserved amino acid residues in a bipartite ATP binding motif are necessary for flagellar assembly.

Cloning and characterization of the Helicobacter pylori flbA gene, which codes for a membrane protein involved in coordinated expression of flagellar genes

Flagellar motility has been shown to be an essential requirement for the ability of Helicobacter pylori to colonize the gastric mucosa. While some flagellar structural components have been studied in molecular detail, nothing was known about factors that play a role in the regulation of flagellar biogenesis. We have cloned and characterized an H. pylori homolog (named flbA) of the lcrD/flbF family of genes. Many proteins encoded by these genes are known to be involved in flagellar biogenesis or secretion of virulence-associated proteins via type III secretion systems. The H. pylori flbA gene (2,196 bp) is capable of coding for a predicted 732-amino-acid, 80.9-kDa protein that has marked sequence similarity with other known members of the LcrD/FlbF protein family. An isogenic strain with a mutation in the flbA gene was constructed by disruption of the gene with a kanamycin resistance cassette and electroporation-mediated allelic exchange mutagenesis. The mutant strain expressed neither the FlaA nor the FlaB flagellin protein. The expression of the FlgE hook protein was reduced in comparison with the wild-type strain, and the extent of this reduction was growth phase dependent. The flbA gene disruption was shown to downregulate the expression of these flagellar genes on the transcriptional level. The flbA mutants were aflagellate and completely nonmotile. Occasionally, assembled hook structures could be observed, indicating that export of axial flagellar filament components was still possible in the absence of the flbA gene product. The hydrophilic part of the FlbA protein was expressed in Escherichia coli, purified, and used to raise a polyclonal rabbit antiserum against the FlbA protein. Western blot experiments with this antiserum indicated that the FlbA protein is predominantly associated with the cytoplasmic membrane in H. pylori. The antiserum cross-reacted with two other proteins (97 and 43 kDa) whose expression was not affected by the flbA gene disruption and which might represent further H. pylori homologs of the LcrD/FlbF protein family.

Identification and sequences of the Treponema pallidum flhA, flhF, and orf304 genes

The recently identified fla operon of Treponema pallidum contains several genes that encode motility-related proteins. We have determined the nucleotide sequences of three genes, designated flhA, flhF, and orf304, that are located immediately downstream of the flhB gene in the fla operon. The flhA gene encodes a 707-amino acid protein that contains five putative membrane spanning domains. FlhA has strong homology with members of a family of proteins that are involved in flagellar biogenesis and regulation/secretion of virulence-related proteins. The flhF gene encodes a 437-amino acid protein that contains three consensus elements that are characteristic of a GTP-binding domain. The orf304 gene encodes a 304-amino acid protein that contains a consensus ATP-binding motif. The order of the flhA, flhF, and orf304 genes is identical to that of corresponding genes in the Bacillus subtilis che/fla operon. Due to the location of the flhA, flhF and orf304 genes in the T. pallidum fla operon, we hypothesize that the FlhA, FlhF, and Orf304 proteins are involved in the biogenesis/assembly of treponemal periplasmic flagella.

Functions of bacterial flagella

Many bacterial species are motile by means of flagella. The structure and implantation of flagella seems related to the specific environments the cells live in. In some cases, the bacteria even adapt their flagellation pattern in response to the environmental conditions they encounter. Swarming cell differentiation is a remarkable example of this phenomenon. Flagella seem to have more functions than providing motility alone. For many pathogenic species, studies have been performed on the contribution of flagella to the virulence, but the result is not clear in all cases. Flagella are generally accepted as being important virulence factors, and expression and repression of flagellation and virulence have in several cases been shown to be linked. Providing motility is always an important feature of flagella of pathogenic bacteria, but adhesive and other properties also have been attributed to these flagella. In nonpathogenic bacterial colonization, flagella are important locomotive and adhesive organelles as well. In several cases where competition between several bacterial species exists, motility by means of flagella is shown to provide a specific advantage for a bacterium. This review gives an overview of studies that have been performed on the significance of flagellation in a wide variety of processes where flagellated bacteria are involved.

Temporal and spatial regulation of fliP, an early flagellar gene of Caulobacter crescentus that is required for motility and normal cell division

In Caulobacter crescentus, the genes encoding a single polar flagellum are expressed under cell cycle control. In this report, we describe the characterization of two early class II flagellar genes contained in the orfX-fliP locus. Strains containing mutations in this locus exhibit a filamentous growth phenotype and fail to express class III and IV flagellar genes. A complementing DNA fragment was sequenced and found to contain two potential open reading frames. The first, orfX, is predicted to encode a 105-amino-acid polypeptide that is similar to MopB, a protein which is required for both motility and virulence in Erwinia carotovora. The deduced amino acid sequence of the second open reading frame, fliP, is 264 amino acids in length and shows significant sequence identity with the FliP protein of Escherichia coli as well as virulence proteins of several plant and mammalian pathogens. The FliP homolog in pathogenic organisms has been implicated in the secretion of virulence factors, suggesting that the export of virulence proteins and some flagellar proteins share a common mechanism. The 5' end of orfX-fliP mRNA was determined and revealed an upstream promoter sequence that shares few conserved features with that of other early Caulobacter flagellar genes, suggesting that transcription of orfX-fliP may require a different complement of trans-acting factors. In C. crescentus, orfX-fliP is transcribed under cell cycle control, with a peak of transcriptional activity in the middle portion of the cell cycle. Later in the cell cycle, orfX-fliP expression occurs in both poles of the predivisional cell. Protein fusions to a lacZ reporter gene indicate that FliP is specifically targeted to the swarmer compartment of the predivisional cell.

Mutations in yscC, yscD, and yscG prevent high-level expression and secretion of V antigen and Yops in Yersinia pestis

The Yersinia pestis low-Ca2+ response stimulon is responsible for the temperature- and Ca(2+)-regulated expression and secretion of plasmid pCD1-encoded antihost proteins (V antigen and Yops). We have previously shown that lcrD and yscR encode proteins that are essential for high-level expression and secretion of V antigen and Yops at 37 degrees C in the absence of Ca2+. In this study, we constructed and characterized mutants with in-frame deletions in yscC, yscD, and yscG of the ysc operon that contains yscA through yscM. All three mutants lost the Ca2+ requirement for growth at 37 degrees c, expressed only basal levels of V antigen and YopM in the presence or absence of Ca2+, and failed to secrete these proteins to the culture supernatant. Overproduction of YopM in these mutants failed to restore YopM export, showing that the mutations had a direct effect on secretion. The protein products of yscC, yscD, and yscG were identified and localized by immunoblot analysis. YscC was localized to the outer membrane of Y. pestis, while YscD was found in the inner membrane. YscG was distributed equally between the soluble and total membrane fractions. Double mutants were characterized to assess where YscC and YscD act in low-Ca2+ response (LCR) regulation. lcrH::cat-yscC and lcrH::cat-yscD double mutants were constitutively induced for expression of V antigen and YopM; however, these proteins were not exported. This finding showed that the ysc mutations did not directly decrease induction of LCR stimulon genes. In contrast, lcrE-yscC, lcrG-yscC, lcrE-yscD, and lcrG-yscD double mutants as well as an lcrE-lcrD double mutant expressed only basal levels of V antigen and YopM and also failed to secrete these proteins to the culture supernatant. These results indicated that a functional LCR secretion system was necessary for high-level expression of LCR stimulon proteins in the lcrE and lcrG mutants but not in an lcrH::cat mutant. Possible models of regulation which incorporate these results are discussed.

Global regulation of a sigma 54-dependent flagellar gene family in Caulobacter crescentus by the transcriptional activator FlbD

Biosynthesis of the Caulobacter crescentus polar flagellum requires the expression of a large number of flagellar (fla) genes that are organized in a regulatory hierarchy of four classes (I to IV). The timing of fla gene expression in the cell cycle is determined by specialized forms of RNA polymerase and the appearance and/or activation of regulatory proteins. Here we report an investigation of the role of the C. crescentus transcriptional regulatory protein FlbD in the activation of sigma 54-dependent class III and class IV fla genes of the hierarchy by reconstituting transcription from these promoters in vitro. Our results demonstrate that transcription from promoters of the class III genes flbG, flgF, and flgI and the class IV gene fliK by Escherichia coli E sigma 54 is activated by FlbD or the mutant protein FlbDS140F (where S140F denotes an S-to-F mutation at position 140), which we show here has a higher potential for transcriptional activation. In vitro studies of the flbG promoter have shown previously that transcriptional activation by the FlbD protein requires ftr (ftr for flagellar transcription regulation) sequence elements. We have now identified multiple ftr sequences that are conserved in both sequence and spatial architecture in all known class III and class IV promoters. These newly identified ftr elements are positioned ca. 100 bp from the transcription start sites of each sigma 54-dependent fla gene promoter, and our studies indicate that they play an important role in controlling the levels of transcription from different class III and class IV promoters. We have also used mutational analysis to show that the ftr sequences are required for full activation by the FlbD protein both in vitro and in vivo. Thus, our results suggest that FlbD, which is encoded by the class II flbD gene, is a global regulator that activates the cell cycle-regulated transcription from all identified sigma 54-dependent promoters in the C. crescentus fla gene hierarchy.

A mutation that uncouples flagellum assembly from transcription alters the temporal pattern of flagellar gene expression in Caulobacter crescentus

The transcription of flagellar genes in Caulobacter crescentus is regulated by cell cycle events that culminate in the synthesis of a new flagellum once every cell division. Early flagellar gene products regulate the expression of late flagellar genes at two distinct stages of the flagellar trans-acting hierarchy. Here we investigate the coupling of early flagellar biogenesis with middle and late flagellar gene expression. We have isolated mutants (bfa) that do not require early class II flagellar gene products for the transcription of middle or late flagellar genes. bfa mutant strains are apparently defective in a negative regulatory pathway that couples early flagellar biogenesis to late flagellar gene expression. The bfa regulatory pathway functions solely at the level of transcription. Although flagellin promoters are transcribed in class II/bfa double mutants, there is no detectable flagellin protein on immunoblots prepared from mutant cell extracts. This finding suggests that early flagellar biogenesis is coupled to gene expression by two distinct mechanisms: one that negatively regulates transcription, mediated by bfa, and another that functions posttranscriptionally. To determine whether bfa affects the temporal pattern of late flagellar gene expression, cell cycle experiments were performed in bfa mutant strains. In a bfa mutant strain, flagellin expression fails to shut off at its normal time in the cell division cycle. This experimental result indicates that bfa may function as a regulator of flagellar gene transcription late in the cell cycle, after early flagellar structures have been assembled.

The fliA gene encoding sigma 28 in Yersinia enterocolitica

Yersinia enterocolitica is an enterobacterium responsible for gastrointestinal syndromes. Its pathogenicity depends on the presence of the 70-kb pYV plasmid, which directs Yop secretion. The Yop secretion machinery, consisting of the YscA-U and LcrD proteins, presents some structural similarity with the flagellum assembly machinery characterized in other bacteria. Flagellum assembly requires sigma 28, an alternative sigma factor. The region upstream of the lcrD gene resembles promoters recognized by sigma 28, suggesting that the similarity between Yop secretion and flagellum assembly could extend to their regulation. The chromosome of Y. enterocolitica also contains pathogenicity determinants such as myfA, which encodes the Myf antigen subunit. The promoter region of myfA also resembles promoters recognized by sigma 28. In an attempt to clarify the role of sigma 28 in the expression of lcrD, myfA, and flagellar genes, we cloned, sequenced, and mutagenized the fliA gene encoding the sigma 28 homolog in Y. enterocolitica. As is the case in other bacteria, fliA was required for motility. However, it was involved neither in fibrilla synthesis nor in Yop secretion. The fliA mutant allowed us to monitor the role of motility in pathogenesis. At least in the mouse model, motility seemed not to be required for Y. enterocolitica pathogenesis.

Regulation of the Caulobacter crescentus rpoN gene and function of the purified sigma 54 in flagellar gene transcription

The sequential transcription of flagellar (fla) genes in the Caulobacter crescentus cell cycle is controlled by the organization of these genes in a regulatory hierarchy of four levels (I-IV). Level III and level IV genes at the bottom of the hierarchy are dependent on level II genes and are transcribed late in the cell cycle from sigma 54-dependent promoters. To study the regulation of genes at levels III and IV, we have isolated and sequenced the rpoN gene in order to analyze its expression, purified the rpoN gene product, and examined the role of the RpoN protein in initiation of transcription from sigma 54-dependent promoters. We report here epistasis experiments that show rpoN is required for transcription of level III genes, but that the expression of the rpoN gene itself is not dependent on any of the fla genes examined; these results place rpoN at level II near the top of the hierarchy. Consistent with this conclusion were nuclease S1 assays that mapped the rpoN transcription start site and identified a sequence centered at -24, GTTA/TACCA/TT, which is similar to the core consensus sequence of the level IIB fliF, fliL, and fliQ promoters. We purified the full-length rpoN gene product to near homogeneity and demonstrated that the RpoN protein is required for transcription from the well-characterized sigma 54-dependent glnAp2 promoter of Escherichia coli and specifically recognizes the level III flbG gene promoter of C. crescentus. These last results confirm that rpoN encodes the C. crescentus sigma 54 factor and opens the way for the biochemical analysis of transcriptional regulation of level III and IV fla genes.

Regulation of cellular differentiation in Caulobacter crescentus

In Caulobacter crescentus, asymmetry is generated in the predivisional cell, resulting in the formation of two distinct cell types upon cell division: a motile swarmer cell and a sessile stalked cell. These progeny cell types differ in their relative programs of gene expression and DNA replication. In progeny swarmer cells, DNA replication is silenced for a defined period, but stalked cells reinitiate chromosomal DNA replication immediately following cell division. The establishment of these differential programs of DNA replication may be due to the polar localization of DNA replication proteins, differences in chromosome higher-order structure, or pole-specific transcription. The best-understood aspect of Caulobacter development is biogenesis of the polar flagellum. The genes encoding the flagellum are expressed under cell cycle control predominantly in the predivisional cell type. Transcription of flagellar genes is regulated by a trans-acting hierarchy that responds to both flagellar assembly and cell cycle cues. As the flagellar genes are expressed, their products are targeted to the swarmer pole of the predivisional cell, where assembly occurs. Specific protein targeting and compartmentalized transcription are two mechanisms that contribute to the positioning of flagellar gene products at the swarmer pole of the predivisional cell.

Requirement for FlhA in flagella assembly and swarm-cell differentiation by Proteus mirabilis

Swarming by Proteus mirabilis is characterized by cycles of rapid population migration across surfaces, following differentiation of typical rods into long, aseptate swarm cells that overexpress flagella and virulence factors, particularly haemolysin. A non-swarming Tn5phoA mutant was unable to synthesize flagella, to fully elongate or to induce high levels of the toxin. The mutation lay within a 2091 bp gene encoding a homologue of the Escherichia coli FlhA belonging to a family of proteins that are required for assembly of flagella or virulence proteins and that are suggested to act either directly in membrane translocation and/or in regulating synthesis of the export apparatus. In trans expression of multicopy flhA restored cell elongation and migration and generated differentiation-specific hyperexpression of flagellin and toxin genes to levels above those seen in the wild-type strain. Transcription of flhA was strongly induced during differentiation, from its own putative sigma 28 promoter. The results suggest a mechanistic coupling of flagella assembly and swarm-cell differentiation.

Multiple structural proteins are required for both transcriptional activation and negative autoregulation of Caulobacter crescentus flagellar genes

The periodic and sequential expression of flagellar (fla) genes in the Caulobacter crescentus cell cycle depends on their organization into levels I to IV of a regulatory hierarchy in which genes at the top of the hierarchy are expressed early in the cell cycle and are required for the later expression of genes below them. In these studies, we have examined the regulatory role of level II fliF operon, which is located near the top of the hierarchy. The last gene in the fliF operon, flbD, encodes a transcriptional factor required for activation of sigma 54-dependent promoters at levels III and IV and negative autoregulation of the level II fliF promoter. We have physically mapped the fliF operon, identified four new genes in the transcription unit, and determined that the organization of these genes is 5'-fliF-fliG-flbE-fliN-flbD-3'. Three of the genes encode homologs of the MS ring protein (FliF) and two switch proteins (FliG and FliN) of enteric bacteria, and the fourth encodes a predicted protein (FlbE) without obvious similarities to known bacterial proteins. We have introduced nonpolar mutations in each of the open reading frames and shown that all of the newly identified genes (fliF, fliG, flbE, and fliN) are required in addition to flbD for activation of the sigma 54-dependent flgK and flbG promoters at level III. In contrast, fliF, fliG, and flbE, but not fliN, are required in addition to flbD for negative autoregulation of the level II fliF promoter. The simplest interpretation of these results is that the requirements of FlbD in transcriptional activation and repression are not identical, and we speculate that FlbD function is subject to dual or overlapping controls. We also discuss the requirement of multiple structural genes for regulation of levels II and III genes and suggest that fla gene expression in C. crescentus may be coupled to two checkpoints in flagellum assembly.

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.

FlbD has a DNA-binding activity near its carboxy terminus that recognizes ftr sequences involved in positive and negative regulation of flagellar gene transcription in Caulobacter crescentus

FlbD is a transcriptional regulatory protein that negatively autoregulates fliF, and it is required for expression of other Caulobacter crescentus flagellar genes, including flaN and flbG. In this report we have investigated the interaction between carboxy-terminal fragments of FlbD protein and enhancer-like ftr sequences in the promoter regions of fliF, flaN, and flbG. FlbDc87 is a glutathione S-transferase (GST)-FlbD fusion protein that carries the carboxy-terminal 87 amino acids of FlbD, and FlbDc87 binds to restriction fragments containing the promoter regions of fliF, flaN, and flbG, whereas a GST-FlbD fusion protein carrying the last 48 amino acids of FlbD failed to bind to these promoter regions. DNA footprint analysis demonstrated that FlbDc87 is a sequence-specific DNA-binding protein that makes close contact with 11 nucleotides in ftr4, and 6 of these nucleotides were shown previously to function in negative regulation of fliF transcription in vivo (S. M. Van Way, A. Newton, A. H. Mullin, and D. A. Mullin, J. Bacteriol. 175:367-376, 1993). Three DNA fragments, each carrying an ftr4 mutation that resulted in elevated fliF transcript levels in vivo, were defective in binding to FlbDc87 in vitro. We also found that a missense mutation in the recognition helix of the putative helix-turn-helix DNA-binding motif of FlbDc87 resulted in defective binding to ftr4 in vitro. These data suggest that the binding of FlbDc87 to ftr4 is relevant to negative transcriptional regulation of fliF and that FlbD functions directly as a repressor. Footprint analysis showed that FlbDc87 also makes close contacts with specific nucleotides in ftr1, ftr2, and ftr3 in the flaN-flbG promoter region, and some of these nucleotides were shown previously to be required for regulated transcription of flaN and flbG (D. A. Mullin and A. Newton, J. Bacteriol. 175:2067-2076, 1993). Footprint analysis also revealed a new ftr-like sequence, ftr5, at -136 from the transcription start site of flbG. Our results demonstrate that FlbD contains a sequence-specific DNA-binding activity within the 87 amino acids at its carboxy terminus, and the results suggest that FlbD exerts its effect as a positive and negative regulator of C. crescentus flagellar genes by binding to ftr sequences.

Information essential for cell-cycle-dependent secretion of the 591-residue Caulobacter hook protein is confined to a 21-amino-acid sequence near the N-terminus

Recent findings suggest that axial flagellar proteins and virulence proteins of Gram-negative bacteria are exported from the cytoplasm via conserved translocation systems. To identify residues essential for secretion of flagellar axial proteins we examined the 591-residue Caulobacter crescentus flagellar hook protein. Western blot assays of the culture media of strains producing mutant hook proteins show that only residues 38-58 are essential for its secretion to the cell surface. We discuss the observation that this unprocessed 21-residue sequence is not conserved in other axial proteins and does not correspond to the SGL-, ANNLAN- and heptad repeat motifs that are located just upstream of the essential secretion information in the hook protein and are conserved near the N-termini of other axial proteins. These motifs, for which an essential role in export or assembly has been proposed, are required for motility. However, we also demonstrate that hook protein can only be secreted when the flagellar basal body is present in the cell envelope. The cell-cycle regulation of hook protein secretion confirms the specificity of the assay used in these studies and suggests that the basal body itself may serve as a secretion channel for the hook protein.

Genetic organization of the region upstream from the Campylobacter jejuni flagellar gene flhA

Campylobacter jejuni (Cj) is a Gram-bacterium that causes a diarrheal disease in humans. A Cj homolog of the LcrD/FlhA family of proteins was recently described [Miller et al., Infect. Immun. 61 (1993) 2930-2936]. This family includes proteins that are involved in flagellar biogenesis, such as the Cj FlhA protein, but also includes proteins found in invasive pathogens, such as the Yersinia pestis LcrD protein, that play a role in the regulation and/or secretion of virulence-related proteins. Hybridization studies indicated that both the flhA gene and upstream DNA are present in several bacterial species closely related to Cj, including C. fetus, C. lari, C. upsaliensis and C. hyointestinalis. The presence of a second flhA/lcrD homolog was not detected in Cj, indicating that a a separate homolog involved in secretion of virulence proteins may not be present. The 4-kb region immediately upstream from Cj flhA was analyzed. Three open reading frames (ORFs) were found: a 408-nucleotide (nt) gene encoding a homolog of proteins present in Escherichia coli and Desulfovibrio vulgaris, but of unknown function, a 266-nt rpsO gene and a 2823-nt gene encoding a homolog of the Bacillus subtilis SpoIIIE protein. The Cj SpoIIIE homolog had 53% similar or identical amino acids when compared to the B. subtilis protein, and like the B. subtilis protein contained a nt-binding domain and potential transmembrane (TM) regions. All three ORFs were expressed in E. coli minicells, apparently from their own promoters.(ABSTRACT TRUNCATED AT 250 WORDS)

The lcrB (yscN/U) gene cluster of Yersinia pseudotuberculosis is involved in Yop secretion and shows high homology to the spa gene clusters of Shigella flexneri and Salmonella typhimurium

Virulent bacteria of the genus Yersinia secrete a number of virulence determinants called Yops. These proteins lack typical signal sequences and are not posttranslationally processed. Two gene loci have been identified as being involved in the specific Yop secretion system (G. Cornelis, p. 231-265, In C. E. Hormache, C. W. Penn, and C. J. Smythe, ed., Molecular Biology of Bacterial Infection, 1992; S. C. Straley, G. V. Plano, E. Skrzypek, P. L. Haddix, and K. A. Fields, Mol. Microbiol. 8:1005-1010, 1993). Here, we have shown that the lcrB/virB locus (yscN to yscU) encodes gene products essential for Yop secretion. As in previously described secretion apparatus mutants, expression of the Yop proteins was decreased in the yscN/U mutants. An lcrH yscR double mutant expressed the Yops at an increased level but did not secrete Yops into the culture supernatant. The block in Yop expression of the ysc mutants was also circumvented by overexpression of the activator LcrF in trans. Although the Yops were expressed in elevated amounts, the Yops were still not exported. This analysis showed that the ysc mutants were unable to secrete Yops and that they were also affected in the negative Ca(2+)-regulated loop. The yscN/U genes showed remarkably high homology to the spa genes of Shigella flexneri and Salmonella typhimurium with respect to both individual genes and gene organization. These findings indicate that the genes originated from a common ancestor.

A low-Ca2+ response (LCR) secretion (ysc) locus lies within the lcrB region of the LCR plasmid in Yersinia pestis

The causative agent of plague, Yersinia pestis, contains a 75-kb plasmid, pCD1, which carries a virulence-related stimulon called the low-Ca2+ response stimulon (LCRS). LCRS operons are regulated by the environmental signals of temperature and Ca2+. This study characterized a portion of the lcrB region of pCD1, known to contain at least one gene necessary for the regulation of LCRS operons by Ca2+. The sequence of a 2-kb region revealed three open reading frames, designated yscQ, yscR, and yscS, predicted to encode acidic proteins of 34.4, 24.4, and 8.5 kDa. All three proteins were homologous to proteins involved in flagellar function or virulence. An antipeptide antibody specific for YscR was used to localize YscR to the inner membrane of Y. pestis. Analysis of yscR-phoA fusions supported a model for yscR which predicts four transmembrane regions and a large, central hydrophilic domain. In-frame deletion mutations of yscQ and yscR were constructed and moved into Y. pestis. Both mutants failed to show the restriction of growth that normally accompanies maximal LCRS induction. Unlike the parent Y. pestis, the yscR mutant did not respond to the absence of Ca2+ by increasing the net transcription or translation of the LCRS-encoded V antigen, YopM, or LcrG. The yscR mutant also was defective for secretion of V antigen, YopM, and LcrG. These findings implicate a dual role for YscR in regulation of LCRS operons and secretion of LCRS proteins and add to the developing picture of how secretion of virulence proteins may be coupled to transcriptional regulation in yersiniae.

Mutation of flgM attenuates virulence of Salmonella typhimurium, and mutation of fliA represses the attenuated phenotype

Salmonella typhimurium ST39 exhibits reduced virulence in mice and decreased survival in mouse macrophages compared with the parent strain SL3201. Strain ST39 is nonmotile, carries an indeterminate deletion in and near the flgB operon, and is defective in the mviS (mouse virulence Salmonella) locus. In flagellum-defective strains, the flgM gene product of S. typhimurium negatively regulates flagellar genes by inhibiting the activity of FliA, the flagellin-specific sigma factor. In this study, flgM of wild-type S. typhimurium LT2 was found to complement the mviS defect in ST39 for virulence in mice and for enhanced survival in macrophages. Transduction of flgM::Tn10dCm into the parent strain SL3201 resulted in attenuation of mouse virulence and decreased survival in macrophages. However, a flgM-fliA double mutant was fully virulent in mice and survived in macrophages at wild-type levels. Thus, the absolute level of FliA activity appears to affect the virulence of S. typhimurium SL3201 in mice. DNA hybridization studies showed that flgM-related sequences were present in species other than Salmonella typhimurium and that sequences related to that of fliA were common among members of the family Enterobacteriaceae. Our results demonstrate that flgM and fliA, two genes previously shown to regulate flagellar operons, are also involved in the regulation of expression of virulence of S. typhimurium and that this system may not be unique to the genus Salmonella.

An unusual promoter controls cell-cycle regulation and dependence on DNA replication of the Caulobacter fliLM early flagellar operon

Transcription of flagellar genes in Caulobacter crecentus is programmed to occur during the predivisional stage of the cell cycle. The mechanism of activation of Class II flagellar genes, the highest identified genes in the Caulobacter flagellar hierarchy, is unknown. As a step toward understanding this process, we have defined cis-acting sequences necessary for expression of a Class II flagellar operon, fliLM. Deletion analysis indicated that a 55 bp DNA fragment was sufficient for normal, temporally regulated promoter activity. Transcription from this promoter-containing fragment was severely reduced when chromosomal DNA replication was inhibited. Extensive mutational analysis of the promoter region from -42 to -5 identified functionally important nucleotides at -36 and -35, between -29 and -22, and at -12, which correlates well with sequences conserved between fliLM and the analogous regions of two other Class II flagellar operons. The promoter sequence does not resemble that recognized by any known bacterial sigma factor. Models for regulation of Caulobacter early flagellar promoters are discussed in which RNA polymerase containing a novel sigma subunit interacts with an activation factor bound to the central region of the promoter.

A Campylobacter jejuni homolog of the LcrD/FlbF family of proteins is necessary for flagellar biogenesis

A Campylobacter jejuni homolog of the lcrD/flbF family of genes was cloned and sequenced. The nucleotide sequence of the gene, called flbA, predicted a protein of 78,864 Da, with significant homology to a group of related proteins including the Yersinia pestis LcrD, Salmonella typhimurium InvA, and Caulobacter crescentus FlbF proteins. The greatest homology was seen with the C. crescentus FlbF protein, with an overall amino acid sequence homology of 57%. An insertion mutation in the C. jejuni 81-176 flbA gene was constructed. The resultant strain did not synthesize flagellin and was nonmotile.

Multiple effects of lcrD mutations in Yersinia pestis

Plasmid pCD1 of Yersinia pestis contains a low-calcium response stimulon responsible for the temperature- and calcium-regulated expression and secretion of proteins involved in virulence, which include the V antigen and Yops. We have previously shown that insertional inactivation of the bicistronic lcrDR operon abolished the calcium requirement for growth at 37 degrees C and reduced expression of the V antigen and Yops. In this study, we constructed and characterized three mutants having nonpolar lcrD deletions. All three mutants lost the two main low-calcium response properties: a calcium requirement for growth at 37 degrees C and strong expression of the V antigen and Yops. The effects on virulence gene expression occurred at both the levels of transcription and secretion. The growth, transcription, and secretion defects could be at least partially complemented for two of the lcrD mutants by providing lcrD in trans. A third mutant could not be complemented, and a plasmid carrying this mutation had a dominant negative effect over normal LcrD function. In the three mutants, the amount of mutant LcrD protein detectable in immunoblots was inversely related to the amount of complementation. Taken together, these data indicate that LcrD function involves the interaction of LcrD with another molecule.

Homology between the HrpO protein of Pseudomonas solanacearum and bacterial proteins implicated in a signal peptide-independent secretion mechanism

A region of approximately 22 kb of DNA defines the large hrp gene cluster of strain GMI1000 of Pseudomonas solanacearum. The majority of mutants that map to this region have lost the ability to induce disease symptoms on tomato plants and are no longer able to elicit a hypersensitive reaction (HR) on tobacco, a non-host plant. In this study we present the complementation analysis and nucleotide sequence of a 4772 bp region of this hrp gene cluster. Three complete open reading frames (ORFs) are predicted within this region. The corresponding putative proteins, HrpN, HrpO and HpaP, have predicted sizes of 357, 690 and 197 amino acids, respectively, and predicted molecular weights of 38,607, 73,990 and 21,959 dalton, respectively. HrpN and HrpO are both predicted to be hydrophobic proteins with potential membrane-spanning domains and HpaP is rich in proline residues. A mutation in hpaP (for hrp associated) does not affect the HR on tobacco or the disease on tomato plants. None of the proteins is predicted to have an N-terminal signal sequence, which would have indicated that the proteins are exported. Considerable sequence similarities were found between HrpO and eight known or predicted prokaryotic proteins: LcrD of Yersinia pestis and Y. enterocolitica, FlbF of Caulobacter crescentus, FlhA of Bacillus subtilis, MxiA and VirH of Shigella flexneri, InvA of Salmonella typhimurium and HrpC2 of Xanthomonas campestris pv. vesicatoria. These homologies suggest that certain hrp genes of phytopathogenic bacteria code for components of a secretory system, which is related to the systems for secretion of flagellar proteins, Ipa proteins of Shigella flexneri and the Yersinia Yop proteins. Furthermore, these homologous proteins have the common feature of being implicated in a distinct secretory mechanism, which does not require the cleavage of a signal peptide. The sequence similarity between HrpO and HrpC2 is particularly high (66% identity and 81% similarity) and the amino acid sequence comparison between these two proteins presented here reveals the first such sequence similarity to be shown between Hrp proteins of P. solanacearum and X. campestris. An efflux of plant electrolytes was found to be associated with the interactions between P. solanacearum and both tomato and tobacco leaves. This phenomenon may be part of the mechanism by which hrp gene products control and determine plant-bacterial interactions, since hrpO mutants induced levels of leakage which were significantly lower than those induced by the wild type on each plant.

Bacillus subtilis FlhA: a flagellar protein related to a new family of signal-transducing receptors

The Bacillus subtilis flhA gene lies in the major che/fla operon, a transcription unit that spans 26 kilobases (kb) of DNA. flhA encodes a 677-amino-acid polypeptide that is a strong candidate for an integral membrane protein. The sequence of FlhA displays substantial homology to a newly identified family of putative signal-transducing receptors that have been implicated in diverse cellular processes. FlhA is the first member of this family to be described from a Gram-positive bacterium. We demonstrate that flhA is a flagellar gene and that FlhA is required in trans for the formation of products from some, but not all, B. subtilis motility-related operons that are regulated by the sigma D form of RNA polymerase. We suggest that FlhA is a component of a signalling system that is involved with the formation of some flagellar gene products during the biosynthesis of the flagellum.

Identification of the promoter and a negative regulatory element, ftr4, that is needed for cell cycle timing of fliF operon expression in Caulobacter crescentus

The fliF operon of Caulobacter crescentus, which was previously designated the flaO locus, is near the top of the flagellar-gene regulatory hierarchy, and it is one of the earliest transcription units to be expressed in the cell cycle. In this report, we have identified two cis-acting sequences that are required for cell cycle regulation of fliF transcription. The first sequence was defined by the effects of three 2-bp deletions and five point mutations, each of which greatly reduced the level of fliF operon transcript in vivo. These eight mutations lie between -37 and -22 within an 18-bp sequence that matches, at 11 nucleotides, sequences in the 5' regions of the flaQR (flaS locus) and fliLM operons, which are also expressed early and occupy a high level in the regulatory hierarchy (A. Dingwall, A. Zhuang, K. Quon, and L. Shapiro, J. Bacteriol. 174:1760-1768, 1992). We propose that this 18-bp sequence contains all or part of the fliF promoter. We have also identified a second sequence, 17 bp long and centered at -8, which we have provisionally designated ftr4 because of its similarity to the enhancer-like ftr sequences required for regulation of sigma 54 promoters flaN and flbG (D. A. Mullin and A. Newton, J. Bacteriol. 171:3218-3227, 1989). Six of the seven mutations in ftr4 examined resulted in a large increase in fliF operon transcript levels, suggesting a role for ftr4 in negative regulation. A 2-bp deletion at -12 and -13 in ftr4 altered the cell cycle pattern of fliF operon transcription; the transcript was still expressed periodically, but the period of its synthesis was extended significantly. We suggest that the ftr4 sequence may form part of a developmental switch which is required to turn off fliF operon transcription at the correct time in the cell cycle.

mxiA of Shigella flexneri 2a, which facilitates export of invasion plasmid antigens, encodes a homolog of the low-calcium-response protein, LcrD, of Yersinia pestis

The plasmid-encoded invasion plasmid antigen (Ipa) export accessory locus of Shigella flexneri 2a, mxiA, was cloned, and the complete DNA sequence of the gene was determined. The mixA open reading frame was found to encode a polypeptide of 74.03 kDa with a pI of 5.02. A hydropathy analysis of the predicted protein revealed a hydrophilic C terminus and an extremely hydrophobic N terminus without a cleavable signal sequence but with several potential membrane-spanning regions. While a homology search did not reveal any significant relatedness of the mxiA DNA sequence to any known bacterial gene sequences, the derived amino acid sequence of MxiA was found to be highly homologous (68%) to the sequence of the protein encoded by the low-calcium-response locus, lcrD, of Yersinia pestis. The lcrD encodes an inner membrane regulatory protein that has an N-terminal membrane anchor and that is implicated in facilitating the export of Y. pestis outer membrane proteins (G. V. Plano, S. S. Barve, and S. C. Straley, J. Bacteriol. 173:7293-7303, 1991). Congo red binding, HeLa cell invasion, and Ipa excretion were restored in two avirulent mxiA fusion mutants when they were transformed with a cloned copy of the mxiA gene. Furthermore, the expression of the cloned mxiA gene was independent of any vector-specified promoter, suggesting that the transcription of mxiA is driven by its own promoter in this clone. In contrast, the overexpression of mxiA that resulted when it was placed under the control of the lac promoter was found to be deleterious in Escherichia coli. We conclude that mxiA is a homolog of the Y. pestis lcrD locus and may function similarly in S. flexneri, either by directly affecting the excretion of virulence factors or by regulating the expression of export accessory genes.