LcrD, a membrane-bound regulator of the Yersinia pestis low-calcium response

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.

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.

YscU, a Yersinia enterocolitica inner membrane protein involved in Yop secretion

Pathogenic yersiniae secrete antihost Yop proteins by a recently discovered secretion pathway which is also encountered in several animal and plant pathogens. The components of the export machinery are encoded by the virA (lcrA), virB (lcrB), and virC (lcrC) loci of the 70-kb pYV plasmid. In the present paper we describe yscU, the last gene of the virB locus. We determined the DNA sequence and mutated the gene on the pYV plasmid. After inactivation of yscU, the mutant strain was unable to secrete Yop proteins. The topology of YscU was investigated by the analysis of YscU-PhoA translational fusions generated by TnphoA transposition. This showed that the 40.3-kDa yscU product contains four transmembrane segments anchoring a large cytoplasmic carboxyl-terminal domain to the inner membrane. YscU is related to Spa40 from Shigella flexneri, to SpaS from Salmonella typhimurium, to FlhB from Bacillus subtilis, and to HrpN from Pseudomonas solanacearum.

The Salmonella typhimurium invasion genes invF and invG encode homologues of the AraC and PulD family of proteins

We have identified two novel Salmonella typhimurium genes, invF and invG, which are required for the efficient entry of these organisms into cultured epithelial cells. invF and invG are located immediately upstream of invE, a previously identified gene also required for Salmonella entry. Non-polar mutations in these genes rendered S. typhimurium severely deficient for entry into cultured epithelial cells. The nucleotide sequences of invF and invG indicated that these genes encode polypeptides with predicted molecular weights of 24,373 and 62,275, respectively. Proteins of similar sizes were observed when invF and invG were expressed in a bacteriophage T7 RNA polymerase-based expression system. Comparison of the predicted sequence of InvF with translated sequences in the existing databases indicated that this protein is homologous to members of the AraC family of prokaryotic transcription regulators. However, mutations in invF did not significantly affect the expression of other members of the inv locus. InvG was found to be homologous to members of the PulD family of specialized translocases. This homology suggests that InvG may be necessary for the export of invasion-related determinants or involved in the assembly of a supramolecular structure that promotes entry.

Virulence protein export systems in Salmonella and Shigella: a new family or lost relatives?

A number of Gram-negative bacterial pathogens secrete 'virulence determinants' directly into the extracellular medium, where they interact with host cells to promote disease. The study of the secretion machinery used by these organisms to transport specific virulence determinants out to the cell surface and beyond is of growing importance in the field of bacterial pathogenesis. Elements of the secretion machinery are shared by several pathogens. These homologous elements may lead to a better understanding of how the machinery works, but the unique elements will tell us more about what distinguishes one bacterial pathogen from another.

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.

Role of YadA in resistance of Yersinia enterocolitica to phagocytosis by human polymorphonuclear leukocytes

Pathogenic Yersinia enterocolitica cells do not induce the chemiluminescence response of human polymorphonuclear leukocytes (PMNs). We tested the chemiluminescence response to Y. enterocolitica mutants affected in the known pYV-encoded factors. We did not detect any influence of the Yops in this phenomenon. By contrast, the presence of YadA correlated with a lack of chemiluminescence. The expression of YadA at the bacterial surface also reduced the phagocytosis by PMNs. Finally, we measured the survival of Y. enterocolitica cells confronted with PMNs by the classical plating method and by a new luminometry assay. We observed that YadA+ bacteria were not killed, while YadA- bacteria were killed. We conclude that the presence of YadA at the surface of Y. enterocolitica cells prevents phagocytosis and killing by PMNs. This conclusion is in good agreement with our recent observation that YadA protects Y. enterocolitica from opsonization by C3b.

YscN, the putative energizer of the Yersinia Yop secretion machinery

Pathogenic yersiniae secrete a set of 11 antihost proteins called Yops. Yop secretion appears as the archetype of the type III secretion pathway. Several components of this machinery are encoded by the virA (lcrA) and virC (lcrC) loci of the 70-kb pYV plasmid. In this paper, we describe yscN, another gene involved in this pathway. It is the first gene of the virB locus. It encodes a 47.8-kDa protein similar to the catalytic subunits of F0F1 and related ATPases, as well as to products of other genes presumed to be involved in a type III secretion pathway. YscN contains the two consensus nucleotide-binding motifs (boxes A and B) described by Walker et al. (J. E. Walker, M. Saraste, M. J. Runswick, and N. J. Gay, EMBO J. 1:945-951, 1982). We engineered a pYV mutant encoding a modified YscN protein lacking box A. This mutant, impaired in Yop secretion, can be complemented in trans by a cloned yscN gene. We conclude that YscN is a component of the Yop secretion machinery using ATP. We hypothesize that it is either the energizer of this machinery or a part of it.

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.

Regulation and polarized transfer of the Yersinia outer proteins (Yops) involved in antiphagocytosis

Pathogenic Yersinia express a number of strictly regulated, plasmid-encoded virulence determinants (Yops), some of which are important in enabling the pathogen to block phagocytosis. The events mediating antiphagocytosis and the regulation of this process are becoming increasingly well understood.

Bacillus subtilis flagellar proteins FliP, FliQ, FliR and FlhB are related to Shigella flexneri virulence factors

The amino acid sequences of the Bacillus subtilis flagellar proteins, FliP, FliQ, FliR and FlhB, as deduced from their respective nucleotide sequences, were found to share significant homology to the Shigella flexneri Spa24, Spa9, Spa29 and Spa40 virulence proteins, respectively. These proteins are required for the presentation of surface plasmid antigens. These results further support the growing hypothesis that a superfamily of proteins exists for the biosynthesis of supramolecular structures that lie in an external to the cell membrane.

HrpI of Erwinia amylovora functions in secretion of harpin and is a member of a new protein family

HrpI, a 78-kDa protein, functions in the secretion of harpin, a proteinaceous elicitor of the hypersensitive response from Erwinia amylovora. The predicted amino acid sequence of HrpI is remarkably similar to that of LcrD of Yersinia species, the first member of a recently described protein family. Other proteins of the family are MixA from Shigella flexneri, InvA from Salmonella typhimurium, FlhA from Caulobacter crescentus, HrpI from Pseudomonas syringae pv. syringae, HrpO from Pseudomonas solanacearum, and HrpC2 from Xanthomonas campestris pv. vesicatoria. Cells of E. amylovora containing mutated hrpI genes or cells of Escherichia coli containing the cloned hrp gene cluster with mutated hrpI produce but do not export harpin. When similar cells with functional hrpI genes were grown at 25 degrees C, but not at 37 degrees C, harpin was exported to the culture supernatant. Direct evidence that HrpI is involved in the secretion of a virulence protein has been offered. Two other loci of the hrp gene cluster are involved in the regulation of harpin, and four other loci also are involved in the secretion of harpin. Since harpin and other proteins likely to be secreted by the LcrD family of proteins lack typical signal peptides, their secretion mechanism is distinct from the general protein export pathway.

Conservation of secretion pathways for pathogenicity determinants of plant and animal bacteria

Extracellular proteins of plant and animal bacteria are important in virulence. Many of these are secreted through the type I sec-independent and the type II sec-dependent pathways. Recently, a third distinct pathway, involved in secretion of Yops, has been discovered in Yersinia. This pathway has homology with pathways in plant pathogenic bacteria that are putatively involved in the secretion of proteins active on plant cells, such as harpin and possibly some avr gene products

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.

LcrG, a secreted protein involved in negative regulation of the low-calcium response in Yersinia pestis

The purpose of this study was to define the function of LcrG, the product of the first gene in the lcrGVHyopBD operon of the low-Ca(2+)-response (LCR) virulence plasmid of Yersinia pestis. We created a Y. pestis strain having an in-frame deletion in lcrG. This nonpolar mutant had an abnormal LCR growth phenotype: it was unable to grow at 37 degrees C in the presence of 2.5 mM Ca2+ ("Ca2+ blind") but was able to grow at 37 degrees C when 18 mM ATP was present. At 37 degrees C it failed to downregulate the expression and secretion of its truncated product (LcrG), V antigen, and YopM. All of these mutant properties were complemented by plasmids carrying normal lcrG. However, a nonpolar lcrE mutation and an lcrH mutation (both also causing a Ca(2+)-blind phenotype) were not complemented in this way. The Y. pestis parent strain expressed LcrG at 37 degrees C in the presence and absence of Ca2+ and transported it to the medium when Ca2+ was absent. We identified two LCR-regulated loci, lcrD and yscDEF, required for this transport. Complementation analysis of the Y. pestis lcrR strain previously shown to lack the expression of LcrG showed that the loss of LcrG but not of LcrR caused the Ca(2+)-blind phenotype of that mutant. Taken together, the results show that LcrG is a negative regulator of the LCR, perhaps functioning in Ca2+ sensing along with LcrE.

Regulation by Ca2+ in the Yersinia low-Ca2+ response

The Yersinia low-Ca2+ response (LCR) is a regulatory response in which a set of plasmid-borne operons is transcriptionally regulated at 37 degrees C in response to the presence or absence of mM concentrations of Ca2+. LCR-regulated operons encode secreted proteins with regulatory and virulence roles as well as non-secreted regulatory proteins and components of the secretion machinery. Downregulation by Ca2+ is imposed by a signalling cascade that includes secreted proteins and possibly also components of the secretion system and is hypothesized to act on membrane-bound inductive components. An important role in LCR induction is played by LcrD, an inner-membrane protein with homologues in several virulence-associated and flagella assembly-related systems in diverse bacterial species. The mechanism of signal transduction in response to Ca2+ is not known, and the proteins that bind DNA to downregulate transcription have not been identified.

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.

Genetic and biochemical analysis of Salmonella typhimurium FliI, a flagellar protein related to the catalytic subunit of the F0F1 ATPase and to virulence proteins of mammalian and plant pathogens

FliI is a Salmonella typhimurium protein that is needed for flagellar assembly and may be involved in a specialized protein export pathway that proceeds without signal peptide cleavage. FliI shows extensive sequence similarity to the catalytic beta subunit of the F0F1 ATPase (A. P. Volger, M. Homma, V. M. Irikura, and R. M. Macnab, J. Bacteriol. 173:3564-3572, 1991). It is even more similar to the Spa47 protein of Shigella flexneri (M. M. Venkatesan, J. M. Buysse, and E. V. Oaks, J. Bacteriol. 174:1990-2001, 1992) and the HrpB6 protein of Xanthomonas campestris (S. Fenselau, I. Balbo, and U. Bonas, Mol. Plant-Microbe Interact. 5:390-396, 1992), which are believed to play a role in the export of virulence proteins. Site-directed mutagenesis of residues in FliI that correspond to catalytically important residues in the F1 beta subunit resulted in loss of flagellation, supporting the hypothesis that FliI is an ATPase. FliI was overproduced and purified almost to homogeneity. It demonstrated ATP binding but not hydrolysis. An antibody raised against FliI permitted detection of the protein in wild-type cells and an estimate of about 1,500 subunits per cell. An antibody directed against the F1 beta subunit of Escherichia coli cross-reacted with FliI, confirming that the proteins are structurally related. The relationship between three proteins involved in flagellar assembly (FliI, FlhA, and FliP) and homologs in a variety of virulence systems is discussed.

Linear chromosomal physical and genetic map of Borrelia burgdorferi, the Lyme disease agent

A physical map of the 952 kbp chromosome of Borrelia burgdorferi Sh-2-82 has been constructed. Eighty-three intervals on the chromosome, defined by the cleavage sites of 15 restriction enzymes, are delineated. The intervals vary in size from 96 kbp to a few hundred bp, with an average size of 11.5 kbp. A striking feature of the map is its linearity; no other bacterial groups are known to have linear chromosomes. The two ends of the chromosome do not hybridize with one another, indicating that there are no large common terminal regions. The chromosome of this strain was found to be stable in culture; passage 6, 165 and 320 cultures have identical chromosomal restriction maps. We have positioned all previously known Borrelia burgdorferi chromosomal genes and several newly identified ones on this map. These include the gyrA/gyrB/dnaA/dnaN gene cluster, the rRNA gene cluster, fla, flgE, groEL (hsp60), recA, the rho/hip cluster, the dnaK (hsp70)/dnaJ/grpE cluster, the pheT/pheS cluster, and the genes which encode the potent immunogen proteins p22A, p39 and p83. Our electrophoretic analysis detects five linear and at least two circular plasmids in B. burgdorferi Sh-2-82. We have constructed a physical map of the 53 kbp linear plasmid and located the operon that encodes the two major outer surface proteins ospA and ospB on this plasmid. Because of the absence of functional genetic tools for this organism, these maps will serve as a basis for future mapping, cloning and sequencing studies of B. burgdorferi.

SycE, a chaperone-like protein of Yersinia enterocolitica involved in Ohe secretion of YopE

Pathogenic yersiniae secrete a set of 11 anti-host proteins called Yops. The yop genes, scattered around the pYV plasmid, constitute a thermoinduced regulon controlled by the product of virF gene. The secretion of the Yops also requires the presence of the products of the other vir genes and operons, namely virA, virB and virC. The large virC operon and presumably some genes of the virA region encode a new secretion system. Mutations in any of these vir genes impair the production of all the Yops. In contrast, mutations in the yerA locus, located close to yopE, specifically abolish the expression of the cytotoxin YopE. We describe here the counterpart of yerA in Yersinia enterocolitica W22703. We demonstrate that the gene product of yerA regulates the production of YopE at a post-transcriptional level. It specifically binds the YopE protein. We consider that it acts as a specific chaperone and we call it SycE (for specific YopE chaperone). We hypothesize that SycE is a link between translation and the specific Yop export machinery. It is the first representative of a new family of pYV-encoded proteins.

Eight genes in region 5 that form an operon are essential for invasion of epithelial cells by Shigella flexneri 2a

The 7-kb region 5 on the large 230-kb plasmid pMYSH6000 in Shigella flexneri 2a YSH6000 is one of the virulence-associated DNA segments required for the invasion of epithelial cells (C. Sasakawa, K. Kamata, T. Sakai, S. Makino, M. Yamada, N. Okada, and M. Yoshikawa, J. Bacteriol. 170:2480-2484, 1988). To elucidate the functional organization of region 5 and to determine the virulence-associated genes encoded by region 5, we performed insertion and deletion mutagenesis, DNA subcloning, and complete nucleotide sequencing of region 5 and found that region 5 contained 11 open reading frames (ORFs) named ORF-1 through ORF-11 which could be translated into proteins with molecular masses of 15.1, 47.5, 13.2, 33.0, 33.4, 24.2, 9.4, 28.5, 39.9, 9.1, and 10.4 kDa, respectively. Complementation tests of the 14 Tn5-induced noninvasive mutants of region 5 with the above plasmid constructs have indicated that region 5 consists of an operon and that ORF-2 through ORF-9, but not ORF-1, ORF-10, and ORF-11, are essential for invasion, and 7 of 8 ORFs (ORF-2 and ORF-4 through ORF-9) and presumably the remaining ORF (ORF-3) are required for secretion of the Ipa proteins. The transcriptional organization, as determined by a promoter-proving vector, S1 nuclease protection, and primer extension RNA sequencing analysis revealed that region 5 is transcribed from a promoter located 47 bp upstream of the 5' end of ORF-2 for the 47.5-kDa protein and that the promoter activity identified was regulated by the virB gene, the transcriptional activator on the 230-kb plasmid.

Signal transduction and virulence regulation in human and animal pathogens

Pathogens have developed many strategies for survival in animals and humans which possess very effective defense mechanisms. Although there are many different ways, in which pathogenic bacteria solved the problem to overcome the host defense, some common features of virulence mechanisms can be detected even in phylogenetically very distant bacteria (Finlay and Falkow (1989) Microb. Rev. 6, 1375-1383). One important feature is that the regulation of expression of virulence factors and the exact timing of their expression is very important for many of the pathogenic bacteria, as most of them have to encounter different growth situations during an infection cycle, which require a fast adaptation to the new situation by the expression of different factors. This review gives an overview about the mechanisms used by pathogenic bacteria to accomplish the difficult task of regulation of their virulence potential in response to environmental changes. In addition, the relationship of these virulence regulatory systems with other signal transduction mechanisms not involved in pathogenicity is discussed.

Role of the transcription activator virF and the histone-like protein YmoA in the thermoregulation of virulence functions in yersiniae

The chromosome of Y. enterocolitica encodes a heat-stable enterotoxin, Yst, being related to STI. The capacity to produce Yst generally disappears during storage of the strains. In these strains, the yst gene is intact but remains silent. The pYV plasmid encodes the eleven secreted antihost proteins called Yops as well as the outer membrane protein YadA. The Yops are secreted by a novel, pYV-encoded secretion mechanism. This mechanism which does not involve the removal of an N-terminal signal sequence, is encoded by the pYV virA and virC loci. The virC locus contains 13 genes called yscA-M. The virA locus encodes the LcrD membrane protein. The yop, yadA and ysc genes form the yop regulon controlled by transcriptional activator VirF. Transcription of the yop, yadA, ysc and virF genes is controlled by temperature. A chromosome-encoded histone-like protein, called YmoA, is involved in the thermoregulation of the yop regulon, which suggests that this thermoregulation could result from temperature-induced changes in DNA topology. The phenotype of ymoA mutants resembles that of osmZ or drdX mutants of E. coli but YmoA is not the Yersinia homologue of the E. coli histone H1. The YmoA histone is also involved in the silencing of the yst gene.

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.

Rapid and specific identification of Yersinia pestis by using a nested polymerase chain reaction procedure

We developed a 4-h nested polymerase chain reaction assay that detected a region of the plasminogen activator gene of Yersinia pestis in 100% of 43 Y. pestis strains isolated from humans, rats, and fleas yet was unreactive with the closely related species Yersinia enterocolitica and Yersinia pseudotuberculosis.

Yersinia pestis YopM: thrombin binding and overexpression

In previous studies, Yersinia pestis YopM has been shown through mutational analysis to be necessary for virulence in mice and found to have homology with the thrombin-binding domain of the platelet receptor GPIb alpha. In this study, YopM was purified and shown by dot blot and chemical cross-linking tests to bind to human alpha-thrombin. No cross-linked product could be detected when human prothrombin was incubated with YopM. As a functional test of thrombin binding, it was shown that native but not boiled YopM inhibits thrombin-induced aggregation of human platelets. Control tests showed that YopM did not inactivate the platelets themselves, nor was its effect a nonspecific consequence of its very acidic isoelectric point. Microsequencing of YopM revealed an intact N terminus, indicating that functional YopM is not processed at the N terminus or secreted by a mechanism involving a cleavable signal sequence. Further characterization was made of an interesting effect on yopM expression that had been noticed in a previous study. A 1.5-kb HaeIII subclone overexpressed YopM in both Y. pestis and Escherichia coli compared with a larger clone containing the 5.3-kb HindIII-F fragment. To search for a possible regulator of YopM expression, the HindIII-F fragment was sequenced, revealing several open reading frames and three large repeated sequences. Deletional analysis showed that these were not involved in regulation of yopM. The data implicated a DNA structure 5' to yopM in moderating yopM expression.

The Pseudomonas syringae pv. syringae 61 hrpH product, an envelope protein required for elicitation of the hypersensitive response in plants

Pseudomonas syringae pv. syringae 61 contains a 25-kb cluster of hrp genes that are required for elicitation of the hypersensitive response (HR) in tobacco. TnphoA mutagenesis of cosmid pHIR11, which contains the hrp cluster, revealed two genes encoding exported or inner-membrane-spanning proteins (H.-C. Huang, S. W. Hutcheson, and A. Collmer, Mol. Plant-Microbe Interact. 4:469-476, 1991). The gene in complementation group X, designated hrpH, was subcloned on a 3.1-kb SalI fragment into pCPP30, a broad-host-range, mobilizable vector. The subclone restored the ability of hrpH mutant P. syringae pv. syringae 61-2089 to elicit the HR in tobacco. DNA sequence analysis of the 3.1-kb SalI fragment revealed a single open reading frame encoding an 81,956-Da preprotein with a typical amino-terminal signal peptide and no likely inner-membrane-spanning hydrophobic regions. hrpH was expressed in the presence of [35S]methionine by using the T7 RNA polymerase-promoter system and vector pT7-3 in Escherichia coli and was shown to encode a protein with an apparent molecular weight of 83,000 on sodium dodecyl sulfate-polyacrylamide gels. The HrpH protein in E. coli was located in the membrane fraction and was absent from the periplasm and cytoplasm. The HrpH protein possessed similarity with several outer membrane proteins that are known to be involved in protein or phage secretion, including the Klebsiella oxytoca PulD protein, the Yersinia enterocolitica YscC protein, and the pIV protein of filamentous coliphages. All of these proteins possess a possible secretion motif, GG(X)12VP(L/F)LXXIPXIGXL(F/L), near the carboxyl terminus, and they lack a carboxyl-terminal phenylalanine, in contrast to other outer membrane proteins with no known secretion function. These results suggest that the P. syringae pv. syringae HrpH protein is involved in the secretion of a proteinaceous HR elicitor.

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.

Structure and regulation of the Yersinia pestis yscBCDEF operon

We have investigated the physical and genetic structure and regulation of the Yersinia pestis yscBCDEF region, previously called lcrC. DNA sequence analysis showed that this region is homologous to the corresponding part of the ysc locus of Yersinia enterocolitica and suggested that the yscBCDEF cistrons belong to a single operon on the low-calcium response virulence plasmid pCD1. Promoter activity measurements of ysc subclones indicated that yscBCDEF constitutes a suboperon of the larger ysc region by revealing promoter activity in a clone containing the 3' end of yscD, intact yscE and yscF, and part of yscG. These experiments also revealed an additional weak promoter upstream of yscD. Northern (RNA) analysis with a yscD probe showed that operon transcription is thermally induced and downregulated in the presence of Ca2+. Primer extension of operon transcripts suggested that two promoters, a moderate-level constitutive one and a stronger, calcium-downregulated one, control full-length operon transcription at 37 degrees C. Primer extension provided additional support for the proposed designation of a yscBCDEF suboperon by identifying a 5' end within yscF, for which relative abundances in the presence and absence of Ca2+ revealed regulation that is distinct from that for transcripts initiating farther upstream. YscB and YscC were expressed in Escherichia coli by using a high-level transcription system. Attempts to express YscD were only partially successful, but they revealed interesting regulation at the translational level.

Molecular and functional characterization of the Salmonella invasion gene invA: homology of InvA to members of a new protein family

One of the earliest steps in the pathogenic cycle of the facultative intracellular pathogen Salmonella spp. is the invasion of the cells of the intestinal epithelium. We have previously identified a genetic locus, inv, that allows Salmonella spp. to enter cultured epithelial cells. invA is a member of this locus, and it is the first gene of an operon consisting of at least two additional invasion genes. We have constructed strains carrying nonpolar mutations in invA and examined the individual contribution of this gene to the invasion phenotype of Salmonella typhimurium. Nonpolar S. typhimurium invA mutants were deficient in invasion of cultured epithelial cells although they were fully capable of attaching to the same cells. In addition, unlike wild-type S. typhimurium, invA mutants did not alter the normal architecture of the microvilli of polarized epithelial cells nor did they cause any alterations in the distribution of actin microfilaments of infected cells. The invasion phenotype of invA mutants was readily rescued by wild-type S. typhimurium when cultured epithelial cells were simultaneously infected with both strains. On the contrary, in a similar experiment, the adherent Escherichia coli strain RDEC-1 was not internalized into cultured cells when coinfected with wild-type S. typhimurium. The invA locus was found to be located at about 59 min on the Salmonella chromosome, 7% linked to mutS. The nucleotide sequence of invA showed an open reading frame capable of encoding a polypeptide of 686 amino acids with eight possible membrane-spanning regions and a predicted molecular weight of 75,974. A protein of this size was visualized when invA was expressed in a bacteriophage T7 RNA polymerase-based expression system. The predicted sequence of InvA was found to be homologous to Caulobacter crescentus FlbF, Yersinia LcrD, Shigella flexneri VirH, and E. coli FlhA proteins. These proteins may form part of a family of proteins with a common function, quite possibly the translocation of specific proteins across the bacterial cell membrane.

The cell cycle-regulated flagellar gene flbF of Caulobacter crescentus is homologous to a virulence locus (lcrD) of Yersinia pestis

We have characterized flbF, a key locus located at the top of the flagellar gene hierarchy of Caulobacter crescentus. This gene is required for transcription from sigma 54 promoters of fla genes expressed late in the cell cycle. We have determined the nucleotide sequence of the gene, mapped the 5' end of the flbF RNA, and examined the pattern of expression in the cell cycle. Our results show that flbF is expressed earlier in the cell cycle than other fla genes, that it is expressed at a low level throughout the stalked cell cycle, and that its 5' regulatory region contains sequences that can be aligned with the sigma 28 promoter consensus reported for enteric bacteria. flbF contains an open reading frame of 700 residues with an amino-terminal half rich in hydrophobic residues that could correspond to six to eight transmembrane domains. The translated flbF sequence is very similar to LcrD (low calcium response) encoded by virulence plasmids of pathogenic Yersinia spp. (G. Plano, S. Barve, and S. Straley, J. Bacteriol. 173:7293-7303, 1991). LcrD and FlbF can be aligned over the entire length of the proteins with the greatest degree of sequence identity (45%) in the hydrophobic amino-terminal region. The high degree of sequence homology of proteins derived from widely differing organisms, including Caulobacter and Yersinia species, suggests that FlbF and LcrD may be representatives of a larger family of regulatory proteins with a common sensor mechanism for modifying responses to appropriate stimuli.