Nucleotide sequence of the invasion plasmid antigen B and C genes (ipaB and ipaC) of Shigella flexneri

Virulence Gene Regulation in Shigella

Shigella species are the causative agents of bacillary dysentery in humans, an invasive disease in which the bacteria enter the cells of the epithelial layer of the large intestine, causing extensive tissue damage and inflammation. They rely on a plasmid-encoded type III secretion system (TTSS) to cause disease; this system and its regulation have been investigated intensively at the molecular level for decades. The lessons learned have not only deepened our knowledge of Shigella biology but also informed in important ways our understanding of the mechanisms used by other pathogenic bacteria to cause disease and to control virulence gene expression. In addition, the Shigella story has played a central role in the development of our appreciation of the contribution of horizontal DNA transfer to pathogen evolution.A 30-kilobase-pair "Entry Region" of the 230-kb virulence plasmid lies at the heart of the Shigella pathogenesis system. Here are located the virB and mxiE regulatory genes and most of the structural genes involved in the expression of the TTSS and its effector proteins. Expression of the virulence genes occurs in response to an array of environmental signals, including temperature, osmolarity, and pH.At the top of the regulatory hierarchy and lying on the plasmid outside the Entry Region isvirF, encoding an AraC-like transcription factor.Virulence gene expression is also controlled by chromosomal genes,such as those encoding the nucleoid-associated proteins H-NS, IHF, and Fis, the two-component regulators OmpR/EnvZ and CpxR/CpxA, the anaerobic regulator Fnr, the iron-responsive regulator Fur, and the topoisomerases of the cell that modulate DNA supercoiling. Small regulatory RNAs,the RNA chaperone Hfq,and translational modulation also affect the expression of the virulence phenotypetranscriptionally and/orposttranscriptionally.

Binding affects the tertiary and quaternary structures of the Shigella translocator protein IpaB and its chaperone IpgC

Shigella flexneri uses its type III secretion system (T3SS) to promote invasion of human intestinal epithelial cells as the first step in causing shigellosis, a life-threatening form of dysentery. The Shigella type III secretion apparatus (T3SA) consists of a basal body that spans the bacterial envelope and an exposed needle that injects effector proteins into target cells. The nascent Shigella T3SA needle is topped with a pentamer of the needle tip protein invasion plasmid antigen D (IpaD). Bile salts trigger recruitment of the first hydrophobic translocator protein, IpaB, to the tip complex where it senses contact with a host membrane. In the bacterial cytoplasm, IpaB exists in a complex with its chaperone IpgC. Several structures of IpgC have been determined, and we recently reported the 2.1 Å crystal structure of the N-terminal domain (IpaB(74.224)) of IpaB. Like IpgC, the IpaB N-terminal domain exists as a homodimer in solution. We now report that when the two are mixed, these homodimers dissociate and form heterodimers having a nanomolar dissociation constant. This is consistent with the equivalent complexes copurified after they had been co-expressed in Escherichia coli. Fluorescence data presented here also indicate that the N-terminal domain of IpaB possesses two regions that appear to contribute additively to chaperone binding. It is also likely that the N-terminus of IpaB adopts an alternative conformation as a result of chaperone binding. The importance of these findings within the functional context of these proteins is discussed.

The structures of coiled-coil domains from type III secretion system translocators reveal homology to pore-forming toxins

Many pathogenic Gram-negative bacteria utilize type III secretion systems (T3SSs) to alter the normal functions of target cells. Shigella flexneri uses its T3SS to invade human intestinal cells to cause bacillary dysentery (shigellosis) that is responsible for over one million deaths per year. The Shigella type III secretion apparatus is composed of a basal body spanning both bacterial membranes and an exposed oligomeric needle. Host altering effectors are secreted through this energized unidirectional conduit to promote bacterial invasion. The active needle tip complex of S. flexneri is composed of a tip protein, IpaD, and two pore-forming translocators, IpaB and IpaC. While the atomic structure of IpaD has been elucidated and studied, structural data on the hydrophobic translocators from the T3SS family remain elusive. We present here the crystal structures of a protease-stable fragment identified within the N-terminal regions of IpaB from S. flexneri and SipB from Salmonella enterica serovar Typhimurium determined at 2.1 Å and 2.8 Å limiting resolution, respectively. These newly identified domains are composed of extended-length (114 Å in IpaB and 71 Å in SipB) coiled-coil motifs that display a high degree of structural homology to one another despite the fact that they share only 21% sequence identity. Further structural comparisons also reveal substantial similarity to the coiled-coil regions of pore-forming proteins from other Gram-negative pathogens, notably, colicin Ia. This suggests that these mechanistically separate and functionally distinct membrane-targeting proteins may have diverged from a common ancestor during the course of pathogen-specific evolutionary events.

The extreme C terminus of Shigella flexneri IpaB is required for regulation of type III secretion, needle tip composition, and binding

Type III secretion systems (T3SSs) are widely distributed virulence determinants of Gram-negative bacteria. They translocate bacterial proteins into host cells to manipulate them during infection. The Shigella T3SS consists of a cytoplasmic bulb, a transmembrane region, and a hollow needle protruding from the bacterial surface. The distal tip of mature, quiescent needles is composed of IpaD, which is topped by IpaB. Physical contact with host cells initiates secretion and leads to assembly of a pore, formed by IpaB and IpaC, in the host cell membrane, through which other virulence effector proteins may be translocated. IpaB is required for regulation of secretion and may be the host cell sensor. However, its mode of needle association is unknown. Here, we show that deletion of 3 or 9 residues at the C terminus of IpaB leads to fast constitutive secretion of late effectors, as observed in a DeltaipaB strain. Like the DeltaipaB mutant, mutants with C-terminal mutations also display hyperadhesion. However, unlike the DeltaipaB mutant, they are still invasive and able to lyse the internalization vacuole with nearly wild-type efficiency. Finally, the mutant proteins show decreased association with needles and increased recruitment of IpaC. Taken together, these data support the notion that the state of the tip complex regulates secretion. We propose a model where the quiescent needle tip has an "off" conformation that turns "on" upon host cell contact. Our mutants may adopt a partially "on" conformation that activates secretion and is capable of recruiting some IpaC to insert pores into host cell membranes and allow invasion.

Secretion and function of Salmonella SPI-2 effector SseF require its chaperone, SscB

Salmonella strains utilize a type III secretion system for their successful survival and replications inside host cells. SseF is one of the several effector proteins that are required for conferring this survival ability by altering the trafficking of the Salmonella-containing vacuoles. These effector proteins often require appropriate chaperones to maintain their stabilities inside the bacteria. These chaperones are also known to assist the subsequent secretion and translocation of their substrates. We report here that SscB acts as the chaperone for SseF, an effector for the Salmonella pathogenicity island 2 (SPI-2). We found that the sscB gene is required for the formation of Salmonella sp.-induced continuous filaments in epithelial cells. Efficient Salmonella replication in macrophages requires SscB function. Intracellular and secretion levels of SseF are greatly reduced in an sscB mutant strain compared to the wild-type strain. A protein stability assay demonstrated that the half-life of SseF is significantly shortened in the absence of SscB. Transcriptional analysis of the sseF gene showed that the effect of SscB on the SseF level is not at the transcriptional level. A coprecipitation experiment indicated that SscB interacts with SseF. In summary, our results indicate that SscB is a chaperone for SPI-2 effector SseF to facilitate its secretion and function inside the host cells.

IcsB, secreted via the type III secretion system, is chaperoned by IpgA and required at the post-invasion stage of Shigella pathogenicity

Shigella deliver a subset of effector proteins such as IpaA, IpaB and IpaC via the type III secretion system (TTSS) into host cells during the infection of colonic epithelial cells. Many bacterial effectors including some from Shigella require specific chaperones for protection from degradation and targeting to the TTSS. In this study, we have investigated the role of the icsB gene located upstream of the ipaBCDA operon in Shigella infection because the role of IcsB as a virulence factor remains unknown. Here, we found that the IcsB protein is secreted via the TTSS of Shigella in vitro and in vivo. We show that IpgA protein encoded by ipgA, the gene immediately downstream of icsB, serves as the chaperone required for the stabilization and secretion of IcsB. We have shown that IcsB binds to IpgA in bacterial cytosol and the binding site is in the middle of the IcsB protein. Intriguingly, although its significance in Shigella pathogenicity is as yet unclear, the icsB gene can be read-through into the ipgA gene to create a translational fusion protein. Furthermore, the contribution of IcsB to the pathogenicity of Shigella was demonstrated by plaque-forming assay and the Sereny test. The ability of the icsB mutant to form plaques was greatly reduced compared with that of the wild type in MDCK cell monolayers. Furthermore, when guinea pig eyes were infected with a non-polar icsB mutant, the bacteria failed to provoke keratoconjunctivitis. These results suggest that IcsB is secreted via the TTSS, chaperoned by IpgA, and required at the post-invasion stage of Shigella pathogenicity

Structure-function analysis of the Shigella virulence factor IpaB

Infection by the gram-negative bacterium Shigella flexneri results in dysentery, an acute inflammatory disease of the colon. Essential events in the pathogenesis of Shigella infections include bacterial invasion of epithelial cells, escape from the phagosome, and induction of apoptosis in macrophages. The Shigella virulence factor invasion plasmid antigen B (IpaB) is required for all of these processes. Induction of apoptosis is dependent on IpaB binding to the cysteine protease caspase-1 (Casp-1). The activation of this enzyme triggers both apoptosis and release of the proinflammatory cytokine interleukin-1beta. Several IpaB mutants were generated to correlate function with protein subdomains. We determined that the N-terminal portion of IpaB is necessary for stable expression of IpaB. A putative amphipathic alpha-helical domain preserves the structure of IpaB. We found 10 consecutive residues within the amino terminus of the hydrophobic region that play a critical role in invasion, phagosomal escape, and cytotoxicity. An IpaB mutant carrying a mutation in this region binds to Casp-1 yet is not cytotoxic, even following direct delivery to the macrophage cytoplasm. These results indicate that the association between IpaB and Casp-1 is only a step in the activation of macrophage apoptosis.

The virulence plasmid pWR100 and the repertoire of proteins secreted by the type III secretion apparatus of Shigella flexneri

Bacteria of Shigella spp. are the causative agents of shigellosis. The virulence traits of these pathogens include their ability to enter into epithelial cells and induce apoptosis in macrophages. Expression of these functions requires the Mxi-Spa type III secretion apparatus and the secreted IpaA-D proteins, all of which are encoded by a virulence plasmid. In wild-type strains, the activity of the secretion apparatus is tightly regulated and induced upon contact of bacteria with epithelial cells. To investigate the repertoire of proteins secreted by Shigella flexneri in conditions of active secretion, we determined the N-terminal sequence of 14 proteins that are secreted by a mutant in which secretion was deregulated. Sequencing of the virulence plasmid pWR100 of the S. flexneri strain M90T (serotype 5) has allowed us to identify the genes encoding these secreted proteins and suggests that approximately 25 proteins are secreted by the type III secretion apparatus. Analysis of the G+C content and the relative positions of genes and open reading frames carried by the plasmid, together with information concerning the localization and function of encoded proteins, suggests that pWR100 contains blocks of genes of various origins, some of which were initially carried by four different plasmids.

Complex function for SicA, a Salmonella enterica serovar typhimurium type III secretion-associated chaperone

Salmonella enterica encodes a type III secretion system within a pathogenicity island located at centisome 63 that is essential for virulence. All type III secretion systems require the function of a family of low-molecular-weight proteins that aid the secretion process by acting as partitioning factors and/or secretion pilots. One such protein is SicA, which is encoded immediately upstream of the type III secreted proteins SipB and SipC. We found that the absence of SicA results in the degradation of both SipB and SipC. Interestingly, in the absence of SipC, SipB was not only stable but also secreted at wild-type levels in a sicA mutant background, indicating that SicA is not required for SipB secretion. We also found that SicA is capable of binding both SipB and SipC. These results are consistent with a SicA role as a partitioning factor for SipB and SipC, thereby preventing their premature association and degradation. We also found that introduction of a sicA null mutation results in the lack of expression of SopE, another type III-secreted protein. Such an effect was shown to be transcriptional. Introduction of a loss-of-function sipC mutation into the sicA mutant background rescued sopE expression. These results indicate that the effect of sicA on sopE expression is indirect and most likely exerted through a regulatory factor(s) partitioned by SicA from SipC. These studies therefore describe a surprisingly complex function for the Salmonella enterica type III secretion-associated chaperone SicA.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

Shigella flexneri YSH6000 induces two types of cell death, apoptosis and oncosis, in the differentiated human monoblastic cell line U937

Shigella flexneri, but not a non-invasive mutant derivative rapidly induced cell death in human monoblastic U937 cells as well as in differentiated cells pretreated with interferon-gamma (IFN gamma) or retinoic acid (RA). We investigated the morphological and biochemical characteristics of bacterial invasion-induced cell death in these differentiated U937 cells. IFN gamma-differentiated cells showed morphological changes typical of apoptosis and their DNA was cleaved giving a ladder-like electrophoretic pattern after infection by Shigellae. In contrast, swelling of the cytoplasm and blebbing of the plasma membrane were observed in RA-differentiated and undifferentiated cells invaded by the bacteria. No condensation of nuclei was observed in these cells by light microscopy, and no internucleosomal fragmentation of DNA was detected on agarose gels, which resembled the features of oncosis. Furthermore, cleavage of poly(ADP-ribose) polymerase, a substrate for apoptotic caspases, was seen only in IFN gamma-pretreated cells but not in RA-pretreated or undifferentiated cells. These findings suggested that virulent Shigella flexneri induces distinct types of cell death in U937 cells depending on their differentiation state.

Genes encoding putative effector proteins of the type III secretion system of Salmonella pathogenicity island 2 are required for bacterial virulence and proliferation in macrophages

The type III secretion system of Salmonella pathogenicity island 2 (SPI-2) is required for systemic infection of this pathogen in mice. Cloning and sequencing of a central region of SPI-2 revealed the presence of genes encoding putative chaperones and effector proteins of the secretion system. The predicted products of the sseB, sseC and sseD genes display weak but significant similarity to amino acid sequences of EspA, EspD and EspB, which are secreted by the type III secretion system encoded by the locus of enterocyte effacement of enteropathogenic Escherichia coli. The transcriptional activity of an sseA::luc fusion gene was shown to be dependent on ssrA, which is required for the expression of genes encoding components of the secretion system apparatus. Strains carrying nonpolar mutations in sseA, sseB or sseC were severely attenuated in virulence, strains carrying mutations in sseF or sseG were weakly attenuated, and a strain with a mutation in sseE had no detectable virulence defect. These phenotypes were reflected in the ability of mutant strains to grow within a variety of macrophage cell types: strains carrying mutations in sseA, sseB or sseC failed to accumulate, whereas the growth rates of strains carrying mutations in sseE, sseF or sseG were only modestly reduced. These data suggest that, in vivo, one of the functions of the SPI-2 secretion system is to enable intracellular bacterial proliferation.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Environmental regulation of Salmonella typhi invasion-defective mutants

Salmonella typhi is the etiologic agent of human typhoid. During infection, S. typhi adheres to and invades epithelial and M cells that line the distal ileum. To survive in the human host, S. typhi must overcome numerous complex extracellular and intracellular environments. Since relatively little is known about S. typhi pathogenesis, studies were initiated to identify S. typhi genes involved in the early steps of interaction with the host and to evaluate the environmental regulation of these genes. In the present study, TnphoA mutagenesis was used to study these early steps. We isolated 16 Salmonella typhi TnphoA mutants that were defective for both adherence and invasion of the human small intestinal epithelial cell line Int407. Twelve of sixteen mutations were identified in genes homologous to the S. typhimurium invG and prgH genes, which are known to be involved in the type III secretion pathway of virulence proteins. Two additional insertions were identified in genes sharing homology with the cpxA and damX genes from Escherichia coli K-12, and two uncharacterized invasion-deficient mutants were nonmotile. Gene expression of TnphoA fusions was examined in response to environmental stimuli. We found that the cpxA, invG, and prgH genes were induced when grown under conditions of high osmolarity (0.3 M NaCl). Expression of invG and prgH genes was optimal at pH 6.5 and strongly reduced at low pH (5.0). Transcription of both invG and prgH TnphoA gene fusions was initiated during the late logarithmic growth phase and was induced under anaerobic conditions. Finally, we show that both invG and prgH genes appear to be regulated by DNA supercoiling, a mechanism influenced by environmental factors. These results are the first to demonstrate that in S. typhi, (i) the prgH and cpxA genes are osmoregulated, (ii) the invG gene is induced under low oxygen conditions, (iii) the invG gene is pH regulated and growth phase dependent, and (iv) the prgH gene appears to be regulated by DNA supercoiling. Since our experimental conditions were designed to mimic the in vivo environmental milieu, our results suggest that specific environmental conditions act as signals to induce the expression of S. typhi invasion genes.

Immunogenicity of IpaC-hybrid proteins expressed in the Shigella flexneri 2a vaccine candidate SC602

We have investigated the capacity of live attenuated Shigella flexneri strains to act as vectors for the induction of local and systemic antibody responses against heterologous epitopes. The S. flexneri IpaC antigen was selected as a carrier protein into which the C3 neutralizing epitope of the poliovirus VP1 protein was inserted in eight sites distributed along IpaC. The resulting IpaC-C3 hybrid proteins were expressed from recombinant plasmids in the S. flexneri 2a vaccine candidate, SC602. Their production was similar to that of wild-type IpaC. All of the hybrid proteins but one were secreted as efficiently as wild-type IpaC. Immunization of mice with each of the recombinant SC602 derivatives reveals that one construct is able to induce serum and local anti-C3 antibodies, showing that at least one permissive site of insertion within IpaC can be defined. Furthermore, mouse-to-mouse variability in the anti-C3 response indicates that the amount of hybrid proteins produced in the host by SC602 should be improved for optimal use of S. flexneri live attenuated strains as mucosal vectors for foreign epitopes.

The modified nucleoside 2-methylthio-N6-isopentenyladenosine in tRNA of Shigella flexneri is required for expression of virulence genes

The virulence of the human pathogen Shigella flexneri is dependent on both chromosome- and large-virulence-plasmid-encoded genes. A kanamycin resistance cassette mutation in the miaA gene (miaA::Km Sma), which encodes the tRNA N6-isopentyladenosine (i6A37) synthetase and is involved in the first step of the synthesis of the modified nucleoside 2-methylthio-N6-isopentenyladenosine (ms2i6A), was transferred to the chromosome of S. flexneri 2a by phage P1 transduction. In the wild-type bacterium, ms2i6A37 is present in position 37 (next to and 3' of the anticodon) in a subset of tRNA species-reading codons starting with U (except tRNA(Ser) species SerI and SerV). The miaA::Km Sma mutant of S. flexneri accordingly lacked ms2i6A37 in its tRNA. In addition, the mutant strains showed reduced expression of the virulence-related genes ipaB, ipaC, ipaD, virG, and virF, accounting for sixfold-reduced contact hemolytic activity and a delayed response in the focus plaque assay. A cloned sequence resulting from PCR amplification of the wild-type Shigella chromosome and exhibiting 99% homology with the nucleotide sequence of the Escherichia coli miaA gene complemented the virulence-associated phenotypes as well as the level of the modified nucleoside ms2i6A in the tRNA of the miaA mutants. In the miaA mutant, the level of the virulence-associated protein VirF was reduced 10-fold compared with the wild type. However, the levels of virF mRNA were identical in the mutant and in the wild type. These findings suggest that a posttranscriptional mechanism influenced by the presence of the modified nucleoside ms2i6A in the tRNA is involved in the expression of the virF gene. The role of the miaA gene in the virulence of other Shigella species and in enteroinvasive E. coli was further generalized.

The invasion-associated type-III protein secretion system in Salmonella--a review

The genetic determinants that confer upon Salmonella the ability to enter non-phagocytic cells are largely encoded in a pathogenicity island located at centisome 63 of the bacterial chromosome. Molecular genetic analysis has revealed that this region encodes a specialized protein secretion system that mediates the export and/or translocation of putative signaling proteins into the host cell. This protein secretion system, which has been termed type III or contact-dependent, has also been identified in other plant and animal pathogens that have, in common, the ability to interact with eukaryotic host cells in an intimate manner.

Secretion of Shigella flexneri Ipa invasins on contact with epithelial cells and subsequent entry of the bacterium into cells are growth stage dependent

Upon contact with the surface of epithelial cells, Shigella flexneri secretes Ipa proteins through the Mxi-Spa type III secretion apparatus. Among the Ipa proteins, IpaB and IpaC form a soluble complex in the bacterial supernatant which appears to be sufficient to initiate the cellular rearrangements necessary to achieve bacterial entry. Here, we provide the first evidence that efficiency of bacterial entry into cells depends on the stage of bacterial growth. Bacteria in the early phase of exponential growth are six times more invasive than those in the stationary phase. The entry efficiency of the bacteria present on the cell surface appears to correlate with the percentage of those that are able to secrete their invasins. This suggests that the capacity to activate the Mxi-Spa apparatus is a major factor in the regulation of bacterial entry efficiency. Consistent with these observations, we have further shown that bacteria which have reached the stage of division secrete Ipa proteins more often than those that have not. Also, initial secretion occurs essentially in the area of the septation furrow. The Ipa proteins, secreted in the vicinity of the septation furrow, seem to initiate the early stages of reorganization of the host cell cytoskeleton.

Identification and characterization of phoN-Sf, a gene on the large plasmid of Shigella flexneri 2a encoding a nonspecific phosphatase

A gene encoding a nonspecific phosphatase, named PhoN-Sf, was identified on the large virulence plasmid (pMYSH6000) of Shigella flexneri 2a YSH6000. The phosphatase activity in YSH6000 was observed under high-phosphate conditions. However, it was found that low-phosphate conditions induced a slightly higher level of activity. The nucleotide sequence of the phoN-Sf region cloned from pMYSH6000 possessing the phoN-Sf gene encoded 249 amino acids with a typical signal sequence at the N terminus. The deduced amino acid sequence of the PhoN-Sf protein revealed significant homology to sequences of nonspecific acid phosphatases of other bacteria, such as Providencia stuartii (PhoN, 83.2%), Morganella morganii (PhoC, 80.6%), Salmonella typhimurium (PhoN, 47.8%), and Zymomonas mobilis (PhoC, 34.8%). The PhoN-Sf protein was purified, and its biochemical properties were characterized. The apparent molecular mass of the protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was calculated to be 27 kDa. The 20 amino acids at the N terminus corresponded to the 20 amino acid residues following the putative signal sequence of PhoN-Sf protein deduced from the nucleotide sequence. The PhoN-Sf activity had a pH optimum of 6.6, and the optimum temperature was 37 degrees C. The enzymatic activity was inhibited by diisopropyl fluorophosphate, N-bromosuccinimide, or dithiothreitol but not by EDTA. The subcellular localization of the PhoN-Sf protein in YSH6000 revealed that the protein was found predominantly in the periplasm. Examination of Shigella and enteroinvasive Escherichia coli strains for PhoN-Sf production by immunoblotting with the PhoN-specific antibody and for the presence of phoN-Sf DNA by using a phoN-Sf probe indicated that approximately one-half of the strains possessed the phoN-Sf gene on the large plasmid and expressed the PhoN-Sf protein. The Tn5 insertion mutants of YSH6000 possessing phoN-Sf::Tn5 still retained wild-type levels of invasiveness, as well as the subsequent spreading capacity in MK2 epithelial cell monolayers, thus suggesting that the PhoN-Sf activity is not involved in expression of the virulence phenotypes of Shigella strains under in vitro conditions.

Homologs of the Shigella IpaB and IpaC invasins are required for Salmonella typhimurium entry into cultured epithelial cells

Entry into host cells is an essential feature in the pathogenicity of Salmonella spp. The inv locus of Salmonella typhimurium encodes several proteins which are components of a type III protein secretion system required for these organisms to gain access to host cells. We report here the identification of several proteins whose secretion into the culture supernatant of S. typhimurium is dependent on the function of the inv-encoded translocation apparatus. Nucleotide sequence analysis of the genes encoding two of these secreted proteins, SipB and SipC, indicated that they are homologous to the Shigella sp. invasins IpaB and IpaC, respectively. An additional gene was identified, sicA, which encodes a protein homologous to IpgC, a Shigella protein that serves as a molecular chaperone for the invasins IpaB and IpaC. Nonpolar mutations in sicA, sipB, and sipC rendered S. typhimurium unable to enter cultured epithelial cells, indicating that these genes are required for bacterial internalization.

Enhanced secretion through the Shigella flexneri Mxi-Spa translocon leads to assembly of extracellular proteins into macromolecular structures

Genes required for entry of Shigella flexneri into epithelial cells in vitro are clustered in two adjacent loci, one of which encodes secretory proteins, the IpaA-D proteins, and the other their dedicated secretion apparatus, the Mxi-Spa translocon. Ipa secretion, which is induced upon contact of bacteria with epithelial cells, is prevented during growth in vitro. Here, we show that ipaB and ipaD mutations lead to enhanced secretion of a set of about 15 proteins. These extracellular proteins and some Ipas associate in organized structures consisting of extended sheets. Growth of the wild-type strain in the presence of Congo red is shown to induce protein secretion through the Mxi-Spa translocon. Cultures grown to stationary phase in the presence of Congo red contain extracellular filaments whose composition and morphology are similar to those produced by the hypersecreting ipaB and ipaD mutants.

Shigella flexneri-HeLa cell interactions: a putative role for host cell protein kinases

Epithelial cell invasion has been shown to be a prerequisite for Shigella flexneri virulence. Recently, we have documented the induction of transcription factor DNA binding activities as a result of S. flexneri challenge of HeLa cells. In this report, we show that HeLa cells challenged with S. flexneri display differences in phosphotyrosine-containing proteins. These changes are detected as early as 5 min post-challenge. Challenge with a noninvasive ipaB mutant strain resulted in the induction of a similar, but less intense, profile of phosphotyrosine-containing host cell proteins. Phosphotyrosine-containing proteins could be detected in S. flexneri, but were unique from those detected following HeLa cell challenge. S. flexneri invasion of HeLa cell monolayers was reduced by treatment with protein kinase inhibitors. These data suggest a role for protein kinases in the initial response of host cells to S. flexneri.

Extracellular association and cytoplasmic partitioning of the IpaB and IpaC invasins of S. flexneri

Shigella species cause bacillary dysentery in humans by invading colonic epithelial cells. IpaB and IpaC, two major invasins of these pathogens, are secreted into the extracellular milieu. We show here that IpaB and IpaC form a complex in the extracellular medium and that each binds independently to a 17 kDa polypeptide, IpgC, in the bacterial cytoplasm. The IpgC polypeptide was found to be necessary for bacterial entry into epithelial cells, to stabilize the otherwise unstable IpaB protein, and to prevent the proteolytic degradation of IpaC that occurs through its association with unprotected IpaB. We propose that IpgC, which is not secreted and thus acts as a molecular chaperone, serves as a receptor that prevents premature oligomerization of IpaB and IpaC within the cytoplasm of Shigella cells.

vacC, a virulence-associated chromosomal locus of Shigella flexneri, is homologous to tgt, a gene encoding tRNA-guanine transglycosylase (Tgt) of Escherichia coli K-12

The genetic determinants required for invasion of epithelial cells by Shigella flexneri and for the subsequent bacterial spreading are encoded by the large virulence plasmid. Expression of the virulence genes is under the control of various genes on the large plasmid as well as on the chromosome. We previously identified one of the virulence-associated loci near phoBR in the NotI-C fragment of the chromosome of S. flexneri 2a YSH6000 and designated the locus vacC. The vacC mutant showed decreased levels of IpaC, and IpaD proteins as well as transcription of ipa, an operon essential for bacterial invasion (N. Okada, C. Sasakawa, T. Tobe, M. Yamada, S. Nagai, K. A. Talukder, K. Komatsu, S. Kanegasaki, and M. Yoshikawa, Mol. Microbiol. 5:187-195, 1991). To elucidate the molecular nature of the vacC locus, we cloned the vacC region from YSH6000 on a 1.8-kb SalI-BamHI DNA fragment. The nucleotide sequence of the 1,822-bp vacC clone was highly (> 98%) homologous to the tgt region of Escherichia coli K-12, which is located at 9.3 min on the linkage map. Complementation tests indicated that the vacC function was encoded by an open reading frame expressing a 42.5-kDa protein, which corresponded to the tgt gene of E. coli K-12, coding for tRNA-guanine transglycosylase (Tgt) (K. Reuter, R. Slany, F. Ullrich, and H. Kersten, J. Bacteriol. 173:2256-2264, 1991). The cloned tgt gene from E. coli K-12 restored the virulence phenotype to the vacC mutant of YSH6000. Characterization of the vacC mutant indicated that levels of VirG, a protein essential for bacterial spreading, and VirF, the positive regulator for the expression of the virG and ipaBCD operons, decreased significantly compared with those of the wild type. Similar phenotypic changes occurred in vacC mutants constructed by insertion of a neomycin resistance gene in shigellae and enteroinvasive E. coli strains, consistent with the hypothesis that the vacC (tgt) gene contributes to the pathogenicity of Shigella flexneri.

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.

Shigella flexneri invasion of HeLa cells induces NF-kappa B DNA-binding activity

Although information about the genetic basis and mechanisms of Shigella flexneri cellular invasion is accumulating, little is known about changes in cell signaling and their consequences following bacterium-host cell interactions. A general result of signal transduction is alterations in the levels and/or activities of transcription factors. Alterations in transcription factor binding activities were observed following challenge with S. flexneri. Changes in the DNA-binding activities of cellular transcription factors to AP1, AP2, cyclic AMP response element, CTF1/NF1, NF-kappa B/Rel, OCT1, and SP1 DNA-binding sites were investigated by electrophoretic mobility shift assays. NF-kappa B/Rel DNA-binding activity was enhanced more than 11-fold by cellular invasion; noninvasive S. flexneri strains induced low levels of kappa B DNA binding. Both subunits of the NF-kappa B transcription factor, p50 and p65, but not c-Rel (p85), are components of the kappa B DNA-binding activity. These data suggest that changes in cellular transcription factor binding activity are a consequence of S. flexneri invasion, and these changes could play a role in the initial host response or in the pathogenesis of the disease.

Characterization of B-cell epitopes on IpaB, an invasion-associated antigen of Shigella flexneri: identification of an immunodominant domain recognized during natural infection

The invasion plasmid antigen B (IpaB), a 62-kDa plasmid-encoded protein associated with the ability of shigellae to invade epithelial cells, is the bacterial antigen most strongly and consistently recognized by the host during infection. The strong systemic and mucosal immune responses observed against this invasin prompted us to map its B-cell epitopes. For this purpose, IpaB was first overexpressed in Shigella flexneri and used to raise rabbit polyclonal antiserum and murine monoclonal antibodies, which were subsequently used to screen a lambda gt11 ipaB library. Inserts of recombinant DNA clones that were specifically recognized by the antisera and antibodies were sequenced, and three distinct determinants were identified. Further characterization of these determinants showed that they were recognized by sera from patients convalescent from shigellosis, suggesting that they are relevant to the humoral response during natural infection. Moreover, the IpaB region comprising the three determinants was systematically recognized by all sera from infected patients that we tested, whereas other regions of the protein were not. These data suggest that this region, located between amino acid residues 147 and 258, is the major immunogenic domain of the invasin in the course of natural infection.

Characterization of virulence genes of enteroinvasive Escherichia coli by TnphoA mutagenesis: identification of invX, a gene required for entry into HEp-2 cells

While enteroinvasive Escherichia coli (EIEC) and shigellae are genotypically nearly identical, a difference has been reported in the infective dose to humans: EIEC is 10,000-fold less infectious than shigellae. A possible basis for this difference lies in the inherent invasiveness of these bacteria toward epithelial cells. Thus, despite the high degree of homology between the invasion plasmids of EIEC and shigellae, substantial differences in genetic organization and/or sequence may exist. We have undertaken a systematic genetic analysis of the EIEC plasmid pSF204, using transposon mutagenesis. Congo red-negative TnphoA insertion mutants (Pcr- PhoA-) and TnphoA fusion mutants (PhoA+) were isolated and screened for the ability to invade cultured HEp-2 cells. Most invasion-negative (Inv-) mutations mapped to a 30-kb segment of the invasion plasmid, including homologs of the Shigella flexneri ipa, mxi, and spa genes. Inv- PhoA+ fusions in the EIEC ipaC, mxiG, mxiJ, mxiM, and mxiD homologs and in a proposed new gene, named invX, located downstream of the spa region were identified and characterized. This analysis indicates the presence of the ipaC, mxiG, mxiJ, mxiM, mxiD, and invX gene products in the EIEC cell envelope and demonstrates a strict requirement for these genetic loci in invasion. Overall, our results suggest a high degree of genetic, structural, and functional homology between the EIEC and S. flexneri large invasion plasmids.

Characterization of the Shigella flexneri ipgD and ipgF genes, which are located in the proximal part of the mxi locus

The Shigella flexneri invasion process requires the synthesis of the Ipa proteins and their secretion by specific factors encoded by the mxi and spa genes, which are clustered upstream from the ipa operon. We report here the characterization of the ipgD, ipgE, and ipgF genes, which are located in the 5' end of the mxi locus. Analysis of IpgF-PhoA fusions endowed with high levels of alkaline phosphatase activity confirmed the functionality of a classical signal sequence detected in the sequence of IpgF. The ipgD and ipgF genes were each inactivated on the large virulence plasmid by insertion of a nonpolar cassette; each of the ipgD and ipgF mutants thus constructed showed the same invasive phenotype as the wild-type strain and was able to provoke keratoconjunctivitis in guinea pigs. It thus appears that two genes located at the ipa-proximal part of the mxi locus are not directly involved in invasion. Analysis of concentrated culture supernatants of the wild-type and ipgD strains indicated that secretion of one polypeptide, whose size was consistent with that predicted for the IpgD protein (60 kDa), was abolished in the ipgD mutant.

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.

MxiD, an outer membrane protein necessary for the secretion of the Shigella flexneri lpa invasins

The invasive phenotype of Shigella flexneri is conferred by a 220 kb virulence plasmid, pWR100, that encodes both the lpa proteins, which are involved in the entry process, and factors which are required for the export and correct localization of the lpa proteins. We have characterized the mxiD gene, whose expression, like that of the ipa operon, is regulated by temperature. After inactivation of mxiD, the mutant strain was unable to invade HeLa cells and to provoke keratoconjunctivitis in guinea-pigs. Analysis of culture supernatants indicated that wild-type S. flexneri secretes about nine polypeptides and that secretion of several of these, including lpaA, lpaB, and lpaC, is abolished in the mxiD mutant. Examination of the membrane proteins of the wild-type and mxiD strains suggested that MxiD is an outer membrane protein. Amino acid sequence comparison revealed that MxiD is homologous to the YscC protein of Yersinia enterocolitica and to the C-terminal region of the PulD protein of Klebsiella pneumoniae. Both YscC and PulD are involved in extracellular protein secretion. These results indicate that MxiD is an essential component of the lpa secretion apparatus.

YopB and YopD constitute a novel class of Yersinia Yop proteins

Virulent Yersinia species harbor a common plasmid that encodes essential virulence determinants (Yersinia outer proteins [Yops]), which are regulated by the extracellular stimuli Ca2+ and temperature. The V-antigen-encoding operon has been shown to be involved in the Ca(2+)-regulated negative pathway. The genetic organization of the V-antigen operon and the sequence of the lcrGVH genes were recently presented. The V-antigen operon was shown to be a polycistronic operon having the gene order lcrGVH-yopBD (T. Bergman, S. Håkansson, A. Forsberg, L. Norlander, A. Macellaro, A. Bäckman, I. Bölin, and H. Wolf-Watz, J. Bacteriol. 173:1607-1616, 1991; S. B. Price, K. Y. Leung, S. S. Barve, and S. C. Straley, J. Bacteriol. 171:5646-5653, 1989). We present here the sequence of the distal part of the V-antigen operons of Yersinia pseudotuberculosis and Yersinia enterocolitica. The sequence information encompasses the yopB and yopD genes and a downstream region in both species. We conclude that the V-antigen operon ends with the yopD gene. This conclusion is strengthened by the observation of an insertion-like element downstream of the yopD gene. The translational start codons of YopB and YopD have been identified by N-terminal amino acid sequencing. By computer analysis, the yopB and yopD gene products were found to be possible transmembrane proteins, and YopD was shown to contain an amphipathic alpha-helix in its carboxy terminus. These findings contrast with the general globular pattern observed for other Yops. Homology between Yersinia LcrH and Shigella flexneri IppI and between Yersinia YopB and S. flexneri IpaB was found, suggesting conservation of this locus between these two genera. YopB was also found to have a moderate level of homology, especially within the hydrophobic regions, to members of the RTX protein family of alpha-hemolysins and leukotoxins, indicating that YopB might exhibit a similar function.

MxiJ, a lipoprotein involved in secretion of Shigella Ipa invasins, is homologous to YscJ, a secretion factor of the Yersinia Yop proteins

Shigella flexneri causes bacillary dysentery by invading epithelial cells of the colonic mucosa. The invasion process requires the synthesis and secretion of the virulence plasmid-encoded Ipa proteins. Using TnphoA mutagenesis, we have identified two virulence plasmid genes, mxiJ and mxiM, that encode proteins exported by the general export pathway. Analysis of the MxiJ and MxiM deduced amino acid sequences suggested that mxiJ and mxiM might encode lipoproteins, which was confirmed by [3H]palmitate labeling of MxiJ:PhoA and MxiM:PhoA fusion proteins. A mxiJ mutant was unable to invade HeLa cells, to induce the formation of plaques on confluent monolayers of HeLa cells, and to provoke keratoconjunctivitis in guinea pigs. In addition, secretion of seven polypeptides, including IpaA, IpaB, and IpaC, was abolished in the mxiJ mutant. Sequence comparisons indicated that MxiJ and MxiH, which is encoded by a gene located upstream from mxiJ, are homologous to the Yersinia enterocolitica YscJ and YscF proteins, respectively.

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.

icsB: a Shigella flexneri virulence gene necessary for the lysis of protrusions during intercellular spread

Shigella flexneri causes bacillary dysentery by invading epithelial cells of the colonic mucosa. We have characterized the icsB gene which is located on the virulence plasmid pWR100. After inactivation of icsB, the mutant strain remained invasive, but formed abnormally small plaques on HeLa cell monolayers, colonized only the peripheral cells of Caco-2 islets, and was unable to provoke a keratoconjunctivitis in guinea-pigs. Examination of infected HeLa cells showed that the icsB mutant was able to lyse the phagocytic vacuole and to form protrusions at the surface of infected cells, but, unlike the wild type, remained trapped in protrusions surrounded by two membranes. These results indicate that IcsB is involved in the lysis of the protrusions, a step necessary for intercellular spread.

Identification and characterization of B-cell epitopes of IpaC, an invasion-associated protein of Shigella flexneri

Invasion plasmid antigen C (IpaC) is a 43-kDa plasmid-encoded protein associated with the ability of shigellae to invade epithelial cells. This protein is consistently strongly recognized by sera from convalescent patients and monkeys experimentally infected with shigellae. The strong immunogenicity of IpaC in the course of natural infection makes it a good candidate as a potentially protective antigen. To map the B-cell epitopes of this protein, the gene encoding IpaC was cloned and expressed at a high level in Escherichia coli. The partially purified recombinant protein was used to raise rabbit polyclonal antisera and murine monoclonal antibodies. A lambda gt11 ipaC gene library was screened with the antisera and antibodies. Recombinant DNA clones producing specific antigenic determinants were isolated, and the sequence of their DNA inserts was determined. The amino acid sequence of each determinant was deduced from the minimal overlap of DNA inserts of multiple antibody-positive DNA clones. Two distinct epitopes, located between amino acid residues 25 and 33 and 90 and 97, were identified. Two additional B-cell epitopes which were located between residues 297 and 349, near the carboxy-terminal end of the protein, were characterized. Each of these epitopes was also recognized by sera from convalescent humans and monkeys. Therefore, it seems likely that these epitopes are relevant to the humoral response against IpaC during natural infection.

Nucleotide sequence and transcriptional regulation of a positive regulatory gene of Shigella dysenteriae

A 1,937 bp PstI-HindIII fragment containing the ipaR locus was cloned from the large invasion plasmid of Shigella dysenteriae CG097, and its nucleotide sequence was completely determined. The IpaR protein (35 kDa, calculated from the DNA sequence) was synthesized in Escherichia coli chi 1411 minicells containing the 1,937-bp PstI-HindIII fragment. To determine the regulatory role of ipaR for ipa genes, we applied genetic complementation experiments using chloramphenicol acetyltransferase (CAT) as reporter. Analyses of CAT activity of the recombinant plasmids containing the 5' flanking sequences of the 24-kDa-protein gene and the ippI, ipaB, ipaC, and ipaD genes defined strong promoters upstream of the 24-kDa-protein gene and ipaD gene, weak promoters upstream of the ippI and ipaB genes, and the absence of any promoter activity for the ipaC gene. Complementation analyses showed that the CAT activity only under direction of the ippI promoter region increased 1.8-fold in the presence of IpaR protein. On the basis of our data, we suggest that an operon comprising ippI, ipaB, and ipaC is positively regulated by IpaR protein which has a trans effect on a DNA sequence upstream of the ippI promoter.

Surface presentation of Shigella flexneri invasion plasmid antigens requires the products of the spa locus

An avirulent, invasion plasmid insertion mutant of Shigella flexneri 5 (pHS1059) was restored to the virulence phenotype by transformation with a partial HindIII library of the wild-type invasion plasmid constructed in pBR322. Western immunoblot analysis of pHS1059 whole-cell lysates revealed that the synthesis of the invasion plasmid antigens VirG, IpaA, IpaB, IpaC, and IpaD was similar to that seen in the corresponding isogenic S. flexneri 5 virulent strain, M90T. IpaB and IpaC, however, were not present on the surface of pHS1059 as was found in M90T, suggesting that the transport or presentation of the IpaB and IpaC proteins onto the bacterial surface was defective in the mutant. pHS1059 was complemented by pWR266, which carried contiguous 1.2- and 4.1-kb HindIII fragments of the invasion plasmid. pHS1059(pWR266) cells were positive in the HeLa cell invasion assay as well as colony immunoblot and enzyme-linked immunosorbent assays, using monoclonal antibodies to IpaB and IpaC. These studies established that the antigens were expressed on the surface of the transformed bacteria. In addition, water extraction of pHS1059 and pHS1059(pWR266) whole cells, which can be used to remove IpaB and IpaC antigens from the surface of wild-type M90T bacteria, yielded significant amounts of these antigens from pHS1059(pWR266) but not from pHS1059. Minicell and DNA sequence analysis indicated that several proteins were encoded by pWR266, comprising the spa loci, which were mapped to a region approximately 18 kb upstream of the ipaBCDAR gene cluster. Subcloning and deletion analysis revealed that more than one protein was involved in complementing the Spa- phenotype in pHS1059. One of these proteins, Spa47, showed striking homology to ORF4 of the Bacillus subtilis flaA locus and the fliI gene sequence of Salmonella typhimurium, both of which bear strong resemblance to the alpha and beta subunits of bacterial, mitochondrial, and chloroplast proton-translocating F0F1 ATPases.

Nucleotide sequence of the ipaBCD structural genes of Shigella dysenteriae

A 9 kb EcoRI and two PstI fragments from the virulence plasmid of Shigella dysenteriae CG097 were shown to contain all ipa genes by probing with Shigella flexneri ipaB, -C, -D and -A gene probes. The DNA sequences of S. dysenteriae ipaBC genes were very similar to those of S. flexneri M90T and S. flexneri YSH6000, but ipaD differed by 22 codons from that of S. flexneri. The differences in ipaD may account for the different in vitro host specificities shown by S. dysenteriae and S. flexneri. The nucleotide composition of ipa genes revealed an unusually large number of codons that are rarely used in Escherichia coli chromosomal genes, indicating a different origin.

Two novel virulence loci, mxiA and mxiB, in Shigella flexneri 2a facilitate excretion of invasion plasmid antigens

A bank of over 4,200 lacZ protein fusions in Shigella flexneri 2a was screened for fusions to temperature-regulated promoters. One mutant, BS260, was completely noninvasive on HeLa cells and mapped to a region on the 220-kb virulence plasmid in which we had previously localized several avirulent temperature-regulated operon fusions (A.E. Hromockyj and A.T. Maurelli, Infect. Immun. 57:2963-2970, 1989). The phenotype of BS260 was similar to that of the previously identified mxi (membrane expression of invasion plasmid antigens) mutants, since it made wild-type intracellular levels of the invasion plasmid antigens (Ipa) but was deficient in the surface expression of IpaB and IpaC. Six kilobases of DNA upstream of the BS260 fusion end joint were cloned, but no temperature-regulated promoter was found, whereas the fusion end joint clone of the noninvasive mxi operon fusion mutant BS226 contained a temperature-regulated promoter. The locus defined by BS260 was designated mxiA, and that defined by BS226 was designated mxiB. Closer analysis of the mxiA and mxiB phenotypes by a cell-free enzyme-linked immunosorbent assay revealed that the mutants failed to excrete IpaB and IpaC into the culture medium, whereas wild-type cells actively released these antigens. Excretion of the ipa polypeptides from wild-type bacteria was confirmed by Western blot analysis of culture supernatants. Protease protection experiments revealed that wild-type S. flexneri 2a actually had much lower levels of surface-exposed IpaB and IpaC relative to those in the total antigen pool. In addition, examination of cellular fractions showed that, although there was no IpaB or IpaC in the outer membrane of BS260 and BS226, the antigens did accumulate in the cytoplasmic membrane. A 76-kDa temperature-regulated polypeptide in wild-type S. flexneri was identified as the putative mxiA gene product. These results strongly suggest that IpaB and IpaC represent truly excreted proteins of S. flexneri and that the mxiA and mxiB loci on the plasmid code for accessory proteins required to facilitate their export through the bacterial outer membrane. These data also suggest that mxiA is part of an operon that specifies additional mxi genes. The products of this operon may constitute a unique multicomponent protein secretion apparatus involved in the transport of Shigella virulence determinants.

Genetic basis of virulence in Shigella species

Shigella species and enteroinvasive strains of Escherichia coli cause disease by invasion of the colonic epithelium, and this invasive phenotype is mediated by genes carried on 180- to 240-kb plasmids. In addition, at least eight loci on the Shigella chromosome are necessary for full expression of virulence. The products of these genes can be classified as (i) virulence determinants that directly affect the ability of shigellae to survive in the intestinal tissues, e.g., the aerobactin siderophore (iucABCD and iutA), superoxide dismutase (sodB), and somatic antigen expression (rfa and rfb); (ii) cytotoxins that contribute to the severity of disease, e.g., the Shiga toxin (stx) and a putative analog of this toxin (flu); and (iii) regulatory loci that affect the expression of plasmid genes, e.g., ompR-envZ, which mediates response to changes in osmolarity, virR (osmZ), which mediates response to changes in temperature, and kcpA, which affects the translation of the plasmid virG (icsA) gene which is associated with intracellular bacterial mobility and intracellular bacterial spread. A single plasmid regulatory gene (virF) controls a virulence-associated plasmid regulon including virG (icsA) and two invasion-related loci, i.e., (i) ipaABCD, encoding invasion plasmid antigens that may be structural components of the Shigella invasion determinant; and (ii) invAKJH (mxi), which is necessary for insertion of invasion plasmid antigens into the outer membrane.

Localization of IpaB protein in Escherichia coli K-12 MC1061 strain carrying Shigella sonnei form I plasmid pSS120

We partially purified antigens which reacted with shigellosis convalescent-monkey antisera. Hybridomas, which were constructed from mice immunized by the antigens, produced monoclonal antibodies recognizing IpaB protein. Using the monoclonal antibody against IpaB, evidence indicating that IpaB proteins were localized on the cell surface of invasive bacteria (Escherichia coli K-12 MC1061 harboring pSS120 plasmid) was obtained by immunoelectron microscopy.

Molecular analysis of variant plasmid forms of a bivalent Salmonella typhi-Shigella sonnei vaccine strain

The Salmonella typhi-Shigella sonnei hybrid vaccine strain 5076-IC was constructed to express the S. sonnei form I antigen, which may play an important role in producing protective immunity. Three clonal variants which existed in preparations of the vaccine could be distinguished phenotypically by lactose utilization, S. sonnei form I antigen expression, and restriction enzyme analysis of large plasmid DNA. Since expression of the form I antigen was lost in two of the clonal variants, genetic instability of the 120-MDa vaccine plasmid appeared to be a potential problem. To examine the molecular basis of this genetic instability, we hybridized large plasmid DNA isolated from the clonal variants to a variety of DNA probes encoding virulence-associated antigens and to Escherichia coli lacZ DNA. Results indicated that DNA rearrangement accompanied by deletions of plasmid material occurred in the vaccine plasmid. In addition, the vaccine plasmid did not contain some S. sonnei genetic material encoding antigenic polypeptides necessary for virulence.