Virulence genes regulated at the transcriptional level by Ca2+ in Yersinia pestis include structural genes for outer membrane proteins

Yersinia enterocolitica O:9 as a potential live oral carrier for protective antigens

Yersinia enterocolitica has the capacity to invade the intestinal tissue and to resist the primary host resistance. The former is chromosome coded while the second largely depends on the presence of a 70 kb plasmid called pYV. This plasmid directs the conditional synthesis of high amounts of proteins (YOPs) that are secreted and inserted in the outer membrane. In order to evaluate Y. enterocolitica W22703 as a potential live carrier for immunization, three strains expressing beta-galactosidase (GZ), were tested for their ability to induce an antibody response to this antigen in mice. The first strain contained plasmid pGC1256, a mutated pYV plasmid containing lacZ transcribed from a yop gene promoter. This strain produced high amounts of GZ instead of a YOP protein and was shown to be hypovirulent. The other strains tested were W22703 pYV+ and pYV- containing a derepressed lac operon carried on an independent plasmid. Immunoblot analysis of sera of mice having received by oral inoculation, W22703(pGC1256) or the pYV+ GZ producing strain revealed the presence of antibodies to GZ. The response to GZ after inoculation of W22703(pGC1256) was shown by ELISA to be only slightly inferior to that obtained by subcutaneous injection of GZ. No response was obtained after oral inoculation of the pYV-GZ producing strain. This showed that the presence of pYV was necessary to obtain an antibody response in this system.

A novel suicide vector and its use in construction of insertion mutations: osmoregulation of outer membrane proteins and virulence determinants in Vibrio cholerae requires toxR

The toxR gene of Vibrio cholerae encodes a transmembrane, DNA-binding protein that activates transcription of the cholera toxin operon and a gene (tcpA) for the major subunit of a pilus colonization factor. We constructed site-directed insertion mutations in the toxR gene by a novel method employing the chromosomal integration of a mobilizable suicide plasmid containing a portion of the toxR coding sequence. Mutants containing these new toxR alleles had an altered outer membrane protein profile, suggesting that two major outer membrane proteins (OmpT and OmpU) might be under the control of toxR. Physiological studies indicated that varying the concentration of the amino acids asparagine, arginine, glutamate, and serine caused coordinate changes in the expression of cholera toxin, TcpA, OmpT, and OmpU. Changes in the osmolarity of a tryptone-based medium also produced coordinate changes in the expression of these proteins. Other environmental signals (temperature and pH) had a more pronounced effect on the expression of cholera toxin and TcpA than they did on the outer membrane proteins. These results suggest that certain environmental signals (i.e., osmolarity and the presence of amino acids) are tightly coupled to the expression of toxR-regulated proteins and therefore may be signals that are directly sensed by the ToxR protein.

The plasmid-encoded Yop2b protein of Yersinia pseudotuberculosis is a virulence determinant regulated by calcium and temperature at the level of transcription

The basic Yop2b protein, encoded by the virulence plasmid pIBI of Yersinia pseudotuberculosis, is produced under Ca2+-deficient conditions. A mutant deleted for the entire yopH gene, which encodes the Yop2b protein, was found to be avirulent. Virulence could be restored by trans-complementation. The DNA-sequence of yopH predicted a 50 737 D polypeptide lacking a typical signal peptide. Transcription of yopH is regulated by both temperature and Ca2+-concentration. Mutations within the region of the virulence plasmid known to be involved in regulating gene expression in response to Ca2+ abolished transcription of yopH. Other temperature-sensitive mutations in the Ca2+-regulatory locus showed a high level of transcription regardless of Ca2+-concentration. These responses were similar to those of the yopE gene. The promoter region of the yopE and yopH genes were compared and four conserved motifs identified.

Identification and mapping of the temperature-inducible, plasmid-encoded proteins of Yersinia spp

The structural genes of the outer membrane polypeptides of Yersinia spp. (YOPs) and the V antigen of plasmid pIB1 of Yersinia pseudotuberculosis were recently cloned and mapped (A. Forsberg, I. Bölin, L. Norlander, and H. Wolf-Watz, Microb. Pathogen. 2:123-137, 1987). The corresponding genes were localized on pYV019 and pYV8081 of Yersinia pestis and Yersinia enterocolitica, respectively. No obvious differences were observed on comparison of pIB1 and pYV019, whereas pYV8081 showed intragenic as well as extragenic changes. However, one region of plasmid pYV8081, which coded for the V antigen, YOP3, and YOP4a, was essentially conserved among the three plasmids. Since this region is connected with the Ca2+ region, we suggest that the conserved region of the virulence plasmids of Yersinia spp. be extended to include both of these regions. Low amounts of the YOPs were detected in the membrane fraction at 37 degrees C in the presence of 2.5 mM calcium. Only minor differences were noticed when the individual YOPs of Y. pestis and Y. pseudotuberculosis were compared. Several differences were observed when the YOPs of Y. enterocolitica were included for comparison. All Y. enterocolitica proteins, except YOP1, YOP4b, and the V antigen, exhibited changes in their characteristic molecular sizes. Although these differences were within a range of +/- 2 kilodaltons, the isoelectric point was retained for each protein type.

Iron regulation of Shiga-like toxin expression in Escherichia coli is mediated by the fur locus

Shiga-like toxin is an iron-regulated cytotoxin quite similar to Shiga toxin from Shigella dysenteriae 1. The structural genes for Shiga-like toxin in Escherichia coli (sltA and sltB) appear to be transcribed as an operon from a promoter upstream of sltA. We used a gene fusion between the promoter and proximal portion of sltA with the gene for bacterial alkaline phosphatase to assess the regulation of toxin expression. Growth in low-iron conditions resulted in a 13- to 16-fold increase in alkaline phosphatase activity. In the presence of a null mutation in the fur locus, however, alkaline phosphatase activity was constitutively high regardless of the iron concentration. These data indicate negative regulation of the slt operon by the fur gene product. We used deletion analysis of the region upstream of the gene fusion to localize the promoter of the slt operon and to show that a region of DNA between the -35 and -10 boxes is necessary for iron regulation of slt expression. In this region, there is a 21-base-pair dyad repeat that is homologous to similar dyads in the promoter regions of three other fur-regulated genes. This region of dyad symmetry may represent an operator binding site for the Fur protein in the presence of iron.

Post-translational regulation of Lcr plasmid-mediated peptides in pesticinogenic Yersinia pestis

The low calcium response of wild type Yersinia pestis, the causative agent of bubonic plague, and of enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica is known to be mediated by a shared Lcr plasmid of about 70 kb. At 37 degrees C in Ca2+-deficient medium, this element promotes restriction of growth with concomitant production of virulence functions including the common V antigen and a set of yersiniae outer membrane peptides termed YOPs (Lcr+). The latter are expressed by the enteropathogenic species but not by wild type Y. pestis which possesses a unique 10 kb Pst plasmid associated with pesticinogeny (Pst+). We show in this report that, after pulse with 35S-methionine, peptides with molecular weights corresponding to YOPs of 78, 47, 45, 44, 36, and 26 kDa are synthesized during the low calcium response by both Lcr+, Pst+ and Lcr+, Pst- cells of Y. pestis. Although stable in the latter, radioactivity in YOPs of wild type was rapidly chased into lower molecular weight degradation products. At least four soluble peptides, including V, were also labeled during starvation for Ca2+; these structures were stable in both Lcr+, Pst+ and Lcr+, Pst- yersiniae. These findings suggest that a product encoded by the Pst plasmid of Y. pestis is required for post-translational regulation of outer membrane but not soluble peptides mediated by a second unrelated Lcr plasmid.

Plasmid-mediated surface fibrillae of Yersinia pseudotuberculosis and Yersinia enterocolitica: relationship to the outer membrane protein YOP1 and possible importance for pathogenesis

The cell surface properties of Yersinia pseudotuberculosis and Yersinia enterocolitica mutants, constructed by insertional inactivation of genes located on the 40- to 50-megadalton virulence plasmid, were examined. Electron microscopy revealed an absolute correlation between expression of four plasmid-dependent, temperature-inducible properties related to the bacterial surface: (i) a fibrillar matrix covering the outer membrane, (ii) outer membrane protein YOP1, (iii) spontaneous autoagglutination, and (iv) mannose-resistant hemagglutination of guinea pig erythrocytes. Immunoelectron microscopy indicated that YOP1 is a structural component of the fibrillae. Experiments demonstrating inhibition of hemagglutination by anti-YOP1 monoclonal antibody suggested a potential role for YOP1 in adhesion. Insertional inactivation of the gene coding for YOP1, with resultant loss of the ability to express fibrillae, led to a significant reduction in the capacity of Y. enterocolitica, but not Y. pseudotuberculosis, to colonize the ileum of orogastrically infected mice. In both Y. enterocolitica and Y. pseudotuberculosis, inactivation of the genes coding for Ca2+ dependency reduced the ability to maintain intestinal colonization, regardless of the ability to express fibrillae. Both surface fibrillae and Ca2+ dependency seem to reflect pathogenic determinants which are required for the establishment of Y. enterocolitica infection. In Y. pseudotuberculosis, however, no clinical significance of the fibrillae has so far been defined.

Identification of invasin: a protein that allows enteric bacteria to penetrate cultured mammalian cells

Bacterial strains harboring the Yersinia pseudotuberculosis inv locus were analyzed in order to investigate the mechanism of host cell penetration by an invasive pathogen. The inv locus was found to be necessary for Y. pseudotuberculosis to enter HEp-2 cells and sufficient to convert E. coli into a microorganism able to penetrate cultured cells. Both E. coli and Y. pseudotuberculosis strains harboring inv mutations were defective for entry into HEp-2 cells. Furthermore, molecular clones containing inv, and little additional DNA, converted E. coli into a microorganism that was indistinguishable from the parental Yersinia strain with regard to the entry of cultured cells. Data from in vitro protein synthesis indicated that a 103 kd protein was synthesized from inv, saturating the coding capacity of the locus. The nucleotide sequence shows an open reading frame corresponding to a protein of similar size. This protein, called invasin, is necessary for the microorganisms to penetrate HEp-2 cells, and is compartmentalized on the outer surface of the bacterium.

A new method for the physical and genetic mapping of large plasmids: application to the localisation of the virulence determinants on the 90 kb plasmid of Salmonella typhimurium

A new method based on transposon-promoted deletions was used to generate a set of deletions in the 90 kb virulence plasmid of Salmonella typhimurium. The analysis of 16 deletion mutants allowed: (1) construction of the restriction map of the plasmid for HindIII, BamHI and BglII; (2) localisation of the plasmid region involved in virulence; (3) identification of two functional replicons on the plasmid.

Modulation of the low-calcium response in Yersinia pestis via plasmid-plasmid interaction

Virulent cells of Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica are known to exhibit a low-calcium response in vitro characterized by restriction of growth and induction of V antigen at 37 degrees C in Ca2+-deficient media (Lcr+). A shared Lcr plasmid mediates these properties and encodes yersiniae outer membrane peptides (Yops) that are expressed in Lcr+ Y. pseudotuberculosis and Y. enterocolitica but not Y. pestis. We present direct evidence here verifying that synthesis of major Yops in the former two species is, like V, repressed by Ca2+ and that these structures are located primarily in the outer membrane. We also verified that rabbits infected with live Lcr+ Y. pestis can raise antibodies against V and Yops. Similar antisera, however, were recovered after immunization with sterile extracts of Ca2+-starved Lcr+ cells of Y. pestis. Results of immunoblots obtained with these antisera showed that restricted Y. pestis produced Yops of about 46 kDa (YopB) and 44 kDa (YopC) after shiftup by addition of Ca2+ at 37 degrees C or reduction of temperature to 26 degrees C. It is established that virulent cells of Y. pestis also possess a unique plasmid known to mediate pesticinogeny (Pst+). Restricted Lcr+, Pst- Y. pestis expressed YopB and YopC plus additional 76 kDa (YopF), 48 kDa (YopH), 36 kDa (YopD), 32.5 kDa (YopJ), and 27 kDa (YopE) outer membrane structures at concentrations comparable to those in Ca2+-starved Y. pseudotuberculosis and Y. enterocolitica. These findings indicate that carriage of the Pst plasmid prevents expression of the Lcr plasmid-mediated Yops in wild type Y. pestis.

Transcription of the yop regulon from Y. enterocolitica requires trans acting pYV and chromosomal genes

Virulent Y. enterocolitica strains restrict their growth at 37 degrees C, in rich medium deprived of calcium. This property, called calcium dependency, correlates with pathogenicity. It is conditioned by a 70 kb plasmid called pYV. The analysis of calcium independent (Cl) insertion mutants defined a 20 kb region called the calcium region. This region contains at least three transcription units called virA, B and C. In growth restriction conditions, Y. enterocolitica releases and inserts in its outer-membrane several pYV encoded proteins (POMPs). By transposition mutagenesis using a mini-Mu dlac element, we localized plasmid genes encoding POMPs of 84, 51, 30 and 25 kDa. These genes appear to be scattered on pYV, outside the calcium region and to constitute a regulon. Transcription of these yop genes increased by factors of 3.5 (yop84) to 200 (yop51) when temperature was shifted from 25 to 37 degrees C. On the contrary, calcium had only a moderate effect. Transcription was also dependent on the culture medium and on a trans-acting factor encoded by the calcium region. One Cl double mutant severely affected in transcription of yop51 defined a new locus called virF in the calcium region of pYV. Transcription of the yop genes was very poor in E. coli K12, even in the presence of a fully functional calcium region.

Molecular cloning and expression of calcium-regulated, plasmid-coded proteins of Y. pseudotuberculosis

A number of plasmid-associated proteins (YOPs) of Y. pseudotuberculosis are induced and expressed at high levels when the pathogen is grown at 37 degrees C in absence of Ca2+ ions. These proteins were recovered both from the outer membrane fraction and the culture supernatant. Two hours after a temperature-shift the YOPs were only found in the culture supernatant, amounting to about 5% of the total cell protein. After 4 h of incubation they were also detected in the outer membrane fraction. Separation by 2-D gel electrophoresis revealed that the YOPs could be separated into 6 different polypeptides; YOP2a (45 kDa), YOP2b (45 kDa), YOP3 (41-42 kDa), YOP4a (34 kDa), YOP4b (34 kDa) and YOP5 (26 kDa). The structural genes of all of these YOPs, except the YOP2a gene, were cloned to pBR322 and their respective genetic localization was established. It was found that the genes were not part of a common operon but scattered around plasmid plB1. Only the YOP4b protein was found to map within the Ca2+ region. The hybrid plasmid plB572 coded for a number of plasmid plB1 specific proteins, one of which showed a molecular weight of 38 kDa. This polypeptide could be precipitated by monospecific V-antiserum, showing that this protein is the V-antigen.

Proteolysis of V antigen from Yersinia pestis

Lcr-plasmids of yersiniae are known to mediate a unique low calcium response characterised by restriction of growth in vitro with induction of putative virulence factors including yersiniae outer membrane-peptides (YOPs) and V antigen (Lcr+). A medium was developed that permitted expression of high yields of V by Yersinia pestis KIM in large fermenter vessels. Immunoblots of specific precipitates prepared by prior molecular sieving showed that native unaggregated V exists as a monomeric 37,000 dalton peptide. Fractionation by precipitation with (NH4)2SO4 and chromatography on phenyl-Sepharose, DEAE cellulose, Sephacryl S200, calcium hydroxyapatite, and Sephadex G200 yielded highly purified antigen as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of parallel preparations from Lcr+ and Lcr- yersiniae. However, yields of V obtained by this process were unexpectedly low. As determined from immunoblots with monospecific polyclonal and monoclonal anti-V, this loss of activity occurred as a function of evident degradation at every step of purification yielding antigenic fragments of about 36,000, 34,000, 31,000, 30,000, and 28,000 daltons. Neutral or acidic pH favored hydrolysis; insignificant cleavage occurred in viable Lcr+ cells of Y. pestis or in culture supernatant fluids. V in neutral cytoplasm from Yersinia pseudotuberculosis or Yersinia enterocolitica did not undergo comparable degradation.

A low-Ca2+ response operon encodes the V antigen of Yersinia pestis

Yersinia pestis has a virulence regulon called the low-Ca2+ response that is mediated by the plasmid pCD and manifested as regulation of growth and of expression of several virulence-associated properties by Ca2+ and temperature. We used Mu dI(Ap lac) to obtain a mutation in pCD1 of Y. pestis KIM that rendered the bacteria unable to express one of these properties, the V antigen. This mutant also had lost the Ca2+ requirement for growth at 37 degrees C and was avirulent in mice. Two-dimensional protein gel electrophoresis showed that the Mu dI(Ap lac) insertion had eliminated 13,000- and 18,000-molecular-weight proteins in addition to the V antigen. We mapped the Mu dI(Ap lac) insertion within pCD1, cloned the HindIII fragment spanning the insertion location, prepared two subclones of this fragment, and identified the proteins these clones expressed in Escherichia coli minicells. The data indicated that the V gene lies within an operon containing three genes; lcrG (encoding the 13,000-molecular-weight protein), lcrV (encoding the 38,000-molecular-weight V antigen), and lcrH (encoding the 18,000-molecular-weight protein). Therefore, the V operon contains the structural gene for V antigen, at least one virulence gene, and at least one Ca2+-dependence gene.

Immunochemical analysis of plasmid-encoded proteins released by enteropathogenic Yersinia sp. grown in calcium-deficient media

Enteropathogenic Yersinia sp. releases plasmid-associated proteins of low molecular mass (26-67 kilodaltons) at 37 degrees C. In this study, the optimum conditions for the release of proteins were assessed and the released proteins (RPs) were analyzed for the manner of release, immunochemical characteristics, and the location of the genes necessary for their synthesis. Protein release was strongly enhanced when growth media were markedly depleted of calcium ions by precipitation with oxalate or chelation with EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid]. RP yields were greatest when Yersinia spp. were in the exponential growth phase. The RPs appeared to be released from the Yersinia spp. by secretion rather than by pinching off of membrane vesicles, because the RPs did not sediment during high-speed centrifugation nor were they contaminated to any significant degree with lipopolysaccharide. Moreover, immunoblot analysis revealed only traces of protein species related to RPs within the outer membranes of plasmid-positive Yersinia spp. grown at 37 degrees C under calcium-restricted conditions. Immunoblot studies also showed that the RPs of Y. enterocolitica serotypes O:3, O:8, and O:9 and the RP of Y. pseudotuberculosis serotype I are highly cross-reactive. Finally, the immunoprecipitates of the products of minicells which harbor Yersinia plasmids were used to demonstrate that at least three proteins immunochemically related to the released fraction were plasmid encoded. These results suggest that at least three of the RPs may be related to or identical with previously described plasmid-encoded Yersinia outer membrane proteins.