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

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.

Nucleotide sequence of the rhamnose biosynthetic operon of Shigella flexneri 2a and role of lipopolysaccharide in virulence

N1308, a chromosomal Tn5 mutant of Shigella flexneri 2a, was described previously as a lipopolysaccharide (LPS) mutant with a short O side chain. N1308 formed foci, but not plaques, in LLC-MK2 cell monolayers and was negative in the Serény test. In this study, the wild-type locus inactivated in N1308 was cloned and further defined by means of complementation analysis. A 4.3-kb BstEII-XhoI fragment of S. flexneri 2a YSH6200 DNA was sufficient to restore both normal LPS and virulence phenotype to the mutant. DNA sequencing of this region revealed four genes, rfbA, rfbB, rfbC, and rfbD, encoding the enzymes required for the biosynthesis of activated rhamnose. The four genes were expressed in Escherichia coli, and the expected protein products were visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. N1308 was shown to have normal levels of surface IpaC and IpaD, while a Western blot (immunoblot) of whole-cell lysates or outer membrane fractions indicated an elevated level of appropriately localized VirG. An in vitro invasion assay revealed that N1308 had normal primary invasive capacity and was able to multiply and move normally within the initial infected cell. However, it exhibited a significant reduction in its ability to spread from cell to cell in the monolayer. A double immunofluorescence assay revealed differences between LLC-MK2 cells infected with the wild-type YSH6000 and those infected with N1308. The wild-type bacteria elicited the formation of the characteristic F-actin tails, whereas N1308 failed to do so. However, N1308 was capable of inducing deposition of F-actin, which accumulated in a peribacterial fashion with only slight, if any, unipolar accumulation of the cytoskeletal protein.

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.

The origin and evolution of species differences in Escherichia coli and Salmonella typhimurium

Since diverging from a common ancestor some 120 million years, Escherichia coli and Salmonella typhimurium have accumulated numerous phenotypic characteristics which have traditionally been used to distinguish these enteric species. While most of the genetic differences between these species are due to the accumulation of point mutations, the majority of the observed variation in phenotypic characters is attributable to segments of the genome confined to only one of the species. We have analyzed the map positions, G+C contents, nucleotide sequences and functions of regions unique to the Salmonella chromosome in an attempt to determine the ancestry of species-specific sequences. Some of the Salmonella-specific regions had uncharacteristically low base compositions and contained open reading frames of atypical codon usage patterns suggesting that portions of the genome were acquired by horizontal transfer from distantly-related bacterial species. The role of these species-specific sequences was assayed by constructing mutant strains harboring deletions in the corresponding regions of the genome. Several functions were ascribed to these unique portions of the Salmonella chromosome, including one encoding proteins involved in virulence and invasion of host epithelial cells.

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.

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.