SummaryADP-ribosylating enzymes, such as cholera and diphtheria toxins, are key virulence factors for a variety of extracellular bacterial pathogens but have not been implicated previously during intracellular pathogenesis. Salmonella strains are capable of invading epithelial cells and localizing in macrophages during infection. The spvB virulence gene of Salmonella is required for human macrophage cytotoxicity in vitro and for enhancing intracellular bacterial proliferation during infection. Here, we present evidence that spvB encodes an ADP-ribosylating enzyme that uses actin as a substrate and depolymerizes actin filaments when expressed in CHO cells. Furthermore, site-directed mutagenesis demonstrates that the ADPribosylating activity of SpvB is essential for Salmonella virulence in mice. As spvB is expressed by Salmonella strains after invasion of epithelial cells or phagocytosis by macrophages, these results suggest that SpvB functions as an intracellular ADP-ribosylating toxin critical for the pathogenesis of Salmonella infections.

SummaryThe pathogenesis of serious systemic Salmonella infections is characterized by survival and proliferation of bacteria inside macrophages. Infection of human monocyte-derived macrophages in vitro with S. typhimurium or S. dublin produces cytopathology characterized by detachment of cells that contain large numbers of proliferating bacteria. This cytopathology is dependent on the expression of the bacterial spv genes, a virulence locus previously shown to markedly enhance the ability of Salmonella to produce systemic disease. After 24 h of infection, macrophage cultures contain two populations of bacteria: (i) proliferating organisms present in a detached cell fraction; and (ii) a static bacterial population in macrophages remaining attached to the culture well. Mutations in either the essential transcriptional activator SpvR or the key SpvB protein markedly reduce the cytopathic effect of Salmonella infection. The spvdependent cytopathology in macrophages exhibits characteristics of apoptosis, with release of nucleosomes into the cytoplasm, nuclear condensation and DNA fragmentation. The current ®ndings suggest that the mechanism of the spv effect is through induction of increased cytopathology in host macrophages.

Infection of human macrophages with Salmonella enterica serovar Typhimurium or Salmonella enterica serovar Dublin produces delayed cytotoxicity characterized by cell detachment and associated apoptosis. Using a site-specific mutant in the SpvB active site, we verify that the ADP-ribosylation activity of SpvB is required for delayed cytotoxicity in human macrophages infected with Salmonella. SipB and the type III protein secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1) are not involved, whereas the SPI2 TTSS is absolutely required for SpvB-dependent cytotoxicity. Furthermore, we show that infection of macrophage cultures with wild-type or sipB mutant bacteria led to a complete loss of polymerized actin in over half of the cells after 24 h. In contrast, macrophages infected with the spvB or SPI2 (ssaV or ssaJ) mutant strain retained normal F-actin filaments, despite similar numbers of intracellular bacteria. We conclude that SpvB and a functional SPI2 TTSS are essential for Salmonella-induced delayed cytotoxicity of human macrophages.Salmonella enterica subspecies I strains are responsible for enteric fever, gastroenteritis, and bacteremia in humans and a broad array of illnesses in animals, including enteritis and septicemia in livestock. A number of subspecies I serovars carry virulence plasmids encoding the highly conserved spv locus associated with disease syndromes of greater severity (13). The spv genes are found more frequently in human extraintestinal isolates than in environmental or fecal isolates, and most cases of human nontyphoidal bacteremia are caused by serovars that contain the spv genes (12, 13).The plasmid-encoded spv locus consists of the spvR regulatory gene and four structural genes, spvABCD (26). spvR encodes a transcriptional regulator activating its own expression and the spvABCD operon (9, 27). Transcription of the spv operon also requires the stationary-phase alternate factor RpoS (5, 10). Since both rpoS and spv are upregulated after phagocytosis by macrophages (4), expression of the spv genes appears to be strongly induced by the intracellular environment of the host cell. The spv genes appear to promote the macrophage phase of the disease process, avoiding destruction by neutrophils and facilitating proliferation of Salmonella strains at extraintestinal sites of infection (13).In vivo experiments suggest that the spv genes enhance bacterial replication in macrophages (18). In human macrophages, spv gene expression is required for induction of cytotoxicity characterized by cell detachment and eventual apoptosis (29). The spvB gene within the spv operon is essential for the spv virulence phenotype, as shown by mutational analysis (35). The spvB gene encodes a 65.6-kDa protein (recently characterized as an ADP-ribosyl transferase) that modifies actin and blocks polymerization to F-actin filaments (28, 38). Site-directed mutagenesis of the conserved NAD-binding site sequence demonstrates that this ADP-ribosylation activity is essential for virulence in mice (28)...

Two transcription factors, C1 (a Myb-domain protein) and B (a basic-helix-loop-helix protein), mediate transcriptional activation of the anthocyanin-biosynthetic genes of maize (Zea mays). To begin to assess the mechanism of activation, the sequences required for C1-and B-mediated induction have been determined for the a2 promoter, which encodes an anthocyanin-biosynthetic enzyme. Analysis of a series of 7-to 13-base-pair substitutions revealed two regions crucial for activation. One region, centered at ؊99, contained a C1-binding site that abolished C1 binding. The other crucial region was adjacent, centered at ؊91. C1 binding was not detected at this site, and mutation of this site did not prevent C1 binding at ؊99. An oligonucleotide dimer containing these two crucial elements was sufficient for C1 and B activation of a heterologous promoter. These data suggest that activation of the anthocyanin genes involves C1 and another factor binding at closely adjacent sites. Mutating a previously postulated anthocyanin consensus sequence within a2 did not significantly reduce activation by C1 and B. However, sequence comparisons of the crucial a2 regions with sequences important for C1-and B-mediated activation in two other anthocyanin promoters led to a revised consensus element shared by these promoters.Anthocyanins are purple pigments that are ubiquitous in plants, and their production is regulated by a variety of developmental, environmental, and genetic cues (van der Meer et al., 1993). The enzymatic pathway that produces anthocyanins has been studied in a diverse array of plants, with the majority of genetic experiments performed in maize (Zea mays), petunia, and snapdragon (Dooner et al., 1991;Quattrocchio et al., 1993;Holton and Cornish, 1995). The long history of study, along with available transposon systems in these species, have led to the identification and cloning of most of the biosynthetic genes that constitute the anthocyanin pathway, as well as the identification and cloning of many regulatory genes (Dooner et al., 1991; van der Meer et al., 1993).Regulation of the anthocyanin pathway in maize requires two classes of transcription factors. One class of regulators contains a bHLH motif (B and R), and the other contains a Myb domain (C1 and Pl). To activate the genes of the anthocyanin pathway, a protein from each class must be expressed; neither alone is sufficient for induction (Goff et al., 1990). The C1 and B proteins directly interact with one another via the two-hybrid assay (Goff et al., 1992), suggesting that these proteins physically act together to activate the genes of this pathway. The precise role of the B protein in activating the anthocyanin-biosynthetic genes is uncertain. Experiments have not revealed either specific DNA-binding activity (L.A. Tolar, M.L. Lesnick, and V.L. Chandler, unpublished data) or an activation domain (Goff et al., 1992). In contrast, the C1 protein binds via its Myb domain to the promoter of the a1 anthocyanin-biosynthetic gene (Sainz et al., 1997) and contain...

No abstract