The second STE12 homologue of Cryptococcus neoformans is MATa-specific and plays an important role in virulence

Cryptococcus neoformans STE12α, a homologue of Saccharomyces cerevisiae STE12, exists only in MATα strains. We identified another STE12 homologue, STE12a, which is MATa specific. As in the case with Δste12α, the mating efficiency for Δste12a was reduced significantly. The Δste12a strains surprisingly still mated with Δste12α strains. In MATα strains, STE12a functionally complemented STE12α for mating efficacy, haploid fruiting, and regulation of capsule size in the mouse brain. Furthermore, when STE12a was replaced with two copies of STE12α, the resulting MATa strain produced hyphae on filament agar. STE12a regulates mRNA levels of several genes that are important for virulence including CNLAC1 and CAP genes. STE12a also modulates enzyme activities of phospholipase and superoxide dismutase. Importantly, deletion of STE12a markedly reduced the virulence in mice, as is the case with STE12α. Brain smears of mice infected with the Δste12a strain showed yeast cells with a considerable reduction in capsule size compared with those infected with STE12a strains. When the disrupted locus of ste12a was replaced with a wild-type STE12a gene, both in vivo and in vitro mutant phenotypes were reversed. These results suggest that STE12a and STE12α have similar functions, and that the mating type of the cells influences the alleles to exert their biological effects. C. neoformans, thus, is the first fungal species that contains a mating-type-specific STE12 homologue in each mating type. Our results demonstrate that mating-type-specific genes are not only important for saprobic reproduction but also play an important role for survival of the organism in host tissue.

The wing in yeast heat shock transcription factor (HSF) DNA-binding domain is required for full activity

The yeast heat shock transcription factor (HSF) belongs to the winged helix family of proteins. HSF binds DNA as a trimer, and additional trimers can bind DNA co-operatively. Unlike other winged helix–turn–helix proteins, HSF’s wing does not appear to contact DNA, as based on a previously solved crystal structure. Instead, the structure implies that the wing is involved in protein–protein interactions, possibly within a trimer or between adjacent trimers. To understand the function of the wing in the HSF DNA-binding domain, a Saccharomyces cerevisiae strain was created that expresses a wingless HSF protein. This strain grows normally at 30°C, but shows a decrease in reporter gene expression during constitutive and heat-shocked conditions. Removal of the wing does not affect the stability or trimeric nature of a protein fragment containing the DNA-binding and trimerization domains. Removal of the wing does result in a decrease in DNA-binding affinity. This defect was mainly observed in the ability to form the first trimer-bound complex, as the formation of larger complexes is unaffected by the deletion. Our results suggest that the wing is not involved in the highly co-operative nature of HSF binding, but may be important in stabilizing the first trimer bound to DNA.

Comparative assessment of virulence of recombinant vaccinia viruses expressing IL-2 and IL-15 in immunodeficient mice

IL-2 and -15 belong to the four α-helix bundle family of cytokines and display a spectrum of overlapping immune functions because of shared signal transducing receptor components of the IL-2 receptor complex. However, recent evidence suggests a nonredundant unique role for IL-15 in the establishment and perhaps maintenance of peripheral natural killer (NK) cell populations in vivo. To explore the contribution of locally released IL-15 on peripheral NK-cell-mediated innate immune responses, we generated a recombinant vaccinia virus that expresses IL-15 and evaluated the course of vaccinial disease in athymic nude mice. Coexpression of IL-15 resulted in the attenuation of virulence of vaccinia virus, and mice inoculated with 105 plaque-forming units or less resolved the infection successfully. In contrast, mice inoculated with a similar dose of the control vaccinia virus failed to eliminate the virus and died of generalized vaccinial disease. Enhanced expression of IL-12 and IFN-γ as well as induction of chemokines were evident in the mice inoculated with IL-15-expressing vaccinia virus in addition to an increase in NK cells in the spleen. However, in this model system, the degree of attenuation in viral virulence attained with coexpression of IL-15 was much less than that achieved with coexpression of IL-2, suggesting that the peripheral NK-cell-mediated events are more responsive to IL-2 than to IL-15.

Virulence of a Mycobacterium tuberculosis clinical isolate in mice is determined by failure to induce Th1 type immunity and is associated with induction of IFN-α/β

To understand how virulent mycobacteria subvert host immunity and establish disease, we examined the differential response of mice to infection with various human outbreak Mycobacterium tuberculosis clinical isolates. One clinical isolate, HN878, was found to be hypervirulent, as demonstrated by unusually early death of infected immune-competent mice, compared with infection with other clinical isolates. The differential effect on survival required lymphocyte function because severe combined immunodeficiency (SCID) mice infected with HN878 or other clinical isolates all died at the same rate. The hypervirulence of HN878 was associated with failure to induce M. tuberculosis-specific proliferation and IFN-γ production by spleen and lymph node cells from infected mice. In addition, 2- to 4-fold lower levels of tumor necrosis factor-α (TNF-α), IL-6, IL-12, and IFN-γ mRNAs were observed in lungs of HN878-infected mice. IL-10, IL-4, and IL-5 mRNA levels were not significantly elevated in lungs of HN878 infected mice. In contrast, IFN-α mRNA levels were significantly higher in lungs of these mice. To further investigate the role of Type 1 IFNs, mice infected with HN878 were treated intranasally with purified IFN-α/β. The treatment resulted in increased lung bacillary loads and even further reduced survival. These results suggest that the hypervirulence of HN878 may be due to failure of this strain to stimulate Th1 type immunity. In addition, the lack of development of Th1 immunity in response to HN878 appears to be associated with increased induction of Type 1 IFNs.

Structure and function of pectic enzymes: Virulence factors of plant pathogens

The structure and function of Erwinia chrysanthemi pectate lysase C, a plant virulence factor, is reviewed to illustrate one mechanism of pathogenesis at the molecular level. Current investigative topics are discussed in this paper.

Plants and animals share functionally common bacterial virulence factors

By exploiting the ability of Pseudomonas aeruginosa to infect a variety of vertebrate and nonvertebrate hosts, we have developed model systems that use plants and nematodes as adjuncts to mammalian models to help elucidate the molecular basis of P. aeruginosa pathogenesis. Our studies reveal a remarkable degree of conservation in the virulence mechanisms used by P. aeruginosa to infect hosts of divergent evolutionary origins.

Pattern of mutation in the genome of influenza A virus on adaptation to increased virulence in the mouse lung: Identification of functional themes

The genetic basis for virulence in influenza virus is largely unknown. To explore the mutational basis for increased virulence in the lung, the H3N2 prototype clinical isolate, A/HK/1/68, was adapted to the mouse. Genomic sequencing provided the first demonstration, to our knowledge, that a group of 11 mutations can convert an avirulent virus to a virulent variant that can kill at a minimal dose. Thirteen of the 14 amino acid substitutions (93%) detected among clonal isolates were likely instrumental in adaptation because of their positive selection, location in functional regions, and/or independent occurrence in other virulent influenza viruses. Mutations in virulent variants repeatedly involved nuclear localization signals and sites of protein and RNA interaction, implicating them as novel modulators of virulence. Mouse-adapted variants with the same hemagglutinin mutations possessed different pH optima of fusion, indicating that fusion activity of hemagglutinin can be modulated by other viral genes. Experimental adaptation resulted in the selection of three mutations that were in common with the virulent human H5N1 isolate A/HK/156/97 and that may be instrumental in its extreme virulence. Analysis of viral adaptation by serial passage appears to provide the identification of biologically relevant mutations.

Selection for in vivo regulators of bacterial virulence

We devised a noninvasive genetic selection strategy to identify positive regulators of bacterial virulence genes during actual infection of an intact animal host. This strategy combines random mutagenesis with a switch-like reporter of transcription that confers antibiotic resistance in the off state and sensitivity in the on state. Application of this technology to the human intestinal pathogen Vibrio cholerae identified several regulators of cholera toxin and a central virulence gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell envelope, biosynthesis, and adherence. We show that phenotypes that appear genetically independent in cell culture become interrelated in the host milieu.

Multilocus analysis of extracellular putative virulence proteins made by group A Streptococcus: Population genetics, human serologic response, and gene transcription

Species of pathogenic microbes are composed of an array of evolutionarily distinct chromosomal genotypes characterized by diversity in gene content and sequence (allelic variation). The occurrence of substantial genetic diversity has hindered progress in developing a comprehensive understanding of the molecular basis of virulence and new therapeutics such as vaccines. To provide new information that bears on these issues, 11 genes encoding extracellular proteins in the human bacterial pathogen group A Streptococcus identified by analysis of four genomes were studied. Eight of the 11 genes encode proteins with a LPXTG(L) motif that covalently links Gram-positive virulence factors to the bacterial cell surface. Sequence analysis of the 11 genes in 37 geographically and phylogenetically diverse group A Streptococcus strains cultured from patients with different infection types found that recent horizontal gene transfer has contributed substantially to chromosomal diversity. Regions of the inferred proteins likely to interact with the host were identified by molecular population genetic analysis, and Western immunoblot analysis with sera from infected patients confirmed that they were antigenic. Real-time reverse transcriptase–PCR (TaqMan) assays found that transcription of six of the 11 genes was substantially up-regulated in the stationary phase. In addition, transcription of many genes was influenced by the covR and mga trans-acting gene regulatory loci. Multilocus investigation of putative virulence genes by the integrated approach described herein provides an important strategy to aid microbial pathogenesis research and rapidly identify new targets for therapeutics research.

DNA repeats identify novel virulence genes in Haemophilus influenzae.

The whole genome sequence (1.83 Mbp) of Haemophilus influenzae strain Rd was searched to identify tandem oligonucleotide repeat sequences. Loss or gain of one or more nucleotide repeats through a recombination-independent slippage mechanism is known to mediate phase variation of surface molecules of pathogenic bacteria, including H. influenzae. This facilitates evasion of host defenses and adaptation to the varying microenvironments of the host. We reasoned that iterative nucleotides could identify novel genes relevant to microbe-host interactions. Our search of the Rd genome sequence identified 9 novel loci with multiple (range 6-36, mean 22) tandem tetranucleotide repeats. All were found to be located within putative open reading frames and included homologues of hemoglobin-binding proteins of Neisseria, a glycosyltransferase (IgtC gene product) of Neisseria, and an adhesin of Yersinia. These tetranucleotide repeat sequences were also shown to be present in two other epidemiologically different H. influenzae type b strains, although the number and distribution of repeats was different. Further characterization of the IgtC gene showed that it was involved in phenotypic switching of a lipopolysaccharide epitope and that this variable expression was associated with changes in the number of tetranucleotide repeats. Mutation of IgtC resulted in attenuated virulence of H. influenzae in an infant rat model of invasive infection. These data indicate the rapidity, economy, and completeness with which whole genome sequences can be used to investigate the biology of pathogenic bacteria.

Unique chromosomal regions associated with virulence of an avian pathogenic Escherichia coli strain.

The avian pathogenic Escherichia coli strain (chi)7122 (serotype O78:K80:H9) causes airsacculitis and colisepticemia in chickens. To identify genes associated with avian disease, a genomic subtraction technique was performed between strain (chi)7122 and the E. coli K-12 strain (chi)289. The DNA isolated using this method was found only in strain (chi)7122 and was used to identify cosmid clones carrying unique DNA from a library of (chi)7122 that were then used to map the position of unique DNA on the E. coli chromosome. A total of 12 unique regions were found, 5 of which correspond to previously identified positions for unique DNA sequence in E. coli strains. To assess the role each unique region plays in virulence, mutants of (chi)7122 were constructed in which a segment of unique DNA was replaced with E. coli K-12 DNA by cotransduction of linked transposon insertions in DNA flanking the unique sequence. The resulting replacement mutants were assessed for inability to colonize the air sac and cause septicemia in 2-week-old white Leghorn chickens. Two mutants were found to be avirulent when injected into the right caudal air sac of 2-week-old chickens. One avirulent mutant, designated (chi)7145, carries a replacement of the rfb locus at 44 min, generating a rough phenotype. The second mutant is designated (chi)7146, and carries a replacement at position 0.0 min on the genetic map. Both mutants could be complemented to partial virulence by cosmids carrying sequences unique to (chi)7122.

Large-scale isolation of candidate virulence genes of Pseudomonas aeruginosa by in vivo selection.

Pseudomonas aeruginosa, an opportunistic human pathogen, is a major causative agent of mortality and morbidity in immunocompromised patients and those with cystic fibrosis genetic disease. To identify new virulence genes of P. aeruginosa, a selection system was developed based on the in vivo expression technology (IVET) that was first reported in Salmonella system. An adenine-requiring auxotrophic mutant strain of P. aeruginosa was isolated and found avirulent on neutropenic mice. A DNA fragment that can complement the mutant strain, containing purEK operon that is required for de novo biosynthesis of purine, was sequenced and used in the IVET vector construction. By applying the IVET selection system to a neutropenic mouse infection model, genetic loci that are specifically induced in vivo were identified. Twenty-two such loci were partially sequenced and analyzed. One of them was a well-studied virulence factor, pyochelin receptor (FptA), that is involved in iron acquisition. Fifteen showed significant homology to reported sequences in GenBank, while the remaining six did not. One locus, designated np20, encodes an open reading frame that shares amino acid sequence homology to transcriptional regulators, especially to the ferric uptake regulator (Fur) proteins of other bacteria. An insertional np20 null mutant strain of P. aeruginosa did not show a growth defect on laboratory media; however, its virulence on neutropenic mice was significantly reduced compared with that of a wild-type parent strain, demonstrating the importance of the np20 locus in the bacterial virulence. The successful isolation of genetic loci that affect bacterial virulence demonstrates the utility of the IVET system in identification of new virulence genes of P. aeruginosa.

Type 1 fimbrial expression enhances Escherichia coli virulence for the urinary tract.

Type 1 fimbriae are adhesion organelles expressed by many Gram-negative bacteria. They facilitate adherence to mucosal surfaces and inflammatory cells in vitro, but their contribution to virulence has not been defined. This study presents evidence that type 1 fimbriae increase the virulence of Escherichia coli for the urinary tract by promoting bacterial persistence and enhancing the inflammatory response to infection. In a clinical study, we observed that disease severity was greater in children infected with E. coli O1:K1:H7 isolates expressing type 1 fimbriae than in those infected with type 1 negative isolates of the same serotype. The E. coli O1:K1:H7 isolates had the same electrophoretic type, were hemolysin-negative, expressed P fimbriae, and carried the fim DNA sequences. When tested in a mouse urinary tract infection model, the type 1-positive E. coli O1:K1:H7 isolates survived in higher numbers, and induced a greater neutrophil influx into the urine, than O1:K1:H7 type 1-negative isolates. To confirm a role of type 1 fimbriae, a fimH null mutant (CN1016) was constructed from an O1:K1:H7 type 1-positive parent. E. coli CN1016 had reduced survival and inflammatogenicity in the mouse urinary tract infection model. E. coli CN1016 reconstituted with type 1 fimbriae (E. coli CN1018) had restored virulence similar to that of the wild-type parent strain. These results show that type 1 fimbriae in the genetic background of a uropathogenic strain contribute to the pathogenesis of E. coli in the urinary tract.

The full-length leptin receptor has signaling capabilities of interleukin 6-type cytokine receptors.

The leptin receptor (OB-R) is a single membrane-spanning protein that mediates the weight regulatory effects of leptin (OB protein). The mutant allele (db) of the OB-R gene encodes a protein with a truncated cytoplasmic domain that is predicted to be functionally inactive. Several mRNA splice variants encoding OB-Rs with different length cytoplasmic domains have been detected in various tissues. Here we demonstrate that the full-length OB-R (predominantly expressed in the hypothalamus), but not a major naturally occurring truncated form or a mutant from found in db/db mice, can mediate activation of signal transducer and activator of transcription (STAT) proteins and stimulate transcription through interleukin 6 responsive gene elements. Reconstitution experiments suggest that, although OB-R mediates intracellular signals with a specificity similar to interleukin 6-type cytokine receptors, signaling appears to be independent of the gp130 signal transducing component of the interleukin 6-type cytokine receptors.

Extensive alteration of fungal gene transcript accumulation and elevation of G-protein-regulated cAMP levels by a virulence-attenuating hypovirus.

Persistent infection of the chestnut blight fungus Cryphonectria parasitica with the prototypic hypovirus CHVI-713 results in attenuation of fungal virulence (hypo-virulence) and reduced accumulation of the GTP-binding (G) protein a subunit CPG-1. Transgenic cosuppression of CPG-1 accumulation in the absence of virus infection also confers hypovirulence. We now report the use of mRNA differential display to examine the extent to which virus infection alters fungal gene transcript accumulation and to assess the degree to which modification of CPG-1 signal transduction contributes to this alteration. More than 400 PCR products were identified that either increased (296 products) or decreased (127 products) in abundance as a result of virus infection. Significantly, 65% of these products exhibited similar changes as a result of CPG-1 cosuppression in the absence of virus infection. We also report that both virus infection and CPG-1 cosuppression elevate cAMP levels 3- to 5-fold. Additionally, it was possible to mimic the effect of virus infection and CPG-1 cosuppression on transcript accumulation for representative fungal genes by drug-induced elevation of cAMP levels. These results strengthen and extend previous indications that hypovirus infection causes a significant and persistent alteration of fungal gene expression/transcript accumulation. They further show that this alteration is primarily mediated through modification of the CPG-1 signaling pathway and suggest that, similar to mammalian Gi alpha subunits, CPG-1 functions as a negative modulator of adenylyl cyclase. Finally, these results suggest a role for G-protein-regulated cAMP accumulation in hypovirus-mediated alteration of fungal gene expression.