Characterization of the tRNA ligases of pathogenic fungi Aspergillus fumigatus and Coccidioides immitis

Yeast tRNA ligase (Trl1) is an essential trifunctional enzyme that repairs RNA breaks with 2',3'-cyclic-PO4 and 5'-OH ends. Trl1 is composed of C-terminal cyclic phosphodiesterase and central polynucleotide kinase domains that heal the broken ends to generate the 3'-OH, 2'-PO4, and 5'-PO4 termini required for sealing by an N-terminal ligase domain. Trl1 enzymes are found in all human fungal pathogens and they are promising targets for antifungal drug discovery because: (i) their domain structures and biochemical mechanisms are unique compared to the mammalian RtcB-type tRNA splicing enzyme; and (ii) there are no obvious homologs of the Trl1 ligase domain in mammalian proteomes. Here we characterize the tRNA ligases of two human fungal pathogens: Coccidioides immitis and Aspergillus fumigatus The biological activity of CimTrl1 and AfuTrl1 was verified by showing that their expression complements a Saccharomyces cerevisiae trl1Δ mutant. Purified recombinant AfuTrl1 and CimTrl1 proteins were catalytically active in joining 2',3'-cyclic-PO4 and 5'-OH ends in vitro, either as full-length proteins or as a mixture of separately produced healing and sealing domains. The biochemical properties of CimTrl1 and AfuTrl1 are similar to those of budding yeast Trl1, particularly with respect to their preferential use of GTP as the phosphate donor for the polynucleotide kinase reaction. Our findings provide genetic and biochemical tools to screen for inhibitors of tRNA ligases from pathogenic fungi.

Structural Divergence of the Group I Intron Binding Surface in Fungal Mitochondrial Tyrosyl-tRNA Synthetases That Function in RNA Splicing

The mitochondrial tyrosyl-tRNA synthetases (mtTyrRSs) of Pezizomycotina fungi, a subphylum that includes many pathogenic species, are bifunctional proteins that both charge mitochondrial tRNA(Tyr) and act as splicing cofactors for autocatalytic group I introns. Previous studies showed that one of these proteins, Neurospora crassa CYT-18, binds group I introns by using both its N-terminal catalytic and C-terminal anticodon binding domains and that the catalytic domain uses a newly evolved group I intron binding surface that includes an N-terminal extension and two small insertions (insertions 1 and 2) with distinctive features not found in non-splicing mtTyrRSs. To explore how this RNA binding surface diverged to accommodate different group I introns in other Pezizomycotina fungi, we determined x-ray crystal structures of C-terminally truncated Aspergillus nidulans and Coccidioides posadasii mtTyrRSs. Comparisons with previous N. crassa CYT-18 structures and a structural model of the Aspergillus fumigatus mtTyrRS showed that the overall topology of the group I intron binding surface is conserved but with variations in key intron binding regions, particularly the Pezizomycotina-specific insertions. These insertions, which arose by expansion of flexible termini or internal loops, show greater variation in structure and amino acids potentially involved in group I intron binding than do neighboring protein core regions, which also function in intron binding but may be more constrained to preserve mtTyrRS activity. Our results suggest a structural basis for the intron specificity of different Pezizomycotina mtTyrRSs, highlight flexible terminal and loop regions as major sites for enzyme diversification, and identify targets for therapeutic intervention by disrupting an essential RNA-protein interaction in pathogenic fungi.

The Protein Maker: an automated system for high-throughput parallel purification

The Protein Maker is an automated purification system developed by Emerald BioSystems for high-throughput parallel purification of proteins and antibodies. This instrument allows multiple load, wash and elution buffers to be used in parallel along independent lines for up to 24 individual samples. To demonstrate its utility, its use in the purification of five recombinant PB2 C-terminal domains from various subtypes of the influenza A virus is described. Three of these constructs crystallized and one diffracted X-rays to sufficient resolution for structure determination and deposition in the Protein Data Bank. Methods for screening lysis buffers for a cytochrome P450 from a pathogenic fungus prior to upscaling expression and purification are also described. The Protein Maker has become a valuable asset within the Seattle Structural Genomics Center for Infectious Disease (SSGCID) and hence is a potentially valuable tool for a variety of high-throughput protein-purification applications.

Structures of a putative ζ-class glutathione S-transferase from the pathogenic fungus Coccidioides immitis

Coccidioides immitis is a pathogenic fungus populating the southwestern United States and is a causative agent of coccidioidomycosis, sometimes referred to as Valley Fever. Although the genome of this fungus has been sequenced, many operons are not properly annotated. Crystal structures are presented for a putative uncharacterized protein that shares sequence similarity with ζ-class glutathione S-transferases (GSTs) in both apo and glutathione-bound forms. The apo structure reveals a nonsymmetric homodimer with each protomer comprising two subdomains: a C-terminal helical domain and an N-terminal thioredoxin-like domain that is common to all GSTs. Half-site binding is observed in the glutathione-bound form. Considerable movement of some components of the active site relative to the glutathione-free form was observed, indicating an induced-fit mechanism for cofactor binding. The sequence homology, structure and half-site occupancy imply that the protein is a ζ-class glutathione S-transferase, a maleylacetoacetate isomerase (MAAI).

Phenotypic characterization and ecological features of Coccidioides spp. from Northeast Brazil

This study extends phenotypic and ecological knowledge of Coccidioides spp., by describing its recovery from soils of Ceará State (Northeast Brazil) and analyzing the in vitro features of the growth of its vegetative phase. Following a human coccidioidomycosis case, Coccidioides spp. strains were isolated from 3 of 14 soil samples collected in an armadillo's burrow. Mycological analysis showed colonies with glabrous, velvety or cottony texture and an increasing quantity of arthroconidia. The overall growth rates of the strains were slower in 8% NaCl medium, maximum growth rate was obtained at 30 degrees C, and their pH tolerance ranged from 4.0 to 11.0. Several carbohydrates and polyalcohol sources could be efficiently metabolized by Coccidioides spp. strains in the mycelial form. Total absence of growth was observed in media supplemented with either L-aspartic acid or L-histidine. Whereas intense growth was found when strains were incubated with any other aminoacid sources studied. Coccidioides spp. strains did not grow in the presence of Tween 60 and Tween 80, but exhibited intense growth in Tween 20. Nicotinic acid and the toxic compounds caffeic acid and phenol could not be metabolized by any strain. All of the strains were positive for urease production and displayed intense growth in media containing cycloheximide concentrations ranging from 0.01 and 0.05%, but did not grow at 0.1 and 0.2%. The present findings confirm the importance of armadillos burrows in the ecology of Coccidioides spp. in Northeast Brazil and indicate that the fungus is a very physiologically versatile organism.

Coccidioides posadasii contains single chitin synthase genes corresponding to classes I to VII

Coccidioides posadasii is a dimorphic fungal pathogen of humans and other mammals. The switch between saprobic and parasitic growth involves synthesis of new cell walls of which chitin is a significant component. To determine whether particular subsets of chitin synthases (CHSes) are responsible for production of chitin at different stages of differentiation, we have isolated six CHS genes from this fungus. They correspond, together with another reported CHS gene, to single members of the seven defined classes of chitin synthases (classes I-VII). Using Real-Time RT-PCR we show their pattern of expression during morphogenesis. CpCHS2, CpCHS3, and CpCHS6 are preferentially expressed during the saprobic phase, while CpCHS1 and CpCHS4 are more highly expressed during the parasitic phase. CpCHS5 and CpCHS7 expression is similar in both saprobic and parasitic phases. Because C. posadasii contains single members of the seven classes of CHSes found in fungi, it is a good model to investigate the putatively different roles of these genes in fungal growth and differentiation.

Global control of dimorphism and virulence in fungi

Microbial pathogens that normally inhabit our environment can adapt to thrive inside mammalian hosts. There are six dimorphic fungi that cause disease worldwide, which switch from nonpathogenic molds in soil to pathogenic yeast after spores are inhaled and exposed to elevated temperature. Mechanisms that regulate this switch remain obscure. We show that a hybrid histidine kinase senses host signals and triggers the transition from mold to yeast. The kinase also regulates cell-wall integrity, sporulation, and expression of virulence genes in vivo. This global regulator shapes how dimorphic fungal pathogens adapt to the mammalian host, which has broad implications for treating and preventing systemic fungal disease.

Urease produced by Coccidioides posadasii contributes to the virulence of this respiratory pathogen

Urease activity during in vitro growth in the saprobic and parasitic phases of Coccidioides spp. is partly responsible for production of intracellular ammonia released into the culture media and contributes to alkalinity of the external microenvironment. Although the amino acid sequence of the urease of Coccidioides posadasii lacks a predicted signal peptide, the protein is transported from the cytosol into vesicles and the central vacuole of parasitic cells (spherules). Enzymatically active urease is released from the contents of mature spherules during the parasitic cycle endosporulation stage. The endospores, together with the urease and additional material which escape from the ruptured parasitic cells, elicit an intense host inflammatory response. Ammonia production by the spherules of C. posadasii is markedly increased by the availability of exogenous urea found in relatively high concentrations at sites of coccidioidal infection in the lungs of mice. Direct measurement of the pH at these infection sites revealed an alkaline microenvironment. Disruption of the urease gene of C. posadasii resulted in a marked reduction in the amount of ammonia secreted in vitro by the fungal cells. BALB/c mice challenged intranasally with the mutant strain showed increased survival, a well-organized granulomatous response to infection, and better clearance of the pathogen than animals challenged with either the parental or the reconstituted (revertant) strain. We conclude that ammonia and enzymatically active urease released from spherules during the parasitic cycle of C. posadasii contribute to host tissue damage, which exacerbates the severity of coccidioidal infection and enhances the virulence of this human respiratory pathogen.

A recombinant aspartyl protease of Coccidioides posadasii induces protection against pulmonary coccidioidomycosis in mice

Coccidioidomycosis is a respiratory disease of humans caused by the desert soil-borne fungal pathogens Coccidioides spp. Recurrent epidemics of this mycosis in the southwestern United States have contributed significantly to escalated health care costs. Clinical and experimental studies indicate that prior symptomatic coccidioidomycosis induces immunity against subsequent infection, and activation of T cells is essential for containment of the pathogen and its clearance from host tissue. Development of a human vaccine against coccidioidomycosis has focused on recombinant T-cell-reactive antigens which elicit a durable protective immune response against pulmonary infection in mice. In this study we fractionated a protective multicomponent parasitic cell wall extract in an attempt to identify T-cell antigens. Immunoblots of electrophoretic separations of this extract identified patient seroreactive proteins which were subsequently excised from two-dimensional polyacrylamide gel electrophoresis gels, trypsin digested, and sequenced by tandem mass spectrometry. The full-length gene which encodes a dominant protein in the immunoblot was identified using established methods of bioinformatics. The gene was cloned and expressed, and the recombinant protein was shown to stimulate immune T cells in vitro. The deduced protein was predicted to contain epitopes that bind to human major histocompatibility complex class II molecules using a TEPITOPE-based algorithm. Synthetic peptides corresponding to the predicted T-cell epitopes induced gamma interferon production by immune T lymphocytes. The T-cell-reactive antigen, which is homologous to secreted fungal aspartyl proteases, protected mice against pulmonary infection with Coccidioides posadasii. We argue that this immunoproteomic/bioinformatic approach to the identification of candidate vaccines against coccidioidomycosis is both efficient and productive.

A metalloproteinase of Coccidioides posadasii contributes to evasion of host detection

Coccidioides posadasii is a fungal respiratory pathogen of humans that can cause disease in immunocompetent individuals. Coccidioidomycosis ranges from a mild to a severe infection. It is frequently characterized either as a persistent disease that requires months to resolve or as an essentially asymptomatic infection that can reactivate several years after the original insult. In this report we describe a mechanism by which the pathogen evades host detection during the pivotal reproductive (endosporulation) phase of the parasitic cycle. A metalloproteinase (Mep1) secreted during endospore differentiation digests an immunodominant cell surface antigen (SOWgp) and prevents host recognition of endospores during the phase of development when these fungal cells are most vulnerable to phagocytic cell defenses. C57BL/6 mice were immunized with recombinant SOWgp and then challenged with a mutant strain of C. posadasii in which the MEP1 gene was disrupted. The animals showed a significant increase in percent survival compared to SOWgp-immune mice challenged with the parental strain. To explain these results, we proposed that retention of SOWgp on the surfaces of endospores of the mutant strain in the presence of high titers of antibody to the immunodominant antigen contributes to opsonization, increased phagocytosis, and killing of the fungal cells. In vitro studies of the interaction between a murine alveolar macrophage cell line and parasitic cells coated with SOWgp showed that the addition of anti-SOWgp antibody could enhance phagocytosis and killing of Coccidioides. We suggest that Mep1 plays a pivotal role as a pathogenicity determinant during coccidioidal infections and contributes to the ability of the pathogen to persist within the mammalian host.

Coccidioides posadasii contains a single 1,3-beta-glucan synthase gene that appears to be essential for growth

1,3-beta-Glucan synthase is responsible for the synthesis of beta-glucan, an essential cell wall structural component in most fungi. We sought to determine whether Coccidioides posadasii possesses genes homologous to known fungal FKS genes that encode the catalytic subunit of 1,3-beta-glucan synthase. A single gene, designated FKS1, was identified, and examination of its predicted protein product showed a high degree of conservation with Fks proteins from other filamentous fungi. FKS1 is expressed at similar levels in mycelia and early spherulating cultures, and expression decreases as the spherules mature. We used Agrobacterium-mediated transformation to create strains that harbor DeltaFKS1::hygB, a null allele of FKS1, and hypothesize that Fks1p function is essential, due to our inability to purify this allele away from a complementing wild-type FKS1 allele in a heterokaryotic strain. The heterokaryon appears normal with respect to growth rate and arthroconidium production; however, microscopic examination of strains with DeltaFKS1::hygB alleles revealed abnormal swelling of hyphal elements.

Expression of ornithine decarboxylase of Coccidioides immitis in three Escherichia coli strains carrying the lambda DE3 lysogen and an E. coli EWH319 strain odc- null mutant

Ornithine decarboxylase from respiratory fungal pathogen, Coccidioides immitis, cloned in the pETCiODC plasmid under control of T7lac promoter, was produced in E. coli BL21(DE3), BL21(DE3)pLysS, BLR(DE3) and EWH319 transformant strains. E. coli BL21(DE3)pLysS-pETCiODC expressed the highest specific activity of ODC, suggesting that this strain could be successfully used for protein structure and drug testing studies.

Comparative study of the reaction mechanism of family 18 chitinases from plants and microbes

Hydrolytic mechanisms of family 18 chitinases from rice (Oryza sativa L.) and Bacillus circulans WL-12 were comparatively studied by a combination of HPLC analysis of the reaction products and theoretical calculation of reaction time-courses. All of the enzymes tested produced beta-anomers from chitin hexasaccharide [(GlcNAc)(6)], indicating that they catalyze the hydrolysis through a retaining mechanism. The rice chitinases hydrolyzed predominantly the fourth and fifth glycosidic linkages from the nonreducing end of (GlcNAc)(6), whereas B. circulans chitinase A1 hydrolyzed the second linkage from the nonreducing end. In addition, the Bacillus enzyme efficiently catalyzed transglycosylation, producing significant amounts of chitin oligomers larger than the initial substrate, but the rice chitinases did not. The time-courses of (GlcNAc)(6) degradation obtained by HPLC were analyzed by theoretical calculation, and the subsite structures of the rice chitinases were identified to be (-4)(-3)(-2)(-1)(+1)(+2). From the HPLC profile of the reaction products previously reported [Terwisscha van Scheltinga et al. (1995) Biochemistry 34, 15619-15623], family 18 chitinase from rubber tree (Hevea brasiliensis) was estimated to have the same type of subsite structure. Theoretical analysis of the reaction time-course for the Bacillus enzyme revealed that the enzyme has (-2)(-1) (+1)(+2)(+3)(+4)-type subsite structure, which is identical to that of fungal chitinase from Coccidioides immitis [Fukamizo et al. (2001) Biochemistry 40, 2448-2454]. The Bacillus enzyme also resembled the fungal chitinase in its transglycosylation activity. Minor structural differences between plant and microbial enzymes appear to result in such functional variations, even though all of these chitinases are classified into the identical family of glycosyl hydrolases.

Purification and characterization of urease isolated from the pathogenic fungus Coccidioides immitis

Coccidioides immitis, the causative agent of San Joaquin Valley fever (coccidioidomycosis), produces a urease which has been suggested to contribute to the virulence of this fungal pathogen. Urease catalyzes the hydrolysis of urea and has been proposed to at least partly account for alkalinity of the microenvironment in which C. immitis grows due to the release of ammonia and ammonium ions. The C. immitis urease was purified to homogeneity (1048-fold) from the mycelial cytosol by chromatographic fractionation. The sequence of 12 N-terminal amino-acid residues of the purified, native polypeptide was identical to that predicted by the translated urease gene sequence which has been reported. The isolated enzyme exhibited a specific activity in the presence of urea of 1750 micromol min(-1) mg(-1) protein, has a native molecular mass of 450 kDa, revealed a Km for urea of 4.1 mM, had a pH optimum of 8.0 and is heat stable. Hydroxyurea, acetohydroxamic acid (AHA) and boric acid each inhibited activity of the purified enzyme. Urease activity was enhanced by the presence of 5-10 mM concentrations of Mg2+ or Mn2+, but inhibited by Li+, Ni2+, Cu2+ or Zn2+. The reversible urease inhibitor, AHA, blocked enzyme activity in the crude mycelial cytosolic fraction when added at a concentration of 10 mM. On the other hand, 10 mM AHA added to 4-day-old mycelial cultures only partially decreased the amount of ammonium detected in the culture medium. It is evident, therefore, that C. immitis urease activity does not account for the total amount of ammonia secreted during in vitro growth of the pathogen. Other metabolic sources of ammonia, which may also contribute to the virulence of C. immitis, are under investigation.

Kinetic properties of chitinase-1 from the fungal pathogen Coccidioides immitis

The endochitinase from Coccidioides immitis (CiX1) is a member of the class 18 chitinase family. Here we show the enzyme functions by a retaining catalytic mechanism; that is, the beta-conformation of the chitin substrate linkages is preserved after hydrolysis. The pattern of cleavage of N-acetyglucosamine (GlcNAc) oligosaccharide substrates has been determined. (GlcNAc)6 is predominantly cleaved into (GlcNAc)2 and (GlcNAc)4, where the (GlcNAc)2 group arises from the nonreducing end of the substrate and is formed as the beta-anomer. With time, transglycosylation occurs, generating (GlcNAc)8 from the product dimer and fresh hexamer. Similar patterns are seen for the cleavage of (GlcNAc)5 and (GlcNAc)4 where dimers cleaved from the nonreducing end reflect the most common binding and hydrolysis pattern. Intrinsic fluorescence measurements suggest the dissociation constant for (GlcNAc)4 is 50 microM. Synthetic substrates with fluorescent leaving groups exhibit complicated profiles in the relationship between initial velocity and substrate concentration, making it difficult to obtain the values of kinetic constants. An improved theoretical analysis of the time-course of (GlcNAc)6 degradation allows the unitary free energy of binding of the individual subsites of the enzyme to be estimated. The free energy values obtained are consistent with the dissociation constant obtained by fluorescence measurements, and generate a model of substrate interaction that can be tested against the crystal structure of the enzyme.

Disruption of the gene which encodes a serodiagnostic antigen and chitinase of the human fungal pathogen Coccidioides immitis

Disruption of genes in medically important fungi has proved to be a powerful tool for evaluation of putative virulence factors and identification of potential protein targets for novel antifungal drugs. Chitinase has been suggested to play a pivotal role in autolysis of the parasitic cell wall of Coccidioides immitis during the asexual reproductive cycle (endosporulation) of this systemic pathogen. Two chitinase genes (CTS1 and CTS2) of C. immitis have been cloned. Preliminary evidence has suggested that expression of CTS1 is markedly increased during endospore formation. The secreted CTS1 chitinase has also been shown to react with patient anti-Coccidioides complement-fixing (CF) antibody and is a valuable aid in the serodiagnosis of coccidioidomycosis. To examine the role of CTS1 in the morphogenesis of parasitic cells, the CTS1 gene was disrupted by a single, locus-specific crossover event. This resulted in homologous integration of a pAN7.1 plasmid construct that contained a 1.1-kb fragment of the chitinase gene into the chromosomal DNA of C. immitis. Results of Southern hybridizations, immunoblot analyses of culture filtrates using both CTS1-specific murine antiserum and serum from a patient with confirmed coccidioidal infection, an immunodiffusion test for CF antigenicity, and substrate gel electrophoresis assays of chitinase activity confirmed that the CTS1 gene was disrupted and nonfunctional. This is the first report of a successful targeted gene disruption in C. immitis. However, loss of CTS1 function had no effect on virulence or endosporulation. Comparative assays of chitinase activity in the parental and Deltacts1 strains suggested that the absence of a functional CTS1 gene can be compensated for by elevated expression of the CTS2 gene. Current investigations are focused on disruption of CTS2 in the Deltacts1 host to further evaluate the significance of chitinase activity in the parasitic cycle of C. immitis.

Sequence, expression and functional analysis of the Coccidioides immitis ODC (ornithine decarboxylase) gene

The ornithine decarboxylase (ODC) gene of the human respiratory fungal pathogen, Coccidioides immitis (Ci) was cloned, sequenced, chromosome-mapped, and expressed in Escherichia coli (Ec). The genomic, cDNA and translated sequences are presented. Transformation of an ODC null mutant strain of Ec (EWH 319) with the Ci ODC gene was conducted to confirm function of the protein encoded by the fungal gene. Activity of the enzyme by the bacterial transformant was inhibited by 1, 4-diamino-2-butanone (DAB), a known inhibitor of eukaryotic ODC. Temporal expression of the Ci ODC gene during the parasitic cell cycle is constitutive, based on results of RT PCR. However, results of enzyme activity assays of cell homogenates obtained at different stages of parasitic cell development in vitro showed that the functional protein is present only during periods of isotropic growth and segmentation, and these morphogenetic events can be arrested by the addition of DAB. The observed absence of a difference in steady-state mRNA transcript amounts, and the developmentally correlated variation in levels of enzyme activity, suggest a translational or post-translational mechanism of ODC regulation. Since no PEST sequence was detected in the Ci ODC, enzyme regulation by programmed protein degradation as reported for many other eukaryotic ODCs may not occur in this case. ODC activity appears to play a key role in the morphogenesis of Ci, and the enzyme could be a rational target for therapy of disseminated coccidioidomycosis.

Identification and cloning of an aspartyl proteinase from Coccidioides immitis

A 45 kDa protein was isolated from a soluble vaccine prepared from formaldehyde-killed spherules of Coccidioides immitis. From the N-terminal amino acid sequence, the protein yielded a 17-amino-acid peptide that was homologous to sequences of other fungal aspartyl proteinases. The coccidioidal cDNA encoding the proteinase was amplified using oligonucleotide primers designed from the 45 kDa N-terminal amino acid sequence and a fungal aspartyl proteinase consensus amino acid sequence. The PCR product was cloned and sequenced, and the remaining 5' upstream and 3' downstream cDNA was amplified, cloned, and sequenced. The cDNA encoding the coccidioidal aspartyl proteinase open reading frame was cloned and the fusion protein containing a C-terminal His-tag expressed in E. coli. The recombinant aspartyl proteinase was purified by immobilized metal affinity chromatography. This recombinant protein will be used for further studies to evaluate its antigenicity, including protective immunogenicity.

Isolation and confirmation of function of the Coccidioides immitis URA5 (orotate phosphoribosyl transferase) gene

The OPRTase (URA5) gene of the human pathogenic fungus, Coccidioides immitis (Ci), was cloned, sequenced, chromosome-mapped and expressed both by transformation of Escherichia coli and by complementation of wdura5Delta, an auxotrophic strain of Wangiella dermatitidis (Wd) with a disrupted URA5 gene. A functional assay of the recombinant URA5 expressed by E. coli was conducted to ensure that the isolated Ci gene encodes the appropriate enzyme. In the absence of a transformation system for Ci, we also used a reported method of introduction of heterologous DNA into cells of the phylogenetically related fungus, Wangiella dermatitidis, to confirm the function of the Ci URA5 gene. Both the genomic and cDNA sequences of the Ci URA5 gene are presented. The transcription start point and two poly(A) addition sites were confirmed. The gene contains a 714-bp ORF that translates a 238-amino-acid (aa) protein of 25.5kDa and pI of 6.5. No introns are present. The translated protein contains a single, putative N-glycosylation site. The deduced Ci protein showed 55-63% aa sequence similarity to reported fungal OPRTases. The URA5 gene was mapped to chromosome IV of Ci, and was shown to be a single copy gene by Southern and Northern hybridizations. Transformation of the wdura5Delta mutant to prototrophy was accomplished by electroporation of Wd yeast cells with the Ci URA5 gene. Cellular uptake of the heterologous DNA was confirmed by Southern hybridization. The stable transformants were unable to grow on a medium containing 5-FOA. Expression of the Ci URA5 gene can be used as a selectable marker for a transformation system, and the latter is essential for molecular studies of this pathogenic fungus.

Crystallization and preliminary X-ray analysis of a chitinase from the fungal pathogen Coccidioides immitis

Chitinase is necessary for fungal growth and cell division and, therefore, is an ideal target for the design of inhibitors which may act as antifungal agents. A chitinase from the fungal pathogen Coccidioides immitis has been expressed as a fusion protein with gluathione-S-transferase (GST), which aids in purification. After cleavage from GST, chitinase was crystallized from 30% PEG 4000 in 0. 1 M sodium acetate pH 4.6. The crystals have a tetragonal crystal lattice and belong to space group P41212 or P43212 and diffract to 2. 2 A resolution. The unit-cell parameters are a = b = 91.2, c = 95.4 A; there is only one chitinase molecule in the asymmetric unit.

Isolation and characterization of the urease gene (URE) from the pathogenic fungus Coccidioides immitis

The urease (URE)-encoding gene from Coccidioides immitis (Ci), a respiratory fungal pathogen of humans, was cloned, sequenced, chromosome-mapped and expressed. Both the genomic and cDNA sequences are reported. The transcription start point and poly(A)-addition site were confirmed. The URE gene contains eight introns and a 2517-bp ORF that translates a 839-amino-acid (aa) protein of 91.5 kDa and pI of 5.5, as deduced by computer analysis of the nucleotide sequence. The translated protein revealed eight putative N-glycosylation sites. The deduced URE showed comparable levels of homology to reported URE of the jack bean plant (Canavalia ensiformis; 71.8%) and URE of several genera of bacteria (Bp, 71.7%; Hp, 68.3%; Ka, 71.6%; Pm, 71.9%). The URE gene was mapped to chromosome III of Ci and was shown to be a single copy gene by Southern hybridization. Expression of a 1687-bp fragment of the URE gene in E. coli resulted in the production of a 63-kDa recombinant protein that was recognized in an immunoblot by antiserum raised against the Ka URE homolog. This is the first report of a fungal URE gene.

Concordance of gene genealogies reveals reproductive isolation in the pathogenic fungus Coccidioides immitis

Simple cladogenetic theory suggests that gene genealogies can be used to detect mixis in a population and delineate reproductively isolated groups within sexual taxa. We have taken this approach in a study of Coccidioides immitis, an ascomycete fungus responsible for a recent epidemic of coccidioidomycosis (Valley fever) in California. To test whether this fungus represents a single sexual species throughout its entire geographic range, we have compared genealogies from fragments of five nuclear genes. The five genealogies show multiple incompatibilities indicative of sex, but also share a branch that partitions the isolates into two reproductively isolated taxa, one centered in California and the other outside California. We conclude that coccidioidomycosis can be caused by two distinct noninterbreeding taxa. This result should aid the future study of the disease and illustrates the utility of the genealogical approach in population genetics.

Use of a recombinant Coccidioides immitis complement fixation antigen-chitinase in conventional serological assays

The coccidioidal complement fixation (CF) antigen has been cloned previously, and the fusion protein has been expressed in Escherichia coli. The recombinant CF (rCF) antigen was affinity purified by adsorption-desorption to chitin, and its reactivity was studied by using sera containing coccidioidal antibodies. The affinity-purified rCF antigen formed a line of identity with an immunodiffusion (ID) CF reference antigen (coccidioidin) derived from mycelial-phase Coccidioides immitis and was reactive with human, canine, and equine sera containing coccidioidal antibody. The affinity-purified rCF antigen yielded no detectable reaction with Blastomyces of Histoplasma antiserum by ID. The affinity-purified rCF antigen fixed complement with positive human sera and, even when used at lower concentrations, yielded titers comparable to those obtained with the coccidioidin. The reactivity of the affinity-purified rCF antigen was further evaluated by enzyme immunoassay, in which it manifested good sensitivity (96.9%) and specificity (100%) when evaluated with 43 human patients' sera. Thus, the affinity-purified rCF antigen has yielded reactions comparable to those of crude coccidioidal antigens in conventional CF, IDCF, and enzyme immunoassay.

Isolation and characterization of two chitinase-encoding genes (cts1, cts2) from the fungus Coccidioides immitis

Two chitinase (CTS)-encoding genes (cts) from Coccidioides immitis (Ci), a respiratory fungal pathogen of humans, were cloned and sequenced. Both the genomic and cDNA sequences are presented. The transcription start points and poly(A)-addition sites were confirmed. The cts1 gene contains five introns and a 1281-bp ORF which translates a 427-amino-acid (aa) protein of 47.4 kDa. The cts2 gene contains two introns and a 2580-bp ORF which translates a 860-aa protein of 91.4 kDa. The deduced CTS1 protein showed highest homology to the Aphanocladium album and Trichoderma harzianum CTS (74% and 76%, respectively), while CTS2 showed highest homology to the CTS of Saccharomyces cerevisiae (Sc) and Candida albicans (47% and 51%, respectively). The putative N-terminal sequence of the mature CTS1 protein also showed 89% homology to the reported N-terminal sequence of a 48-kDa complement fixation antigen (CF-Ag) of Ci which has demonstrated chitinase activity. The CF-Ag is a clinically important antigen used in serodiagnosis of this fungal disease. CTS2 showed several of the conserved features of the Sc CTS, including putative catalytic and Ser/Thr-rich domains, and a C-terminal Cys-rich region. We propose that CTS1 and CTS2 of Ci are members of two distinct classes of fungal chitinases, an observation not previously reported for a single fungus.

Isolation and characterization of the arginase-encoding gene (arg) from Coccidioides immitis

The arginase (ARG)-encoding gene (arg) of Coccidioides immitis, a human fungal pathogen, was cloned and sequenced. Both the genomic and cDNA sequences are provided. The transcription start point and poly(A) sites were confirmed. The arg gene, which was located on chromosome II of C. immitis by Southern hybridization, is a single-copy gene with two introns and a 966-bp ORF which translates a 322-aa protein of 35.1 kDa. The deduced ARG protein showed 44% identity and 68% similarity to the Saccharomyces cerevisiae ARG.

Evidence for a phylogenetic connection between Coccidioides immitis and Uncinocarpus reesii (Onygenaceae)

Coccidioides immitis is an anomaly amongst the human systemic fungal pathogens. Its unique parasitic cycle has contributed to confusion over its taxonomy. Early investigators mistakenly suggested that the pathogen is a protist, while others agreed it to be a fungus but placed it in four different divisions of the Eumycota. The taxonomy of C. immitis is still unresolved. Ultrastructural examinations of its parasitic and saprobic phases have revealed features that are diagnostic of the ascomycetous fungi. Moreover, striking similarities between the kind of asexual reproduction (i.e. arthroconidium formation) of this pathogen and certain anamorphic and teleomorphic members of the genus Malbranchea have suggested a close relationship. Teleomorphs of these Malbranchea species are members of the Onygenaceae (Order, Onygenales). This family also includes teleomorphs of two human respiratory pathogens, Histoplasma capsulatum and Blastomyces dermatitidis. Although the 18S rRNA gene sequences (1713 bp) of these two pathogenic forms differ from that of C. immitis by only 35 and 33 substitutions, respectively, their mode of conidiogenesis is characterized by production of solitary aleurioconidia rather than alternate arthroconidia. In this study we have used characters derived from biochemical, immunological and molecular analyses to compare relatedness between C. immitis, H. capsulatum, B. dermatitidis, and six non-pathogenic species of Malbranchea (the Malbranchea states of Uncinocarpus reesii and Auxarthron zuffianum, as well as M. albolutea, M. dendritica, M. filamentosa and M. gypsea). Evidence is presented which supports inclusion of C. immitis in the Onygenaceae, and indicates that a close phylogenetic relationship exists between the Malbranchea state of U. reesii and this respiratory pathogen.

A seroreactive 120-kilodalton beta-1,3-glucanase of Coccidioides immitis which may participate in spherule morphogenesis

A beta-glucosidase of Coccidioides immitis was identified in electrophoresis gel separations of the concanavalin A-bound mycelial culture-filtrate-plus-lysate preparation. p-Nitrophenol-beta-D-glucopyranoside was used as the substrate to visualize the enzymatically active fraction in nonreducing gels. The gel-isolated, chromatographically purified enzyme has an optimal pH of 8.0 and cleaves beta-1,3-glycosyl linkages. The alkaline beta-glucosidase was further characterized by a pI of 3.8 to 4.0, optimal activity at 37 to 40 degrees C, and molecular size of 120 kDa as identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified beta-glucosidase is identical to a previously reported 120-kDa antigen (Ag) which reacts with immunoglobulin M (IgM) tube precipitin (TP) antibody in sera from patients with coccidioidomycosis. The TP-Ag was described as a valuable serodiagnostic reagent for detection of specific IgM in patients with early coccidioidal infections. The beta-glucosidase, like the TP-Ag, was localized in the cell wall and cytoplasmic vesicles of parasitic cells (spherules) by immunofluorescence and immunoelectron microscopy with specific antiserum raised against the purified enzyme. The boiled cell wall fraction isolated from these same young (presegmented) spherules was partially digested by the beta-glucosidase. Addition of a potent beta-glucosidase inhibitor, 1-deoxynojirimycin, to the parasitic-phase culture medium at a concentration of 200 microM blocked or retarded conversion of arthroconidia to spherules. Antibody was raised in guinea pigs against chromatographically purified 1-deoxynojirimycin which was conjugated with bovine serum albumin. The inhibitor was localized by immunofluorescence in the wall of the 1-deoxynojirimycin-treated cells. We suggest that the spherule wall-associated, alkaline hydrolase functions as a beta-1,3-glucanase to provide for wall plasticity as well as intussusception of newly synthesized wall polymers during the period of rapid diametric growth of parasitic cells of C. immitis.

The coccidioidal complement fixation and immunodiffusion-complement fixation antigen is a chitinase

Culture filtrates and autolysates of Coccidioides immitis have provided suitable crude antigens for the serodiagnosis and prognosis of coccidioidomycosis. One of these, a heat-labile antigen which participates in the immunodiffusion reaction corresponding to the complement fixation reaction (IDCF), has been characterized as a 110-kDa native protein that, when subjected to reducing conditions and heat, yields a 48-kDa component. The present report provides serologic and biochemical evidence that this antigen is a chitinase. This chitinase, isolated from 48-h culture filtrate of the spherule-endospore-phase C. immitis by affinity adsorption to chitin, formed a line of identity with the IDCF reference antigen and participated in the complement fixation reaction with human serum. It lost its enzymatic as well as antigenic activity when heated, but when not heated it retained its enzymatic activity even when precipitated with coccidiodal antibody present in human serum. This chitinase represents a significant serodiagnostic substance and may be important in the morphogenesis of C. immitis.

Isolation and expression of a gene which encodes a wall-associated proteinase of Coccidioides immitis

A chymotrypsinlike serine proteinase of Coccidioides immitis with an estimated molecular size of 34 kDa has been shown by immunoelectron microscopy to be associated with the walls of the parasitic cells of this human respiratory pathogen. The proteinase has been suggested to play a role in spherule development. We report the isolation of a 1.2-kb cDNA from an expression library of C. immitis constructed in the lambda ZAP II phage vector. The cDNA is suggested to encode the 34-kDa protein. We demonstrate identity between segments of the deduced amino acid sequence of the open reading frame of the 1.2-kb cDNA and three distinct sequences obtained from cyanogen bromide cleavage peptides of the purified proteinase. The occurrence of N-glycosyl linkage sites in the deduced sequence of 309 amino acids of the open reading frame (ORF) correlates with our identification of such linkage sites in the native glycosylated proteinase. A protein encoded by an 800-bp fragment of the 1.2-kb cDNA, which was produced by transformed Escherichia coli XL1-Blue, was recognized by the anti-34-kDa protein antibody in a Western blot (immunoblot). Northern (RNA) hybridization of total poly(A)-containing RNA of C. immitis with the labeled 1.2-kb cDNA clone revealed a single band of approximately 1.75 kb. Partial homology was demonstrated between the deduced amino acid sequence of the ORF (927 bp) and reported sequences of alpha-chymotrypsin and chymotrypsinogens. Expression of the proteinase gene was examined by Northern dot blot analysis of total RNA from different stages of parasitic cell development in C. immitis. Maximum levels of specific mRNA were detected during early endospore wall differentiation. The 34-kDa proteinase appears to be concentrated in walls of the parasitic cells at stages of active growth. We suggest that the enzyme may participate in wall plasticization and/or intussusception or in cell wall turnover.

Isolation of antigens with proteolytic activity from Coccidioides immitis

Three antigens with proteolytic activity have been isolated from crude, water-soluble fractions of the saprobic phase of the fungal pathogen Coccidioides immitis. Two proteinases, identified in our immunoelectrophoresis reference system as Ag11 and AgCS, were isolated from the soluble conidial wall fraction (SCWF). Ag11 was previously shown to be a serine proteinase and was characterized in this study as a 60-kilodalton (kDa) fraction by gel filtration (GF). The purified proteinase demonstrated little or no reactivity with 21 serum samples from coccidioidomycosis patients in the enzyme-linked immunosorbent assay; this may be due to limited presentation of this antigen to the host during the course of coccidioidomycosis. AgCS was separated by GF chromatography into two fractions identified by molecular masses of 39 and 19 kDa. Most proteolytic activity was shown by substrate gel electrophoresis to be associated with the lower-molecular-mass fraction. AgCS was reactive with 18 of the 21 serum samples and shown to be the major component of a heat-stable antigen previously reported to be immunospecific for C. immitis. The third antigen with proteolytic activity was isolated from the 5-day mycelial culture filtrate and identified by GF as a 56-kDa fraction. Uniformly high levels of immunoreactivity between 18 of the 21 patient sera and the 56-kDa antigen were demonstrated. Antigens with proteolytic activity may play important roles in fungus-host interactions as well as morphogenesis of the pathogen.

Characterization of a proteinase inhibitor isolated from the fungal pathogen Coccidioides immitis

A proteinase inhibitor was isolated from the cytosol and cell wall of the fungal respiratory pathogen Coccidioides immitis. The inhibitor was purified to apparent homogeneity by acid precipitation, gel filtration and reverse-phase h.p.l.c. It has an Mr of about 5000 under reducing conditions, as revealed by SDS/polyacrylamide-gel electrophoresis, and is both heat-stable and tolerant to low pH. The inhibitor can efficiently block activity of a 36,000-Mr serine proteinase previously isolated from the same cell-wall fraction of C. immitis, and the two molecules react in a 1:1 stoichiometry. The dissociation constant (Ki) of the enzyme-inhibitor complex is 2.3 x 10(-8) M. We suggest that the low-Mr inhibitor may play a role in regulation of the activity of the cell-wall-associated 36,000-Mr proteinase during sporulation of C. immitis.

Possible role of a proteinase in endosporulation of Coccidioides immitis

We previously reported isolation of a serine proteinase from the soluble conidial wall fraction of Coccidioides immitis. The purified proteinase was identified as a polypeptide band of 36,000 Mr by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In this study, we raised monospecific antiserum in rabbits against the purified proteinase for use in immunoelectron microscopy. We showed that immunolabel was localized in the cell wall of both the saprobic and parasitic phases but was most concentrated in the wall of the segmentation apparatus of spherules just prior to endospore differentiation. The total wall fractions of the mycelial phase, as well as those of presegmented and endosporulating spherules, were isolated from in vitro grown cells and then treated with a proteinase inhibitor (phenylmethylsulfonyl fluoride [PMSF]) which irreversibly binds to the residual proteolytic enzyme in the wall isolates. Each fraction was dialyzed, lyophilized, and separately incubated with the active, purified 36,000-Mr proteinase. The reaction mixtures were examined spectrophotometrically (A280) for decomposition of the substrates. Only the PMSF-treated wall isolated from endosporulating spherules was significantly digested. Active, 36,000-Mr proteinase was isolated from intact and viable, endosporulating spherules by brief extraction of the cells with 1% octyl-beta-D-thioglucoside, a nonionic detergent. The serine proteinase may be partly responsible for autolysis of the segmentation apparatus of mature spherules, a morphogenetic process which is pivotal for release of endospores and subsequent proliferation of the pathogen.

Proteinase production by the parasitic cycle of the pathogenic fungus Coccidioides immitis

Coccidioides immitis is the causative agent of coccidioidomycosis (valley fever), a potentially disseminated fungal disease. We hypothesized that proteinases are expressed by the parasitic life cycle of C. immitis and that they might play an important role in the pathogenesis of coccidioidomycosis by facilitating spherule rupture, endospore dissemination, and tissue invasion and destruction. Filtrate from cultures of the parasitic life cycle of C. immitis was therefore assayed for proteolytic activity at neutral pH. The filtrate degraded 68% of a radiolabeled model of an elastin-rich extracellular matrix. The principal activity was against elastin and glycoprotein in the matrix. Degradation of purified elastin by filtrate was 222 micrograms/h per mg of filtrate protein at 37 degrees C. Denatured type I collagen (Azocoll) degradation was 13.5 mg/h per mg of filtrate protein at 37 degrees C. Proteinase activity peaked at 60 h of culture, correlating with release of endospores from mature spherules in the in vitro culture system. Elastase activity was attributed to a serine proteinase which exhibited an active-site preference for phenylalanine at the P1 site. The subunit molecular mass of the elastase determined by [3H]diisopropylfluorophosphate labeling was approximately 25 kilodaltons. Inhibition of the azocollytic activity of crude filtrate by 2 mM 1,10-phenanthroline and 10 mM EDTA, and stimulation by 2 mM CaCl2, suggested that a metalloproteinase was also present. Gelatin substrate gel electrophoresis with and without inhibitors confirmed that two proteinases were expressed, and they were separated by fast protein liquid chromatography.

Isolation and characterization of an extracellular proteinase of Coccidioides immitis

A proteinase isolated from the respiratory pathogen, Coccidioides immitis, was shown to have collagenolytic and elastinolytic activity, as well as the ability to cleave human serum immunoglobulin G and secretory immunoglobulin A. Proteolytic activity was demonstrated with a bovine casein digestion assay in conidial culture exudates, mycelial and spherule culture filtrates, conidial and spherule wall material, and Sephacryl S-300 fractions of the isolated soluble conidial wall material described previously. One of the latter fractions (fraction 2) demonstrated high proteolytic activity. The proteinase was purified from this chromatographic fraction by cold acetone extraction followed by Sephadex G-50 gel filtration and was identified as a polypeptide band of 36,000 Mr by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By means of tandem two-dimensional immunoelectrophoresis, the proteinase was identified as antigen 11 on the basis of its reaction in the coccidioidin/anticoccidioidin reference system. The proteinase is characterized by a broad substrate specificity, optimal activity at 35 to 40 degrees C (pH 8.0) in the presence of human collagen, elastin, or hemoglobin, an isoelectric point of pH 4.5, and inhibition by organofluorides, N-tosyl-L-phenylalanine chloromethyl ketone, chymostatin, and alpha-1-antitrypsin. These features of the enzyme are comparable to those of chymotrypsinlike serine proteinases. Demonstration that the proteinase can cleave human immunoglobulins and digest ubiquitous tissue structural proteins (e.g., collagen and elastin) suggests that it may play a role in the virulence of the fungal pathogen.

Collagenolytic activity of Coccidioides immitis

Coccidioides immitis appears to be unable to digest particulate collagen when cultured on collagen-containing semisolid culture media. However, all C. immitis strains solubilized collagen when the fungus was grown in liquid suspension cultures. Moreover, sterile culture filtrates were collagenolytic in collagen-buffer-agar plate assays.

Elastase activity of fungi with anamorphs similar to Coccidioides immitis

The elastin digestion assay was examined to determine if it would facilitate the identification of Coccidioides immitis when non-pathogenic fungi resembling C. immitis are encountered. Fungal isolants tested have anamorphs that closely resemble the macroscopic or microscopic morphology of C. immitis. Elastin hydrolysis was measured by elastin-agar plate assays. Approximately 80% of the isolants hydrolyzed elastin; thus, the elastin digestion assay as a differential test appears to have little value.

Elastase activity of Coccidioides immitis

Twenty-two strains of Coccidioides immitis were tested for the ability to hydrolyse elastin. Screening assays with Czapek's or Tryptic Soy Agar supplemented with 0.5% elastin demonstrated that 21 strains (95%) were elastolytic. In broth cultures, elastase activity was induced by incorporation of insoluble elastin into the medium and induction was suppressed by supplementation with yeast extract. C. immitis appears to be unique amongst dimorphic fungal pathogens in its digestion of elastin.

[Peptidase activity of Coccidioides immitis]

Glycyl-L-leucinehydrolase consisting of three molecular units was extracted from C. immitis solid cultural medium. During fractionation in polyacrylamide gel of the enzyme-containing extract a 50-fold purification of the enzyme isoform with molecular weight 12,800 is achieved. The enzyme is heat-stable, active in the narrow pH range and hydrolizes peptide bonds containing glycine. Its activity is not inhibited by none of the protease inhibitors tested.

[Immunochemical properties of the extracellular hydrolases (protease and alkaline phosphatase) of Coccidioides immitis]

Extracellular hydrolases (protease and alkaline phosphatase) of the coccidioidal fungus possessed antigenic properties and caused production of the corresponding antibodies. Phosphatase-antiphosphatase-substrate system apparently has future prospects for the elaboration of immunobiochemical methods for the diagnosis of coccidioidomycosis.

[Study of the conditions for protease formation by Coccidioides immitis]

The authors determined conditions for C. immitis protease synthesis during growing on various nutrient media. Four to five weeks could serve as the optimal period of protease formation. Of six nutrient media tested Saburo's medium, containing peptone, proved to be the most favourable for the enzyme production.