Threonine biosynthetic genes are essential in Cryptococcus neoformans

A Limited Number of Amino Acid Permeases Are Crucial for Cryptococcus neoformans Survival and Virulence

One unique attribute of Cryptococcus neoformans is its ability to procure essential monomers from its surroundings to survive in diverse environments. Preferentially, sugars are the energy sources for this opportunistic pathogenic fungus under the carbon catabolite repression (CCR); however, sugar restriction induces alternative use of low molecular weight alcohol, organic acids, and amino acids. The expression of transmembrane amino acid permeases (Aaps) allows C. neoformans to utilize different amino acids and their conjugates, notwithstanding under the nitrogen catabolite repression (NCR). Being referred to as global permeases, there is a notion that all cryptococcal Aaps are important to survival and virulence. This functional divergence makes alternative drug targeting against Cryptococcus a challenge. We examine the functions and regulations of C. neoformans Aap variants with the aim of rationalizing their relevance to cryptococcal cell survival and virulence. Based on nutrient bioavailability, we linked the Cac1 pathway to Ras1 activation for thermotolerance that provides a temperature cushion for Aap activity under physiological conditions. Lastly, mutants of Aaps are examined for significant phenotypic deficiencies/advantages, which buttress the specific importance of limited numbers of Aaps involved in cryptococcal infections.

GC/EI/MS and 1H NMR Metabolomics Reveal the Effect of an Olive Tree Endophytic Bacillus sp. Lipopeptide Extract on the Metabolism of Colletotrichum acutatum

The transition to the Green Deal era requires the discovery of alternative sources of bioactivity and an in-depth understanding of their toxicity to target and non-target organisms. Endophytes have recently emerged as a source of bioactivity of high potential for applications in plant protection, used either per se as biological control agents or their metabolites as bioactive compounds. The olive tree endophytic isolate Bacillus sp. PTA13 produces an array of bioactive lipopeptides (LPs), which additionally exhibit reduced phytotoxicity, features that make them candidates for further research focusing on olive tree plant protection. Here, GC/EI/MS and ¹H NMR metabolomics were employed to study the toxicity of a Bacillus sp. PTA13 LP extract on the olive tree pathogen Colletotrichum acutatum, which causes the devastating disease olive anthracnose. The discovery of resistant isolates of the pathogen to the applied fungicides makes the research on the development of improved sources of bioactivity of paramount importance. Analyses revealed that the applied extract affects the metabolism of the fungus by interfering with the biosynthesis of various metabolites and its energy production. LPs had a great impact on the aromatic amino acid metabolism, the energy equilibrium of the fungus and its fatty acid content. Additionally, the applied LPs affected the levels of pathogenesis-related metabolites, a finding that supports their potential for further research as plant protection agents.

Molecular Characterization of Candida auris Isolates at a Major Tertiary Care Center in Lebanon

BACKGROUND

The globally emerging Candida auris pathogens poses heavy burden to the healthcare system. Their molecular analyses assist in understanding their epidemiology, dissemination, treatment, and control. This study was warranted to describe the genomic features and drug resistance profiles using whole genome sequencing (WGS) among C. auris isolates from Lebanon.

METHODS

A total of 28 C. auris clinical isolates, from different hospital units, were phenotypically identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and tested for antifungal resistance using Vitek-2 system and E test. The complete genomes were determined by WGS using long reads sequencing (PacBio) to reveal the clade distribution and antifungal resistance genes.

RESULTS

Candida auris revealed uniform resistance to fluconazole and amphotericin B, with full susceptibility to echinocandins. Among key resistance genes studied, only two mutations were detected: Y132F in ERG11 gene and a novel mutation, D709E, found in CDR1 gene encoding for an ABC efflux pump. Phylogenetically, C. auris genomes belonged to South Asian clade I and showed limited genetic diversity, suggesting person to person transmission.

CONCLUSION

This characterization of C. auris isolates from Lebanon revealed the exclusivity of clade I lineage together with uniform resistance to fluconazole and amphotericin B. The control of such highly resistant pathogen necessitates an appropriate and rapid recovery and identification to contain spread and outbreaks.

Molecular targets for antifungals in amino acid and protein biosynthetic pathways

Fungi cause death of over 1.5 million people every year, while cutaneous mycoses are among the most common infections in the world. Mycoses vary greatly in severity, there are long-term skin (ringworm), nail or hair infections (tinea capitis), recurrent like vaginal candidiasis or severe, life-threatening systemic, multiorgan infections. In the last few years, increasing importance is attached to the health and economic problems caused by fungal pathogens. There is a growing need for improvement of the availability of antifungal drugs, decreasing their prices and reducing side effects. Searching for novel approaches in this respect, amino acid and protein biosynthesis pathways appear to be competitive. The route that leads from amino acid biosynthesis to protein folding and its activation is rich in enzymes that are descriptive of fungi. Blocking the action of those enzymes often leads to avirulence or growth inhibition. In this review, we want to trace the principal processes of fungi vitality. We present the data of genes encoding enzymes involved in amino acid and protein biosynthesis, potential molecular targets in antifungal chemotherapy, and describe the impact of inhibitors on fungal organisms.

The yeast (Saccharomyces cerevisiae) YCF1 vacuole transporter: Evidence on its implication into the yeast resistance to flusilazole as revealed by GC/EI/MS metabolomics

The development of plant protection product (PPPs)-resistant populations of plant pathogens, pests, and weeds, represents a major challenge that the crop protection sector is facing. Focusing on plant pathogenic fungi, the increased efflux of the active ingredients (a.i.) from the cytoplasm is highly correlated to elevated resistance levels to the applied fungicides. Such mechanism is regulated by ATP-binding cassette transporters (ABC transporters), and although it has been investigated for the past two decades, the latest developments in "omics" technologies could provide new insights with potential applications in crop protection. Within this context, and based on results from preliminary experiments, we have undertaken the task of mining the involvement of the ABC transporter YCF1, which is located in the vacuole membrane, in the fungicide resistance development, applying a functional genomics approach and using yeast (Saccharomyces cerevisiae) as the model organism. Among the fungicides being assessed, flusilazole, which belongs to the azole group of dimethylation inhibitors (DMIs), was discovered as a possible substrate of the YCF1. GC/EI/MS metabolomics analysis revealed the effect of the fungicide's toxicity and that of genotype on yeast's metabolism, confirming the role of this transporter. Fluctuations in the activity of various yeast biosynthetic pathways associated with stress responses were recorded, and corresponding metabolites-biomarkers of flusilazole toxicity were discovered. The metabolites α,α-trehalose, glycerol, myo-inositol-1-phosphate, GABA, l-glutamine, l-tryptophan, l-phenylalanine, l-tyrosine, and phosphate, were the major identified biomarkers of toxicity. Among these, are metabolites that play important roles in fungal metabolism (e.g., cell responses to osmotic stress) or serve as signaling molecules. To the best of our knowledge, this is the first report on the implication of YCF1 in fungal resistance to PPPs. Additionally, the results of GC/EI/MS yeast metabolomics confirmed the robustness of the method and its applicability in the high-throughput study of fungal resistance to fungicides.

The regulation of the sulfur amino acid biosynthetic pathway in Cryptococcus neoformans: the relationship of Cys3, Calcineurin, and Gpp2 phosphatases

Cryptococcosis is a fungal disease caused by C. neoformans. To adapt and survive in diverse ecological niches, including the animal host, this opportunistic pathogen relies on its ability to uptake nutrients, such as carbon, nitrogen, iron, phosphate, sulfur, and amino acids. Genetic circuits play a role in the response to environmental changes, modulating gene expression and adjusting the microbial metabolism to the nutrients available for the best energy usage and survival. We studied the sulfur amino acid biosynthesis and its implications on C. neoformans biology and virulence. CNAG_04798 encodes a BZip protein and was annotated as CYS3, which has been considered an essential gene. However, we demonstrated that CYS3 is not essential, in fact, its knockout led to sulfur amino acids auxotroph. Western blots and fluorescence microscopy indicated that GFP-Cys3, which is expressed from a constitutive promoter, localizes to the nucleus in rich medium (YEPD); the addition of methionine and cysteine as sole nitrogen source (SD-N + Met/Cys) led to reduced nuclear localization and protein degradation. By proteomics, we identified and confirmed physical interaction among Gpp2, Cna1, Cnb1 and GFP-Cys3. Deletion of the calcineurin and GPP2 genes in a GFP-Cys3 background demonstrated that calcineurin is required to maintain Cys3 high protein levels in YEPD and that deletion of GPP2 causes GFP-Cys3 to persist in the presence of sulfur amino acids. Global transcriptional profile of mutant and wild type by RNAseq revealed that Cys3 controls all branches of the sulfur amino acid biosynthesis, and sulfur starvation leads to induction of several amino acid biosynthetic routes. In addition, we found that Cys3 is required for virulence in Galleria mellonella animal model.

Amino acid permeases in Cryptococcus neoformans are required for high temperature growth and virulence; and are regulated by Ras signaling

Cryptococcosis is an Invasive Fungal Infection (IFI) caused by Cryptococcus neoformans, mainly in immunocompromised patients. Therapeutic failure due to pathogen drug resistance, treatment inconstancy and few antifungal options is a problem. The study of amino acid biosynthesis and uptake represents an opportunity to explore possible development of novel antifungals. C. neoformans has 10 amino acids permeases, two of them (Aap3 and Aap7) not expressed at the conditions tested, and five were studied previously (Aap2, Aap4, Aap5, Mup1 and Mup3). Our previous results showed that Aap4 and Aap5 are major permeases with overlapping functions. The aap4Δ/aap5Δ double mutant fails to grow in amino acids as sole nitrogen source and is avirulent in animal model. Here, we deleted the remaining amino acid permeases (AAP1, AAP6, AAP8) that showed gene expression modulation by nutritional condition and created a double mutant (aap1Δ/aap2Δ). We studied the virulence attributes of these mutants and explored the regulatory mechanism behind amino acid uptake in C. neoformans. The aap1Δ/aap2Δ strain had reduced growth at 37°C in L-amino acids, reduced capsule production and was hypovirulent in the Galleria mellonella animal model. Our data, along with previous studies, (i) complement the analysis for all 10 amino acid permeases mutants, (ii) corroborate the idea that these transporters behave as global permeases, (iii) are required during heat and nutritional stress, and (iv) are important for virulence. Our study also indicates a new possible link between Ras1 signaling and amino acids uptake.

Implication of Fusarium graminearum primary metabolism in its resistance to benzimidazole fungicides as revealed by 1H NMR metabolomics

Fungal metabolomics is a field of high potential but yet largely unexploited. Focusing on plant-pathogenic fungi, no metabolomics studies exist on their resistance to fungicides, which represents a major issue that the agrochemical and agricultural sectors are facing. Fungal infections cause quantitative, but also qualitative yield losses, especially in the case of mycotoxin-producing species. The aim of the study was to correlate metabolic changes in Fusarium graminearum strains' metabolomes with their carbendazim-resistant level and discover corresponding metabolites-biomarkers, with primary focus on its primary metabolism. For this purpose, comparative ¹H NMR metabolomics was applied to a wild-type and four carbendazim-resistant Fusarium graminearum strains following or not exposure to the fungicide. Results showed an excellent discrimination between the strains based on their carbendazim-resistance following exposure to low concentration of the fungicide (2 mg L^-1). Both genotype and fungicide treatments had a major impact on fungal metabolism. Among the signatory metabolites, a positive correlation was discovered between the content of F. graminearum strains in amino acids of the aromatic and pyruvate families, l-glutamate, l-proline, l-serine, pyroglutamate, and succinate and their carbendazim-resistance level. In contrary, their content in l-glutamine and l-threonine, had a negative correlation. Many of these metabolites play important roles in fungal physiology and responses to stresses. This work represents a proof-of-concept of the applicability of ¹H NMR metabolomics for high-throughput screening of fungal mutations leading to fungicide resistance, and the study of its biochemical basis, focusing on the involvement of primary metabolism. Results could be further exploited in programs of resistance monitoring, genetic engineering, and crop protection for combating fungal resistance to fungicides.

Nutritional Requirements and Their Importance for Virulence of Pathogenic Cryptococcus Species

Cryptococcus sp. are basidiomycete yeasts which can be found widely, free-living in the environment. Interactions with natural predators, such as amoebae in the soil, are thought to have promoted the development of adaptations enabling the organism to survive inside human macrophages. Infection with Cryptococcus in humans occurs following inhalation of desiccated yeast cells or spore particles and may result in fatal meningoencephalitis. Human disease is caused almost exclusively by the Cryptococcus neoformans species complex, which predominantly infects immunocompromised patients, and the Cryptococcus gattii species complex, which is capable of infecting immunocompetent individuals. The nutritional requirements of Cryptococcus are critical for its virulence in animals. Cryptococcus has evolved a broad range of nutrient acquisition strategies, many if not most of which also appear to contribute to its virulence, enabling infection of animal hosts. In this review, we summarise the current understanding of nutritional requirements and acquisition in Cryptococcus and offer perspectives to its evolution as a significant pathogen of humans.

Antifungal Resistance, Metabolic Routes as Drug Targets, and New Antifungal Agents: An Overview about Endemic Dimorphic Fungi

Diseases caused by fungi can occur in healthy people, but immunocompromised patients are the major risk group for invasive fungal infections. Cases of fungal resistance and the difficulty of treatment make fungal infections a public health problem. This review explores mechanisms used by fungi to promote fungal resistance, such as the mutation or overexpression of drug targets, efflux and degradation systems, and pleiotropic drug responses. Alternative novel drug targets have been investigated; these include metabolic routes used by fungi during infection, such as trehalose and amino acid metabolism and mitochondrial proteins. An overview of new antifungal agents, including nanostructured antifungals, as well as of repositioning approaches is discussed. Studies focusing on the development of vaccines against antifungal diseases have increased in recent years, as these strategies can be applied in combination with antifungal therapy to prevent posttreatment sequelae. Studies focused on the development of a pan-fungal vaccine and antifungal drugs can improve the treatment of immunocompromised patients and reduce treatment costs.

Amino Acid Permeases and Virulence in Cryptococcus neoformans

Fungal opportunistic pathogens colonize various environments, from plants and wood to human and animal tissue. Regarding human pathogens, one great challenge during contrasting niche occupation is the adaptation to different conditions, such as temperature, osmolarity, salinity, pressure, oxidative stress and nutritional availability, which may constitute sources of stress that need to be tolerated and overcome. As an opportunistic pathogen, C. neoformans faces exactly these situations during the transition from the environment to the human host, encountering nutritional constraints. Our previous and current research on amino acid biosynthetic pathways indicates that amino acid permeases are regulated by the presence of the amino acids, nitrogen and temperature. Saccharomyces cerevisiae and Candida albicans have twenty-four and twenty-seven genes encoding amino acid permeases, respectively; conversely, they are scarce in number in Basidiomycetes (C. neoformans, Coprinopsis cinerea and Ustilago maydis), where nine to ten permease genes can be found depending on the species. In this study, we have demonstrated that two amino acid permeases are essential for virulence in C. neoformans. Our data showed that C. neoformans uses two global and redundant amino acid permeases, Aap4 and Aap5 to respond correctly to thermal and oxidative stress. Double deletion of these permeases causes growth arrest in C. neoformans at 37°C and in the presence of hydrogen peroxide. The inability to uptake amino acid at a higher temperature and under oxidative stress also led to virulence attenuation in vivo. Our data showed that thermosensitivity caused by the lack of permeases Aap4 and Aap5 can be remedied by alkaline conditions (higher pH) and salinity. Permeases Aap4 and Aap5 are also required during fluconazole stress and they are the target of the plant secondary metabolite eugenol, a potent antifungal inhibitor that targets amino acid permeases. In summary, our work unravels (i) interesting physiological property of C. neoformans regarding its amino acid uptake system; (ii) an important aspect of virulence, which is the need for amino acid permeases during thermal and oxidative stress resistance and, hence, host invasion and colonization; and (iii) provides a convenient prototype for antifungal development, which are the amino acid permeases Aap4/Aap5 and their inhibitor.

Virulence-Associated Enzymes of Cryptococcus neoformans

Enzymes play key roles in fungal pathogenesis. Manipulation of enzyme expression or activity can significantly alter the infection process, and enzyme expression profiles can be a hallmark of disease. Hence, enzymes are worthy targets for better understanding pathogenesis and identifying new options for combatting fungal infections. Advances in genomics, proteomics, transcriptomics, and mass spectrometry have enabled the identification and characterization of new fungal enzymes. This review focuses on recent developments in the virulence-associated enzymes from Cryptococcus neoformans. The enzymatic suite of C. neoformans has evolved for environmental survival, but several of these enzymes play a dual role in colonizing the mammalian host. We also discuss new therapeutic and diagnostic strategies that could be based on the underlying enzymology.

The Role of Amino Acid Permeases and Tryptophan Biosynthesis in Cryptococcus neoformans Survival

Metabolic diversity is an important factor during microbial adaptation to different environments. Among metabolic processes, amino acid biosynthesis has been demonstrated to be relevant for survival for many microbial pathogens, whereas the association between pathogenesis and amino acid uptake and recycling are less well-established. Cryptococcus neoformans is an opportunistic fungal pathogen with many habitats. As a result, it faces frequent metabolic shifts and challenges during its life cycle. Here we studied the C. neoformans tryptophan biosynthetic pathway and found that the pathway is essential. RNAi indicated that interruptions in the biosynthetic pathway render strains inviable. However, auxotroph complementation can be partially achieved by tryptophan uptake when a non preferred nitrogen source and lower growth temperature are applied, suggesting that amino acid permeases may be the target of nitrogen catabolism repression (NCR). We used bioinformatics to search for amino acid permeases in the C. neoformans and found eight potential global permeases (AAP1 to AAP8). The transcriptional profile of them revealed that they are subjected to regulatory mechanisms which are known to respond to nutritional status in other fungi, such as (i) quality of nitrogen (Nitrogen Catabolism Repression, NCR) and carbon sources (Carbon Catabolism Repression, CCR), (ii) amino acid availability in the extracellular environment (SPS-sensing) and (iii) nutritional deprivation (Global Amino Acid Control, GAAC). This study shows that C. neoformans has fewer amino acid permeases than other model yeasts, and that these proteins may be subjected to complex regulatory mechanisms. Our data suggest that the C. neoformans tryptophan biosynthetic pathway is an excellent pharmacological target. Furthermore, inhibitors of this pathway cause Cryptococcus growth arrest in vitro.

Homoserine and quorum-sensing acyl homoserine lactones as alternative sources of threonine: a potential role for homoserine kinase in insect-stage Trypanosoma brucei

De novo synthesis of threonine from aspartate occurs via the β-aspartyl phosphate pathway in plants, bacteria and fungi. However, the Trypanosoma brucei genome encodes only the last two steps in this pathway: homoserine kinase (HSK) and threonine synthase. Here, we investigated the possible roles for this incomplete pathway through biochemical, genetic and nutritional studies. Purified recombinant TbHSK specifically phosphorylates L-homoserine and displays kinetic properties similar to other HSKs. HSK null mutants generated in bloodstream forms displayed no growth phenotype in vitro or loss of virulence in vivo. However, following transformation into procyclic forms, homoserine, homoserine lactone and certain acyl homoserine lactones (AHLs) were found to substitute for threonine in growth media for wild-type procyclics, but not HSK null mutants. The tsetse fly is considered to be an unlikely source of these nutrients as it feeds exclusively on mammalian blood. Bioinformatic studies predict that tsetse endosymbionts possess part (up to homoserine in Wigglesworthia glossinidia) or all of the β-aspartyl phosphate pathway (Sodalis glossinidius). In addition S. glossinidius is known to produce 3-oxohexanoylhomoserine lactone which also supports trypanosome growth. We propose that T. brucei has retained HSK and threonine synthase in order to salvage these nutrients when threonine availability is limiting.

Inhibitors of amino acids biosynthesis as antifungal agents

Fungal microorganisms, including the human pathogenic yeast and filamentous fungi, are able to synthesize all proteinogenic amino acids, including nine that are essential for humans. A number of enzymes catalyzing particular steps of human-essential amino acid biosynthesis are fungi specific. Numerous studies have shown that auxotrophic mutants of human pathogenic fungi impaired in biosynthesis of particular amino acids exhibit growth defect or at least reduced virulence under in vivo conditions. Several chemical compounds inhibiting activity of one of these enzymes exhibit good antifungal in vitro activity in minimal growth media, which is not always confirmed under in vivo conditions. This article provides a comprehensive overview of the present knowledge on pathways of amino acids biosynthesis in fungi, with a special emphasis put on enzymes catalyzing particular steps of these pathways as potential targets for antifungal chemotherapy.

The role of the de novo pyrimidine biosynthetic pathway in Cryptococcus neoformans high temperature growth and virulence

Fungal infections are often difficult to treat due to the inherent similarities between fungal and animal cells and the resulting host toxicity from many antifungal compounds. Cryptococcus neoformans is an opportunistic fungal pathogen of humans that causes life-threatening disease, primarily in immunocompromised patients. Since antifungal therapy for this microorganism is limited, many investigators have explored novel drug targets aim at virulence factors, such as the ability to grow at mammalian physiological temperature (37°C). To address this issue, we used the Agrobacterium tumefaciens gene delivery system to create a random insertion mutagenesis library that was screened for altered growth at elevated temperatures. Among several mutants unable to grow at 37°C, we explored one bearing an interruption in the URA4 gene. This gene encodes dihydroorotase (DHOase) that is involved in the de novo synthesis of pyrimidine ribonucleotides. Loss of the C. neoformans Ura4 protein, by targeted gene interruption, resulted in an expected uracil/uridine auxotrophy and an unexpected high temperature growth defect. In addition, the ura4 mutant displayed phenotypic defects in other prominent virulence factors (melanin, capsule and phospholipase) and reduced stress response compared to wild type and reconstituted strains. Accordingly, this mutant had a decreased survival rate in macrophages and attenuated virulence in a murine model of cryptococcal infection. Quantitative PCR analysis suggests that this biosynthetic pathway is induced during the transition from 30°C to 37°C, and that transcriptional regulation of de novo and salvage pyrimidine pathway are under the control of the Ura4 protein.

Development of a tightly regulatable copper-mediated gene switch system in dermatophytes

Targeted gene deletion is now available for molecular genetic research of dermatophytes, and the physiological roles of several genes have been elucidated. However, this method cannot be applied to essential genes, which can be potential drug targets. To overcome this limitation, we have developed a conditional gene knockdown system using a copper-responsive promoter. The promoter sequence of the copper transporter gene CTR4 (P(CTR4)) and that of the copper efflux pump gene CRP1 (P(CRP1)) derived from Trichophyton rubrum were examined for their response to copper in Arthroderma vanbreuseghemii. P(CTR4) was demonstrated to repress expression of a reporter gene in the presence of copper, while the activity of P(CRP1) was induced by addition of copper. Importantly, P(CTR4) regulated the gene expression more tightly. Furthermore, when P(CTR4) was applied to regulate the expression of the endogenous genes ERG1 and TRP5, their conditional mutants exhibited decreased growth activity under the repressive conditions. These results suggest that the P(CTR4)-based gene regulation system represents a powerful tool for identification and characterization of a broad range of genes, including essential genes, in dermatophytes.

Development of a continuous assay and steady-state characterization of Escherichia coli threonine synthase

Threonine synthase (TS) catalyzes the hydrolysis of O-phospho-L-homoserine (OPHS) to produce L-threonine (L-Thr) and inorganic phosphate. Here, we report a simplified purification protocol for the OPHS substrate and a continuous, coupled-coupled, spectrophotometric TS assay. The sequential actions of threonine deaminase (TD) and hydroxyisocaproate dehydrogenase (HO-HxoDH) convert the L-Thr product of TS to α-ketobutyrate (α-KB) and then to 2-hydroxybutyrate, respectively, and are monitored as the decrease in absorbance at 340 nm resulting from the concomitant oxidation of β-nicotinamide adenine dinucleotide (NADH) to NAD(+) by HO-HxoDH. The effect of pH on the activities of Escherichia coli TD and Lactobacillus delbrueckii HO-HxoDH was determined to establish this continuous assay as suitable for steady-state characterization and to facilitate the optimization of coupling enzyme concentrations under different assay conditions to enable studies of TS across phyla. To validate this assay, TS from E. coli was characterized. The kinetic parameters (k(cat)=4s(-1) and K(m)=0.34 mM) and the pH optimum of 8.7, determined using the continuous assay, are consistent with values reported for this enzyme based on the discontinuous malachite green assay. The k(cat)/K(m)(OPHS) versus pH profile of E. coli TS is bell-shaped, and the apparent pK(a) values for the acidic and basic limbs are 7.1 and 10.4, respectively.

The GATA-type transcriptional activator Gat1 regulates nitrogen uptake and metabolism in the human pathogen Cryptococcus neoformans

Nitrogen uptake and metabolism are essential to microbial growth. Gat1 belongs to a conserved family of zinc finger containing transcriptional regulators known as GATA-factors. These factors activate the transcription of Nitrogen Catabolite Repression (NCR) sensitive genes when preferred nitrogen sources are absent or limiting. Cryptococcus neoformans GAT1 is an ortholog to the Aspergillus nidulans AreA and Candida albicans GAT1 genes. In an attempt to define the function of this transcriptional regulator in C. neoformans, we generated null mutants (gat1Δ) of this gene. The gat1 mutant exhibited impaired growth on all amino acids tested as sole nitrogen sources, with the exception of arginine and proline. Furthermore, the gat1 mutant did not display resistance to rapamycin, an immunosuppressant drug that transiently mimics a low-quality nitrogen source. Gat1 is not required for C. neoformans survival during macrophage infection or for virulence in a mouse model of cryptococcosis. Microarray analysis allowed the identification of target genes that are regulated by Gat1 in the presence of proline, a poor and non-repressing nitrogen source. Genes involved in ergosterol biosynthesis, iron uptake, cell wall organization and capsule biosynthesis, in addition to NCR-sensitive genes, are Gat1-regulated in C. neoformans.

Fungal homoserine kinase (thr1Delta) mutants are attenuated in virulence and die rapidly upon threonine starvation and serum incubation

The fungally conserved subset of amino acid biosynthetic enzymes not present in humans offer exciting potential as an unexploited class of antifungal drug targets. Since threonine biosynthesis is essential in Cryptococcus neoformans, we further explored the potential of threonine biosynthetic enzymes as antifungal drug targets by determining the survival in mice of Saccharomyces cerevisiae homoserine kinase (thr1Delta) and threonine synthase (thr4Delta) mutants. In striking contrast to aspartate kinase (hom3Delta) mutants, S. cerevisiae thr1Delta and thr4Delta mutants were severely depleted after only 4 h in vivo. Similarly, Candida albicans thr1Delta mutants, but not hom3Delta mutants, were significantly attenuated in virulence. Consistent with the in vivo phenotypes, S. cerevisiae thr1Delta and thr4Delta mutants as well as C. albicans thr1Delta mutants were extremely serum sensitive. In both species, serum sensitivity was suppressed by the addition of threonine, a feedback inhibitor of Hom3p. Because mutation of the HOM3 and HOM6 genes, required for the production of the toxic pathway intermediate homoserine, also suppressed serum sensitivity, we hypothesize that serum sensitivity is a consequence of homoserine accumulation. Serum survival is critical for dissemination, an important virulence determinant: thus, together with the essential nature of C. neoformans threonine synthesis, the cross-species serum sensitivity of thr1Delta mutants makes the fungus-specific Thr1p, and likely Thr4p, ideal antifungal drug targets.

Homoserine toxicity in Saccharomyces cerevisiae and Candida albicans homoserine kinase (thr1Delta) mutants

In addition to threonine auxotrophy, mutation of the Saccharomyces cerevisiae threonine biosynthetic genes THR1 (encoding homoserine kinase) and THR4 (encoding threonine synthase) results in a plethora of other phenotypes. We investigated the basis for these other phenotypes and found that they are dependent on the toxic biosynthetic intermediate homoserine. Moreover, homoserine is also toxic for Candida albicans thr1Delta mutants. Since increasing levels of threonine, but not other amino acids, overcome the homoserine toxicity of thr1Delta mutants, homoserine may act as a toxic threonine analog. Homoserine-mediated lethality of thr1Delta mutants is blocked by cycloheximide, consistent with a role for protein synthesis in this lethality. We identified various proteasome and ubiquitin pathway components that either when mutated or present in high copy numbers suppressed the thr1Delta mutant homoserine toxicity. Since the doa4Delta and proteasome mutants identified have reduced ubiquitin- and/or proteasome-mediated proteolysis, the degradation of a particular protein or subset of proteins likely contributes to homoserine toxicity.

Cytocidal amino acid starvation of Saccharomyces cerevisiae and Candida albicans acetolactate synthase (ilv2{Delta}) mutants is influenced by the carbon source and rapamycin

The isoleucine and valine biosynthetic enzyme acetolactate synthase (Ilv2p) is an attractive antifungal drug target, since the isoleucine and valine biosynthetic pathway is not present in mammals, Saccharomyces cerevisiae ilv2Delta mutants do not survive in vivo, Cryptococcus neoformans ilv2 mutants are avirulent, and both S. cerevisiae and Cr. neoformans ilv2 mutants die upon isoleucine and valine starvation. To further explore the potential of Ilv2p as an antifungal drug target, we disrupted Candida albicans ILV2, and demonstrated that Ca. albicans ilv2Delta mutants were significantly attenuated in virulence, and were also profoundly starvation-cidal, with a greater than 100-fold reduction in viability after only 4 h of isoleucine and valine starvation. As fungicidal starvation would be advantageous for drug design, we explored the basis of the starvation-cidal phenotype in both S. cerevisiae and Ca. albicans ilv2Delta mutants. Since the mutation of ILV1, required for the first step of isoleucine biosynthesis, did not suppress the ilv2Delta starvation-cidal defects in either species, the cidal phenotype was not due to alpha-ketobutyrate accumulation. We found that starvation for isoleucine alone was more deleterious in Ca. albicans than in S. cerevisiae, and starvation for valine was more deleterious than for isoleucine in both species. Interestingly, while the target of rapamycin (TOR) pathway inhibitor rapamycin further reduced S. cerevisiae ilv2Delta starvation viability, it increased Ca. albicans ilv1Delta and ilv2Delta viability. Furthermore, the recovery from starvation was dependent on the carbon source present during recovery for S. cerevisiae ilv2Delta mutants, reminiscent of isoleucine and valine starvation inducing a viable but non-culturable-like state in this species, while Ca. albicans ilv1Delta and ilv2 Delta viability was influenced by the carbon source present during starvation, supporting a role for glucose wasting in the Ca. albicans cidal phenotype.