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

A fungal ubiquitin ligase and arrestin binding partner contribute to pathogenesis and survival during cellular stress

Cellular responses to external stress allow microorganisms to adapt to a vast array of environmental conditions, including infection sites. The molecular mechanisms behind these responses are studied to gain insight into microbial pathogenesis, which could lead to new antimicrobial therapies. Here, we explore a role for arrestin protein-mediated ubiquitination in stress response and pathogenesis in the pathogenic fungus Cryptococcus neoformans. In a previous study, we identified four arrestin-like proteins in C. neoformans and found that one of these is required for efficient membrane synthesis, likely by directing interaction between fatty acid synthases and the Rsp5 E3 ubiquitin ligase. Here, we further explore Cn Rsp5 function and determine that this single Ub ligase is absolutely required for pathogenesis and survival in the presence of cellular stress. Additionally, we show that a second arrestin-like protein, Ali2, similarly facilitates interaction between Rsp5 and some of its protein targets. Of the four postulated C. neoformans arrestin-like proteins, Ali2 appears to contribute the most to C. neoformans pathogenesis, likely by directing Rsp5 to pathogenesis-related ubiquitination targets. A proteomics-based differential ubiquitination screen revealed that several known cell surface proteins are ubiquitinated by Rsp5 and a subset also requires Ali2 for their ubiquitination. Rsp5-mediated ubiquitination alters the stability and the localization of these proteins. A loss of Rsp5-mediated ubiquitination results in cell wall defects that increase susceptibility to external stresses. These findings support a model in which arrestin-like proteins guide Rsp5 to ubiquitinate specific target proteins, some of which are required for survival during stress.

IMPORTANCE

Microbial proteins involved in human infectious diseases often need to be modified by specific chemical additions to be fully functional. Here, we explore the role of a particular protein modification, ubiquitination, in infections due to the human fungal pathogen Cryptococcus neoformans. We identified a complex of proteins responsible for adding ubiquitin groups to fungal proteins, and this complex is required for virulence. These proteins are fungal specific and might be targets for novel anti-infection therapy.

Discovery of CO2 tolerance genes associated with virulence in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is a ubiquitous soil fungus and airborne pathogen that causes over 180,000 deaths each year. Cryptococcus must adapt to host CO2 levels to cause disease, but the genetic basis for this adaptation is unknown. We utilized quantitative trait loci mapping with 374 progeny from a cross between a CO2-tolerant clinical isolate and a CO2-sensitive environmental isolate to identify genetic regions regulating CO2 tolerance. To identify specific quantitative trait genes, we applied fine mapping through bulk segregant analysis of near-isogenic progeny with distinct tolerance levels to CO2. We found that virulence among near-isogenic strains in a murine model of cryptococcosis correlated with CO2 tolerance. Moreover, we discovered that sensitive strains may adapt in vivo to become more CO2 tolerant and more virulent. These findings highlight the underappreciated role of CO2 tolerance and its importance in the ability of an opportunistic environmental pathogen to cause disease.

Effects of CO2 in fungi

Carbon dioxide supplies carbon for photosynthetic species and is a major product of respiration for all life forms. Inside the human body where CO2 is a by-product of the tricarboxylic acid cycle, its level reaches 5% or higher. In the ambient atmosphere, ∼.04% of the air is CO2. Different organisms can tolerate different CO2 levels to various degrees, and experiencing higher CO2 is toxic and can lead to death. The fungal kingdom shows great variations in response to CO2 that has been documented by different researchers at different time periods. This literature review aims to connect these studies, highlight mechanisms underlying tolerance to high levels of CO2, and emphasize the effects of CO2 on fungal metabolism and morphogenesis.

Omics analysis revealed the possible mechanism of streptococcus disease outbreak in tilapia under high temperature

High temperature is a main cause to result in the outbreak of tilapia streptococcal disease. However, the underlying mechanisms are not well understood. In this study, we first confirmed that tilapia infected with Streptococcus agalactiae (S. agalactiae) had a higher mortality at high temperature (35 °C) than that at normal temperature (28 °C). Subsequently, the effects of high temperature on gene expression pattern of S. agalactiae and intestinal microbiota of tilapia were respectively detected by RNA-seq and 16S rDNA sequencing. RNA-seq identified 357 differentially expressed genes (DEGs) in S. agalactiae cultured at 28 °C and 35 °C. GO and KEGG analysis showed that these DEGs were highly involved in metabolic processes, including glucose, lipid and amino acid metabolisms, which indicates that S. agalactiae have stronger vitality and are likely to be more infectious under high temperature. Microbiota analysis revealed that high temperature could influence the bacterial community composition of tilapia intestine, accompanied by changes in intestinal structure. Compared to feed at 28 °C, the total bacterial species as well as pathogens, such as norank_f__Rhizobiales_Incertae_Sedis, Pseudorhodoplanes, Ancylobacter, in tilapia intestine were significantly increased at 35 °C, which may weaken the immune resistance of tilapia. Taken together, our results suggest that high temperature evoked tilapia susceptible to S. agalactiae should be the combined effect of enhanced S. agalactiae metabolism and dysregulated tilapia intestinal microbiota.

Regulation of the pyrimidine biosynthetic pathway by lysine acetylation of E. coli OPRTase

The de novo pyrimidine biosynthesis pathway is an important route due to the relevance of its products, its implications in health and its conservation among organisms. Here, we investigated the regulation by lysine acetylation of this pathway. To this aim, intracellular and extracellular metabolites of the route were quantified, revealing a possible blockage of the pathway by acetylation of the OPRTase enzyme (orotate phosphoribosyltransferase). Chemical acetylation of OPRTase by acetyl-P involved a decrease in enzymatic activity. To test the effect of acetylation in this enzyme, K26 and K103 residues were selected to generate site-specific acetylated proteins. Several differences were observed in kinetic parameters, emphasizing that the k_cat of these mutants showed a strong decrease of 300 and 150-fold for OPRTase-103AcK and 19 and 6.3-fold for OPRTase-26AcK, for forward and reverse reactions. In vivo studies suggested acetylation of this enzyme by a nonenzymatic acetyl-P-dependent mechanism and a reversion of this process by the CobB deacetylase. A complementation assay of a deficient strain in the pyrE gene with OPRTase-26AcK and OPRTase-103AcK was performed, and curli formation, stoichiometric parameters and orotate excretion were measured. Complementation with acetylated enzymes entailed a profile very similar to that of the ∆pyrE strain, especially in the case of complementation with OPRTase-103AcK. These results suggest regulation of the de novo pyrimidine biosynthesis pathway by lysine acetylation of OPRTase in Escherichia coli. This finding is of great relevance due to the essential role of this route and the OPRTase enzyme as a target for antimicrobial, antiviral and cancer treatments.

Novel Promising Antifungal Target Proteins for Conquering Invasive Fungal Infections

Invasive fungal infections (IFIs) pose a serious clinical problem, but the antifungal arsenal is limited and has many disadvantages, such as drug resistance and toxicity. Hence, there is an urgent need to develop antifungal compounds that target novel target proteins of pathogenic fungi for treating IFIs. This review provides a comprehensive summary of the biological functions of novel promising target proteins for treating IFIs in pathogenic fungi and their inhibitors. Inhibitors of inositol phosphoramide (IPC) synthases (such as Aureobasidin A, Khafrefungin, Galbonolide A, and Pleofungin A) have potent antifungal activities by inhibiting sphingolipid synthesis. Disrupting glycosylphosphatidylinositol (GPI) biosynthesis by Jawsamycin (an inhibitor of Spt14), M720 (an inhibitor of Mcd4), and APX001A (an inhibitor of Gwt1) is a promising strategy for treating IFIs. Turbinmicin is a natural-compound inhibitor of Sec14 and has extraordinary antifungal efficacy, broad-antifungal spectrum, low toxicity, and is a promising new compound for treating IFIs. CMLD013075 targets fungal heat shock protein 90 (Hsp90) and has remarkable antifungal efficacy. Olorofim, as an inhibitor of dihydrolactate dehydrogenase, is a breakthrough drug treatment for IFIs. These novel target proteins and their inhibitors may overcome the limitations of currently available antifungal drugs and improve patient outcomes in the treatment of IFIs.

Resistance profiling of Aspergillus fumigatus to olorofim indicates absence of intrinsic resistance and unveils the molecular mechanisms of acquired olorofim resistance

Olorofim (F901318) is a new antifungal currently under clinical development that shows both in vitro and in vivo activity against a number of filamentous fungi including Aspergillus fumigatus. In this study, we screened A. fumigatus isolates for intrinsic olorofim-resistant A. fumigatus and evaluated the ability of A. fumigatus to acquire an olorofim-resistant phenotype. No intrinsic resistance was found in 975 clinical A. fumigatus isolates. However, we found that isolates with increased olorofim MICs (> 8 mg/L) could be selected using a high number of conidia and olorofim exposure under laboratory conditions. Assessment of the frequency of acquired olorofim resistance development of A. fumigatus was shown to be higher than for voriconazole but lower than for itraconazole. Sequencing the PyrE gene of isogenic isolates with olorofim MICs of >8 mg/L identified various amino acid substitutions with a hotspot at locus G119. Olorofim was shown to have reduced affinity to mutated target protein dihydroorotate dehydrogenase (DHODH) and the effect of these mutations was proven by introducing the mutations directly in A. fumigatus. We then investigated whether G119 mutations were associated with a fitness cost in A. fumigatus. These experiments showed a small but significant reduction in growth rate for strains with a G119V substitution, while strains with a G119C substitution did not exhibit a reduction in growth rate. These in vitro findings were confirmed in an in vivo pathogenicity model.

Current Promising Therapeutic Targets for Aspergillosis Treatment

Aspergillosis is a fungal disease caused by different species of Aspergillus. They live in soil,dust and decomposed material. Number of Aspergillus species found till now is about 300 and more are still to be identified. Only few Aspergillus species can cause human disease and the most common species for human infection is Aspergillus fumigatus, which is a ubiquitous airborne saprophytic fungus. Severity of the disease ranges from an allergic response to life-threatening generalized infection. They grow optimally at 37°C and can grow upto 50°C. The fungal conidia are being constantly inhaled by humans and animals everyday normally gets eliminated by innate immune mechanism. Due to increasing number of immunocompromised patients, severe and fatal Aspergillosis cases have augmented. Currently, available antifungal drug for the treatment of Aspergillosis act on these three molecular target are 14 alpha demethylase for Azoles, ergosterol for Polyene and β-1,3-glucan synthase for Echinocandin. These antifungal drug show high resistance problem and toxicity. So, it is high time to develop new drugs for treatment with reduced toxicity and drug resistant problem. Synthesis of essential amino acid is absent in human as they obtain it from their diet but fungi synthesis these amino acid. Thus, enzymes in this pathway acts as novel drug target. This article summarizes promising drug targets presents in different metabolic pathway of Aspergillus genome and discusses their molecular functions in detail. This review also list down the inhibitors of these novel target. We present a comprehensive review that will pave way for discovery and development of novel antifungals against these drug targets for Aspergillosis treatment.

The Future of Antifungal Drug Therapy: Novel Compounds and Targets

Fungal infections are a universal problem and are routinely associated with high morbidity and mortality rates in immunocompromised patients. Existing therapies comprise five different classes of antifungal agents, four of which target the synthesis of ergosterol and cell wall glucans. However, the currently available antifungals have many limitations, including poor oral bioavailability, narrow therapeutic indices, and emerging drug resistance resulting from their use, thus making it essential to investigate the development of novel drugs which can overcome these limitations and add to the antifungal armamentarium. Advances have been made in antifungal drug discovery research and development over the past few years as evidenced by the presence of several new compounds currently in various stages of development. In the following minireview, we provide a comprehensive summary of compounds aimed at one or more novel molecular targets. We also briefly describe potential pathways relevant for fungal pathogenesis that can be considered for drug development in the near future.

Molecular repertoire of Deinococcus radiodurans after 1 year of exposure outside the International Space Station within the Tanpopo mission

BACKGROUND

The extraordinarily resistant bacterium Deinococcus radiodurans withstands harsh environmental conditions present in outer space. Deinococcus radiodurans was exposed for 1 year outside the International Space Station within Tanpopo orbital mission to investigate microbial survival and space travel. In addition, a ground-based simulation experiment with conditions, mirroring those from low Earth orbit, was performed.

METHODS

We monitored Deinococcus radiodurans cells during early stage of recovery after low Earth orbit exposure using electron microscopy tools. Furthermore, proteomic, transcriptomic and metabolomic analyses were performed to identify molecular mechanisms responsible for the survival of Deinococcus radiodurans in low Earth orbit.

RESULTS

D. radiodurans cells exposed to low Earth orbit conditions do not exhibit any morphological damage. However, an accumulation of numerous outer-membrane-associated vesicles was observed. On levels of proteins and transcripts, a multi-faceted response was detected to alleviate cell stress. The UvrABC endonuclease excision repair mechanism was triggered to cope with DNA damage. Defense against reactive oxygen species is mirrored by the increased abundance of catalases and is accompanied by the increased abundance of putrescine, which works as reactive oxygen species scavenging molecule. In addition, several proteins and mRNAs, responsible for regulatory and transporting functions showed increased abundances. The decrease in primary metabolites indicates alternations in the energy status, which is needed to repair damaged molecules.

CONCLUSION

Low Earth orbit induced molecular rearrangements trigger multiple components of metabolic stress response and regulatory networks in exposed microbial cells. Presented results show that the non-sporulating bacterium Deinococcus radiodurans survived long-term low Earth orbit exposure if wavelength below 200 nm are not present, which mirrors the UV spectrum of Mars, where CO₂ effectively provides a shield below 190 nm. These results should be considered in the context of planetary protection concerns and the development of new sterilization techniques for future space missions. Video Abstract.

Transposon mobilization in the human fungal pathogen Cryptococcus is mutagenic during infection and promotes drug resistance in vitro

When transitioning from the environment, pathogenic microorganisms must adapt rapidly to survive in hostile host conditions. This is especially true for environmental fungi that cause opportunistic infections in immunocompromised patients since these microbes are not well adapted human pathogens. Cryptococcus species are yeastlike fungi that cause lethal infections, especially in HIV-infected patients. Using Cryptococcus deneoformans in a murine model of infection, we examined contributors to drug resistance and demonstrated that transposon mutagenesis drives the development of 5-fluoroorotic acid (5FOA) resistance. Inactivation of target genes URA3 or URA5 primarily reflected the insertion of two transposable elements (TEs): the T1 DNA transposon and the TCN12 retrotransposon. Consistent with in vivo results, increased rates of mutagenesis and resistance to 5FOA and the antifungal drugs rapamycin/FK506 (rap/FK506) and 5-fluorocytosine (5FC) were found when Cryptococcus was incubated at 37° compared to 30° in vitro, a condition that mimics the temperature shift that occurs during the environment-to-host transition. Inactivation of the RNA interference (RNAi) pathway, which suppresses TE movement in many organisms, was not sufficient to elevate TE movement at 30° to the level observed at 37°. We propose that temperature-dependent TE mobilization in Cryptococcus is an important mechanism that enhances microbial adaptation and promotes pathogenesis and drug resistance in the human host.

Dihydroorotate dehydrogenase inhibitors in anti-infective drug research

Pyrimidines are essential for the cell survival and proliferation of living parasitic organisms, such as Helicobacter pylori, Plasmodium falciparum and Schistosoma mansoni, that are able to impact upon human health. Pyrimidine building blocks, in human cells, are synthesised via both de novo biosynthesis and salvage pathways, the latter of which is an effective way of recycling pre-existing nucleotides. As many parasitic organisms lack pyrimidine salvage pathways for pyrimidine nucleotides, blocking de novo biosynthesis is seen as an effective therapeutic means to selectively target the parasite without effecting the human host. Dihydroorotate dehydrogenase (DHODH), which is involved in the de novo biosynthesis of pyrimidines, is a validated target for anti-infective drug research. Recent advances in the DHODH microorganism field are discussed herein, as is the potential for the development of DHODH-targeted therapeutics.

Lighting Up Mutation: a New Unbiased System for the Measurement of Microbial Mutation Rates

Although mutation drives evolution over long and short terms, measuring and comparing mutation rates accurately have been particularly difficult. This is especially true when mutations lead to an alteration in fitness.

Although mutation drives evolution over long and short terms, measuring and comparing mutation rates accurately have been particularly difficult. This is especially true when mutations lead to an alteration in fitness. E. Shor, J. Schuyler, and D. S. Perlin (https://doi.org/10.1128/mBio.00120-19) present a new method to compare mutation rates across fungal strains and under different growth conditions: they employ the green fluorescent protein (GFP) as the reporter and count mutations using fluorescence-activated cell sorting (FACS). The estimates of mutation rates using the GFP-FACS approach are similar to those calculated with other reporters, and the method was used to assess if different alleles of the mismatch repair pathway gene MSH2 impact the mutation rates in the human pathogen Candida glabrata. The approach could be extended to other microbes and applications, opening the way for a better understanding of how mutation rates have impacted speciation and the emergence of antimicrobial resistance.

Two-spotted cricket as an animal infection model of human pathogenic fungi

Invertebrate infection models that can be evaluated at human body temperature are limited. In this study, we utilized the two-spotted cricket, a heat-tolerant insect, as an animal infection model of human pathogenic fungi. Injection of human pathogenic fungi, including Candida albicans, Candida glabrata, and Cryptococcus neoformans killed crickets within 48 h at both 27˚C and 37˚C. The median lethal dose values (LD50 values) of C. albicans and C. glabrata against crickets were decreased at 37˚C compared to that at 27˚C, whereas the LD50 value of C. neoformans was not different between 27˚C and 37˚C. Heat-killed cells of the three different fungi also killed crickets, but the LD50 value of the heat-killed cells was higher than 5-fold that of live fungal cells in the respective species. C. neoformans gene-knockout strains of cna1, gpa1, and pka1, which are required for virulence in mammals, had greater LD50 values than the parent strain in crickets. These findings suggest that the two-spotted cricket is a valuable infection model of human pathogenic fungi that can be used to evaluate fungal virulence at variable temperatures, including 37˚C, and that the killing abilities of C. albicans and C. glabrata against animals are increased at 37˚C.

The role of Aspartyl aminopeptidase (Ape4) in Cryptococcus neoformans virulence and authophagy

In order to survive and cause disease, microbial pathogens must be able to proliferate at the temperature of their infected host. We identified novel microbial features associated with thermotolerance in the opportunistic fungal pathogen Cryptococcus neoformans using a random insertional mutagenesis strategy, screening for mutants with defective growth at 37°C. Among several thermosensitive mutants, we identified one bearing a disruption in a gene predicted to encode the Ape4 aspartyl aminopeptidase protein. Ape4 metalloproteases in other fungi, including Saccharomyces cerevisiae, are activated by nitrogen starvation, and they are required for autophagy and the cytoplasm-to-vacuole targeting (Cvt) pathway. However, none have been previously associated with altered growth at elevated temperatures. We demonstrated that the C. neoformans ape4 mutant does not grow at 37°C, and it also has defects in the expression of important virulence factors such as phospholipase production and capsule formation. C. neoformans Ape4 activity was required for this facultative intracellular pathogen to survive within macrophages, as well as for virulence in an animal model of cryptococcal infection. Similar to S. cerevisiae Ape4, the C. neoformans GFP-Ape4 fusion protein co-localized with intracytoplasmic vesicles during nitrogen depletion. APE4 expression was also induced by the combination of nutrient and thermal stress. Together these results suggest that autophagy is an important cellular process for this microbial pathogen to survive within the environment of the infected host.

De Novo Pyrimidine Biosynthesis Connects Cell Integrity to Amphotericin B Susceptibility in Cryptococcus neoformans

The use of amphotericin B (AmB) in conjunction with 5-fluorocytosine (5-FC) is known to be the optimal therapy for treating cryptococcosis, but the mechanism by which 5-FC synergizes with AmB is unknown. In this study, we generated a Cryptococcus neoformans ura1Δ mutant lacking dihydroorotate dehydrogenase (DHODH), which demonstrated temperature-sensitive growth due to a defect in cell integrity and sensitivity to cell wall-damaging agents. In addition, sensitivity to AmB was greatly increased. Inclusion of uracil or uridine in the medium did not suppress the cell wall or AmB phenotype, whereas complementation with the wild-type URA1 gene complemented the mutant phenotype. As a measure of membrane accessibility, we assayed the rate of association of the lipid-binding dye 3,3'-dihexyloxacarbocyanine iodide (DiOC6) and saw more rapid association in the ura1Δ mutant. We likewise saw an increased rate of DiOC6 association in other AmB-sensitive mutants, including a ura- spontaneous URA5 mutant made by 5-fluoroorotic acid (5-FOA) selection and a bck1Δ mutant defective in cell integrity signaling. Similar results were also obtained by using a specific plasma membrane-binding CellMask live stain, with cell integrity mutants that exhibited increased and faster association of the dye with the membrane. Chitin synthase mutants (chs5Δ and chs6Δ) that lack any reported cell wall defects, in turn, demonstrate neither any increased susceptibility to AmB nor a greater accessibility to either of the dyes. Finally, perturbation of the cell wall of the wild type by treatment with the β-1,6-glucan synthase inhibitor caspofungin was synergistic with AmB in vitro. IMPORTANCE Synergy between AmB and nucleotide biosynthetic pathways has been documented, but the mechanism of this interaction has not been delineated. Results from this study suggest a correlation between uridine nucleotide biosynthesis and cell integrity likely mediated through the pool of nucleotide-sugar conjugates, which are precursor molecules for both capsule and cell wall of C. neoformans. Thus, we propose a mechanism by which structural defects in the cell wall resulting from perturbation of pyrimidine biosynthesis allow faster and increased penetration of AmB molecules into the cell membrane. Overall, our work demonstrates that impairment of pyrimidine biosynthesis in C. neoformans could be a potential target for antifungal therapy, either alone or in combination with AmB.

F901318 represents a novel class of antifungal drug that inhibits dihydroorotate dehydrogenase

There is an important medical need for new antifungal agents with novel mechanisms of action to treat the increasing number of patients with life-threatening systemic fungal disease and to overcome the growing problem of resistance to current therapies. F901318, the leading representative of a novel class of drug, the orotomides, is an antifungal drug in clinical development that demonstrates excellent potency against a broad range of dimorphic and filamentous fungi. In vitro susceptibility testing of F901318 against more than 100 strains from the four main pathogenic Aspergillus spp. revealed minimal inhibitory concentrations of ≤0.06 µg/mL-greater potency than the leading antifungal classes. An investigation into the mechanism of action of F901318 found that it acts via inhibition of the pyrimidine biosynthesis enzyme dihydroorotate dehydrogenase (DHODH) in a fungal-specific manner. Homology modeling of Aspergillus fumigatus DHODH has identified a predicted binding mode of the inhibitor and important interacting amino acid residues. In a murine pulmonary model of aspergillosis, F901318 displays in vivo efficacy against a strain of A. fumigatus sensitive to the azole class of antifungals and a strain displaying an azole-resistant phenotype. F901318 is currently in late Phase 1 clinical trials, offering hope that the antifungal armamentarium can be expanded to include a class of agent with a mechanism of action distinct from currently marketed antifungals.

Exploitation of sulfonylurea resistance marker and non-homologous end joining mutants for functional analysis in Zymoseptoria tritici

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We have constructed Z. tritici ku70 and ku80 null mutants.

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Gene targeting frequency in the ku null strains is greater than 85%.

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Deletion of KU70 and KU80 does not affect in vitro growth or pathogenicity.

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Sulfonylurea resistance was established as a new positive selection marker in Z. tritici.

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Ternary vectors were constructed to enable yeast recombinational cloning in Z. tritici.

The lack of techniques for rapid assembly of gene deletion vectors, paucity of selectable marker genes available for genetic manipulation and low frequency of homologous recombination are major constraints in construction of gene deletion mutants in Zymoseptoria tritici. To address these issues, we have constructed ternary vectors for Agrobacterium tumefaciens mediated transformation of Z. tritici, which enable the single step assembly of multiple fragments via yeast recombinational cloning. The sulfonylurea resistance gene, which is a mutated allele of the Magnaporthe oryzae ILV2 gene, was established as a new dominant selectable marker for Z. tritici. To increase the frequency of homologous recombination, we have constructed Z. tritici strains deficient in the non-homologous end joining pathway of DNA double stranded break repair by inactivating the KU70 and KU80 genes. Targeted gene deletion frequency increased to more than 85% in both Z. tritici ku70 and ku80 null strains, compared to ⩽10% seen in the wild type parental strain IPO323. The in vitro growth and in planta pathogenicity of the Z. tritici ku70 and ku80 null strains were comparable to strain IPO323. Together these molecular tools add significantly to the platform available for genomic analysis through targeted gene deletion or promoter replacements and will facilitate large-scale functional characterization projects in Z. tritici.

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.

Pyrimidine Metabolism: Dynamic and Versatile Pathways in Pathogens and Cellular Development

The importance of pyrimidines lies in the fact that they are structural components of a broad spectrum of key molecules that participate in diverse cellular functions, such as synthesis of DNA, RNA, lipids, and carbohydrates. Pyrimidine metabolism encompasses all enzymes involved in the synthesis, degradation, salvage, interconversion and transport of these molecules. In this review, we summarize recent publications that document how pyrimidine metabolism changes under a variety of conditions, including, when possible, those studies based on techniques of genomics, transcriptomics, proteomics, and metabolomics. First, we briefly look at the dynamics of pyrimidine metabolism during nonpathogenic cellular events. We then focus on changes that pathogen infections cause in the pyrimidine metabolism of their host. Next, we discuss the effects of antimetabolites and inhibitors, and finally we consider the consequences of genetic manipulations, such as knock-downs, knock-outs, and knock-ins, of pyrimidine enzymes on pyrimidine metabolism in the cell.