Iron uptake by fungi: contrasted mechanisms with internal or external reduction

Experimental and DFT Study of Monensinate and Salinomycinate Complexes Containing {Fe3(µ3-O)}7+ Core

Two trinuclear oxo-centred iron(III) coordination compounds of monensic and salinomycinic acids (HL) were synthesized and their spectral properties were studied using physicochemical/thermal methods (FT-IR, TG-DTA, TG-MS, EPR, Mössbauer spectroscopy, powder XRD) and elemental analysis. The data suggested the formation of [Fe3(µ3-O)L3(OH)4] and the probable complex structures were modelled using the DFT method. The computed spectral parameters of the optimized constructs were compared to the experimentally measured ones. In each complex, three metal centres were joined together at the axial position by a μ3-O unit to form a {Fe3O}7+ core. The antibiotics monoanions served as bidentate ligands through the carboxylate and hydroxyl groups located at the termini. The carboxylate moieties played a dual role bridging each two metal centres. Hydroxide anions secured the overall neutral character of the coordination species. Mössbauer spectra displayed asymmetric quadrupole doublets that were consistent with the existence of two types of high-spin iron(III) sites with different environments-two Fe[O5] and one Fe[O6] centres. The solid-state EPR studies confirmed the +3 oxidation state of iron with a total spin St = 5/2 per trinuclear cluster. The studied complexes are the first iron(III) coordination compounds of monensin and salinomycin reported so far.

Production of IgY against iron permease Ftr1 from Candida albicans and evaluation of its antifungal activity using Galleria mellonella as a model of systemic infection

Candida albicans is one of the leading pathological agents of mucosal and deep tissue infections. Considering that the variety of antifungals is restricted and that toxicity limits their use, immunotherapies against pathogenic fungi have been viewed as alternatives with reduced adverse effects. In this context, C. albicans has a protein used to capture iron from the environment and the host, known as the high-affinity iron permease Ftr1. This protein may be a new target of action for novel antifungal therapies, as it influences the virulence of this yeast. Thus, the aim of the present study was to produce and conduct the biological characterization of IgY antibodies against C. albicans Ftr1. Immunization of laying hens with an Ftr1-derived peptide resulted in IgY antibodies extracted from egg yolks capable of binding to the antigen with high affinity (avidity index = 66.6 ± 0.3%). These antibodies reduced the growth and even eliminated C. albicans under iron restriction, a favorable condition for the expression of Ftr1. This also occurred with a mutant strain that does not produce Ftr1 in the presence of iron, a circumstance in which the protein analog of iron permease, Ftr2, is expressed. Furthermore, the survival of G. mellonella larvae infected with C. albicans and treated with the antibodies was 90% higher than the control group, which did not receive treatment (p < 0.0001). Therefore, our data suggest that IgY antibodies against Ftr1 from C. albicans can inhibit yeast propagation by blocking iron uptake.

Biosynthesis Pathways, Transport Mechanisms and Biotechnological Applications of Fungal Siderophores

Iron (Fe) is the fourth most abundant element on earth and represents an essential nutrient for life. As a fundamental mineral element for cell growth and development, iron is available for uptake as ferric ions, which are usually oxidized into complex oxyhydroxide polymers, insoluble under aerobic conditions. In these conditions, the bioavailability of iron is dramatically reduced. As a result, microorganisms face problems of iron acquisition, especially under low concentrations of this element. However, some microbes have evolved mechanisms for obtaining ferric irons from the extracellular medium or environment by forming small molecules often regarded as siderophores. Siderophores are high affinity iron-binding molecules produced by a repertoire of proteins found in the cytoplasm of cyanobacteria, bacteria, fungi, and plants. Common groups of siderophores include hydroxamates, catecholates, carboxylates, and hydroximates. The hydroxamate siderophores are commonly synthesized by fungi. L-ornithine is a biosynthetic precursor of siderophores, which is synthesized from multimodular large enzyme complexes through non-ribosomal peptide synthetases (NRPSs), while siderophore-Fe chelators cell wall mannoproteins (FIT1, FIT2, and FIT3) help the retention of siderophores. S. cerevisiae, for example, can express these proteins in two genetically separate systems (reductive and nonreductive) in the plasma membrane. These proteins can convert Fe (III) into Fe (II) by a ferrous-specific metalloreductase enzyme complex and flavin reductases (FREs). However, regulation of the siderophore through Fur Box protein on the DNA promoter region and its activation or repression depend primarily on the Fe availability in the external medium. Siderophores are essential due to their wide range of applications in biotechnology, medicine, bioremediation of heavy metal polluted environments, biocontrol of plant pathogens, and plant growth enhancement.

Systematic metadata analysis of brown rot fungi gene expression data reveals the genes involved in Fenton's reaction and wood decay process

Brown-rot fungi are rapid holocellulose degraders and are the most predominant degraders of coniferous wood products in North America. Brown-rot fungi degrades wood by both enzymatic (plant biomass degrading carbohydrate active enzymes-CAZymes) and non-enzymatic systems (Fenton’s reaction) mechanisms. Identifying the genes and molecular mechanisms involved in Fenton’s reaction would significantly improve our understanding about brown-rot decay. Our present study identifies the common gene expression patterns involved in brown rot decay by performing metadata analysis of fungal transcriptome datasets. We have also analyzed and compared the genome-wide annotations (InterPro and CAZymes) of the selected brown rot fungi. Genes encoding for various oxidoreductases, iron homeostasis, and metabolic enzymes involved in Fenton’s mechanism were found to be significantly expressed among all the brown-rot fungal datasets. Interestingly, a higher number of hemicellulases encoding genes were differentially expressed among all the datasets, while a fewer number of cellulases and peroxidases were expressed (especially haem peroxidase and chloroperoxidase). Apart from these lignocellulose degrading enzymes genes encoding for aldo/keto reductases, 2-nitro dioxygenase, aromatic-ring dioxygenase, dienelactone hydrolase, alcohol dehydrogenase, major facilitator superfamily, cytochrome-P450 monoxygenase, cytochrome b5, and short-chain dehydrogenase were common and differentially up regulated among all the analyzed brown-rot fungal datasets.

Oxidative Damage Control during Decay of Wood by Brown Rot Fungus Using Oxygen Radicals

Brown rot wood-degrading fungi deploy reactive oxygen species (ROS) to loosen plant cell walls and enable selective polysaccharide extraction. These ROS, including Fenton-generated hydroxyl radicals (HO˙), react with little specificity and risk damaging hyphae and secreted enzymes. Recently, it was shown that brown rot fungi reduce this risk, in part, by differentially expressing genes involved in HO˙ generation ahead of those coding carbohydrate-active enzymes (CAZYs). However, there are notable exceptions to this pattern, and we hypothesized that brown rot fungi would require additional extracellular mechanisms to limit ROS damage. To assess this, we grew Postia placenta directionally on wood wafers to spatially segregate early from later decay stages. Extracellular HO˙ production (avoidance) and quenching (suppression) capacities among the stages were analyzed, along with the ability of secreted CAZYs to maintain activity postoxidation (tolerance). First, we found that H2O2 and Fe2+ concentrations in the extracellular environment were conducive to HO˙ production in early (H2O2:Fe2+ ratio 2:1) but not later (ratio 1:131) stages of decay. Second, we found that ABTS radical cation quenching (antioxidant capacity) was higher in later decay stages, coincident with higher fungal phenolic concentrations. Third, by surveying enzyme activities before/after exposure to Fenton-generated HO˙, we found that CAZYs secreted early, amid HO˙, were more tolerant of oxidative stress than those expressed later and were more tolerant than homologs in the model CAZY producer Trichoderma reesei Collectively, this indicates that P. placenta uses avoidance, suppression, and tolerance mechanisms, extracellularly, to complement intracellular differential expression, enabling this brown rot fungus to use ROS to degrade wood.IMPORTANCE Wood is one of the largest pools of carbon on Earth, and its decomposition is dominated in most systems by fungi. Wood-degrading fungi specialize in extracting sugars bound within lignin, either by removing lignin first (white rot) or by using Fenton-generated reactive oxygen species (ROS) to "loosen" wood cell walls, enabling selective sugar extraction (brown rot). Although white rot lignin-degrading pathways are well characterized, there are many uncertainties in brown rot fungal mechanisms. Our study addressed a key uncertainty in how brown rot fungi deploy ROS without damaging themselves or the enzymes they secrete. In addition to revealing differentially expressed genes to promote ROS generation only in early decay, our study revealed three spatial control mechanisms to avoid/tolerate ROS: (i) constraining Fenton reactant concentrations (H2O2, Fe2+), (ii) quenching ROS via antioxidants, and (iii) secreting ROS-tolerant enzymes. These results not only offer insight into natural decomposition pathways but also generate targets for biotechnological development.

Melanization Affects the Content of Selected Elements in Parmelioid Lichens

Lichens belonging to Parmeliaceae are highly diversified, but most of them share an extremely conserved morpho-chemical trait: the lower cortex is heavily melanized. The adaptive value of this character is still uncertain. Melanins are ubiquitous compounds found in most organisms since they fulfil several biological functions including defense against UV radiation, oxidizing agents, microbial stress, and metal complexation. This work aims to establish whether melanization can affect the elemental content of lichen thalli. The relative abundance of macro- (Ca, K and S) and micro- (Fe, Mn and Zn) nutrients in melanized and non-melanized pseudotissues of nine species was first evaluated by a non-destructive micro-X-ray fluorescence elemental analysis on either the upper and lower cortex, and on the internal medulla, which was artificially exposed to the mechanical removal of the lower cortex. Afterwards, the total concentration of the same elements was measured in composite samples by inductively coupled plasma atomic emission spectroscopy after acidic digestion. In order to verify whether Fe and Zn are chemically bound to the melanized pseudotissues, a sequential elution experiment was performed on two species: the two-side heavily melanized Melanelixia glabratula and the one-side lightly melanized Punctelia subrudecta. The content of Fe and Zn was higher in the melanized species than in the non-melanized ones. Species deprived of their melanized lower cortex showed a sharp decrease in Fe but not in Zn, suggesting that the melanized lower cortex is involved in Fe complexation, whereas Zn is homogeneously distributed throughout the thallus.

The Fungal Pathogen Candida glabrata Does Not Depend on Surface Ferric Reductases for Iron Acquisition

Iron acquisition is a crucial virulence determinant for many bacteria and fungi, including the opportunistic fungal pathogens Candida albicans and C. glabrata. While the diverse strategies used by C. albicans for obtaining iron from the host are well-described, much less is known about the acquisition of this micronutrient from host sources by C. glabrata – a distant relative of C. albicans with closer evolutionary ties to Saccharomyces cerevisiae, which nonetheless causes severe clinical symptoms in humans. Here we show that C. glabrata is much more restricted than C. albicans in using host iron sources, lacking, for example, the ability to grow on transferrin and hemin/hemoglobin. Instead, C. glabrata is able to use ferritin and non-protein-bound iron (FeCl₃) as iron sources in a pH-dependent manner. As in other fungal pathogens, iron-dependent growth requires the reductive high affinity (HA) iron uptake system. Typically highly conserved, this uptake mechanism normally relies on initial ferric reduction by cell-surface ferric reductases. The C. glabrata genome contains only three such putative ferric reductases, which were found to be dispensable for iron-dependent growth. In addition and in contrast to C. albicans and S. cerevisiae, we also detected no surface ferric reductase activity in C. glabrata. Instead, extracellular ferric reduction was found in this and the two other fungal species, which was largely dependent on an excreted low-molecular weight, non-protein ferric reductant. We therefore propose an iron acquisition strategy of C. glabrata which differs from other pathogenic fungi, such as C. albicans, in that it depends on a limited set of host iron sources and that it lacks the need for surface ferric reductases. Extracellular ferric reduction by a secreted molecule possibly compensates for the loss of surface ferric reductase activity in the HA iron uptake system.

Fungal Ferromanganese Mineralisation in Cretaceous Dinosaur Bones from the Gobi Desert, Mongolia

Well-preserved mycelia of fungal- or saprolegnia-like biota mineralised by ferromanganese oxides were found for the first time in long bones of Late Cretaceous dinosaurs from the Gobi Desert (Nemegt Valley, Mongolia). The mycelia formed a biofilm on the wall of the bone marrow cavity and penetrated the osteon channels of the nearby bone tissue. Optical microscopy, Raman, SEM/EDS, SEM/BSE, electron microprobe and cathodoluminescence analyses revealed that the mineralisation of the mycelia proceeded in two stages. The first stage was early post-mortem mineralisation of the hyphae by Fe/Mn-oxide coatings and microconcretions. Probably this proceeded in a mildly acidic to circumneutral environment, predominantly due to heterotrophic bacteria degrading the mycelial necromass and liberating Fe and Mn sorbed by the mycelia during its lifetime. The second stage of mineralisation, which proceeded much later following the final burial of the bones in an alkaline environment, resulted from the massive precipitation of calcite and occasionally barite on the iron/manganese-oxide-coated mycelia. The mineral phases produced by fungal biofilms colonising the interiors of decaying dinosaur bones not only enhance the preservation (fossilisation) of fungal remains but can also be used as indicators of the geochemistry of the dinosaur burial sites.

Coordinate responses to alkaline pH stress in budding yeast

Alkalinization of the medium represents a stress condition for the budding yeast Saccharomyces cerevisiae to which this organism responds with profound remodeling of gene expression. This is the result of the modulation of a substantial number of signaling pathways whose participation in the alkaline response has been elucidated within the last ten years. These regulatory inputs involve not only the conserved Rim101/PacC pathway, but also the calcium-activated phosphatase calcineurin, the Wsc1-Pkc1-Slt2 MAP kinase, the Snf1 and PKA kinases and oxidative stress-response pathways. The uptake of many nutrients is perturbed by alkalinization of the environment and, consequently, an impact on phosphate, iron/copper and glucose homeostatic mechanisms can also be observed. The analysis of available data highlights cases in which diverse signaling pathways are integrated in the gene promoter to shape the appropriate response pattern. Thus, the expression of different genes sharing the same signaling network can be coordinated, allowing functional coupling of their gene products.

Involvement of Trichoderma asperellum strain T6 in regulating iron acquisition in plants

Iron (Fe) deficiency is a major plant nutritional disorder in many parts of the world, particularly in areas with saline soils. Among the numerous root-associated microbes that are beneficial for plant nutrient uptake, Trichoderma spp. are the most effective rhizosphere fungi for enhancing plant growth and plant resistance to biotic and abiotic stresses. To investigate the potential mechanisms of action of Trichoderma on insoluble Fe in the soil, which is difficult for plants to absorb and utilize, a high siderophore-producing strain of Trichoderma T6, was isolated from the rhizosphere of cucumber plants. The strain was identified as T. asperellum based on the morphological features and molecular phylogenetic analyses. Applying strain T6 to sterile soil could increase soil levels of Fe(2+) and siderophores, as well as increase Fe(2+) and Fe(3+)-chelate reductase (FCR) activity in cucumber tissues. Purified siderophore eluent (PSE) increased plant growth, thus confirming its role in plant growth promotion. Moreover, extracellular Fe(3+) reducing activity and three kinds of organic acids were detected in the culture filtrate of strain T6. These results indicate that strain T6 influences plant Fe absorption in several ways. Siderophore-based Fe chelation is effective in providing Fe to plants, organic acids, and Fe(3+) reducing enzymes may participate in the solubilization and reduction of insoluble Fe(3+) to Fe(2+).

Different iron sources to study the physiology and biochemistry of iron metabolism in marine micro-algae

We compared ferric EDTA, ferric citrate and ferrous ascorbate as iron sources to study iron metabolism in Ostreococcus tauri, Phaeodactlylum tricornutum and Emiliania huxleyi. Ferric EDTA was a better iron source than ferric citrate for growth and chlorophyll levels. Direct and indirect experiments showed that iron was much more available to the cells when provided as ferric citrate as compared to ferric EDTA. As a consequence, growth media with iron concentration in the range 1–100 nM were rapidly iron-depleted when ferric citrate—but not ferric EDTA was the iron source. When cultured together, P. tricornutum cells overgrew the two other species in iron-sufficient conditions, but E. huxleyi was able to compete other species in iron-deficient conditions, and when iron was provided as ferric citrate instead of ferric EDTA, which points out the critical influence of the chemical form of iron on the blooms of some phytoplankton species. The use of ferric citrate and ferrous ascorbate allowed us to unravel a kind of regulation of iron uptake that was dependent on the day/night cycles and to evidence independent uptake systems for ferrous and ferric iron, which can be regulated independently and be copper-dependent or independent. The same iron sources also allowed one to identify molecular components involved in iron uptake and storage in marine micro-algae. Characterizing the mechanisms of iron metabolism in the phytoplankton constitutes a big challenge; we show here that the use of iron sources more readily available to the cells than ferric EDTA is critical for this task.

The homeostasis of iron, copper, and zinc in paracoccidioides brasiliensis, cryptococcus neoformans var. Grubii, and cryptococcus gattii: a comparative analysis

Iron, copper, and zinc are essential for all living organisms. Moreover, the homeostasis of these metals is vital to microorganisms during pathogenic interactions with a host. Most pathogens have developed specific mechanisms for the uptake of micronutrients from their hosts in order to counteract the low availability of essential ions in infected tissues. We report here an analysis of genes potentially involved in iron, copper, and zinc uptake and homeostasis in the fungal pathogens Paracoccidioides brasiliensis, Cryptococcus neoformans var. grubii, and Cryptococcus gattii. Although prior studies have identified certain aspects of metal regulation in Cryptococcus species, little is known regarding the regulation of these elements in P. brasiliensis. We also present amino acid sequences analyses of deduced proteins in order to examine possible conserved domains. The genomic data reveals, for the first time, genes associated to iron, copper, and zinc assimilation and homeostasis in P. brasiliensis. Furthermore, analyses of the three fungal species identified homologs to genes associated with high-affinity uptake systems, vacuolar and mitochondrial iron storage, copper uptake and reduction, and zinc assimilation. However, homologs to genes involved in siderophore production were only found in P. brasiliensis. Interestingly, in silico analysis of the genomes of P. brasiliensisPb01, Pb03, and Pb18 revealed significant differences in the presence and/or number of genes involved in metal homeostasis, such as in genes related to iron reduction and oxidation. The broad analyses of the genomes of P. brasiliensis, C. neoformans var. grubii, and C. gattii for genes involved in metal homeostasis provide important groundwork for numerous interesting future areas of investigation that are required in order to validate and explore the function of the identified genes and gene pathways.

Growth, Fe3+ reductase activity, and siderophore production by Paenibacillus polymyxa SQR-21 under differential iron conditions

An experiment was planned to evaluate the behavior of Paenibacillus polymyxa SQR-21 under differential iron availability. P. polymyxa was grown under three differential iron conditions (0, 2, 20 μM). Iron starvation slowed bacterial growth and at all iron levels, pH of liquid culture was decreased, but maximum decrease was observed at highest iron level. Cell surface ferrireductase activity decreased as culture aged, while extracellular Fe(3+)-reducing activity constantly increased. Hydroxamates type siderophores production was increased with the decrease in iron levels. Numerous cellular proteins were expressed by P. polymyxa in the range of 5-140 kDa and several of them showed conspicuous differential iron regulation. P. polymyxa seems to have more than one type of iron acquisition mechanism including gradual release of organic acids, cell surface ferrireductases, extracellular reductants, and secretion of low molecular weight hydroxamates chelators. This article is the first to report the kinetic study of P. polymyxa under differential iron availability. The information provided here gives initial information about the iron uptake mechanism of P. polymyxa.

Iron transport by proteoliposomes containing mitochondrial F(1)F(0) ATP synthase isolated from rat heart

In this work, we present evidence of Fe(2+) transport by rat heart mitochondrial F(1)F(0) ATP synthase. Iron uptake by the vesicles containing the enzyme was concentration- and temperature-dependent, with an optimum temperature of 37 degrees C. Both ATP and ADP stimulated iron uptake in a concentration-dependent manner, whereas AMP, AMPPCP, and mADP did not. Inhibitors of the enzyme, oligomycin, and resveratrol similarly blocked iron transport. The iron uptake was confirmed by inhibition using specific antibodies against the alpha, beta, and c subunits of the enzyme. Interestingly, slight transport of common divalent and trivalent metal ions such as Mg(+2), Ca(+2), Mn(+2), Zn(+2), Cu(+2), Fe(+3), and Al(+3) was observed. Moreover, Cu(+2), even in the nM range, inhibited iron uptake and attained maximum inhibition of approximately 56%. Inorganic phosphate (Pi) in the medium exerted an opposite effect depending on the type of adenosine nucleotide, which was suppressed with ATP, but enhanced with ADP. A similarly stimulating effect of ATP and ADP with an inverse effect of Pi suggests that the activity of ATPase and ATP synthase may be associated with iron uptake in a different manner, probably via antiport of H(+).

Trichoderma asperellum strain T34 controls Fusarium wilt disease in tomato plants in soilless culture through competition for iron

Trichoderma asperellum strain T34 has been reported to control the disease caused by Fusarium oxysporum f.sp. lycopersici (Fol) on tomato plants. To study the importance of iron concentration in the growth media for the activity and competitiveness of T34 and the pathogen, we tested four iron concentrations in the nutrient solution [1, 10, 100, and 1000 microM provided as EDTA/Fe(III)] in a biological control experiment with T34 and Fol in tomato plants. The reduction of the Fusarium-infected shoot by T34 was only significant at 10 microM Fe. We hypothesized that Fe competition is one of the key factors in the biocontrol activity exerted by T34 against Fol, as an increase in Fe concentration over 10 microM would lead to the suppression of T34 siderophore synthesis and thus inhibition of Fe competition with Fol. T34 significantly reduced the populations of Fol at all the doses of Fe assayed. In contrast, Fol enhanced the populations of T34 at 1 and 10 microM Fe. Nevertheless, several plant physiological parameters like net CO(2) assimilation (A), stomatal conductance (g(s)), relative quantum efficiency of PSII (Phi(PSII)), and efficiency of excitation energy capture by open PSII reactive centers (Fv'/Fm') demonstrated the protection against Fol damage by treatment with T34 at 100 microM Fe. The first physiological parameter affected by the disease progression was g(s). Plant dry weight was decreased by Fe toxicity at 100 and 1,000 microM. T34-treated plants had significantly greater heights and dry weights than control plants at 1,000 microM Fe, even though T34 did not reduce the Fe content in leaves or stems. Furthermore, T34 enhanced plant height even at the optimal Fe concentration (10 microM) compared to control plants. In conclusion, T. asperellum strain T34 protected tomato plants from both biotic (Fusarium wilt disease) and abiotic stress [Fe(III) toxic effects].

the hyphal-associated adhesin and invasin Als3 of Candida albicans mediates iron acquisition from host ferritin

Iron sequestration by host iron-binding proteins is an important mechanism of resistance to microbial infections. Inside oral epithelial cells, iron is stored within ferritin, and is therefore not usually accessible to pathogenic microbes. We observed that the ferritin concentration within oral epithelial cells was directly related to their susceptibility to damage by the human pathogenic fungus, Candida albicans. Thus, we hypothesized that host ferritin is used as an iron source by this organism. We found that C. albicans was able to grow on agar at physiological pH with ferritin as the sole source of iron, while the baker's yeast Saccharomyces cerevisiae could not. A screen of C. albicans mutants lacking components of each of the three known iron acquisition systems revealed that only the reductive pathway is involved in iron utilization from ferritin by this fungus. Additionally, C. albicans hyphae, but not yeast cells, bound ferritin, and this binding was crucial for iron acquisition from ferritin. Transcriptional profiling of wild-type and hyphal-defective C. albicans strains suggested that the C. albicans invasin-like protein Als3 is required for ferritin binding. Hyphae of an Δals3 null mutant had a strongly reduced ability to bind ferritin and these mutant cells grew poorly on agar plates with ferritin as the sole source of iron. Heterologous expression of Als3, but not Als1 or Als5, two closely related members of the Als protein family, allowed S. cerevisiae to bind ferritin. Immunocytochemical localization of ferritin in epithelial cells infected with C. albicans showed ferritin surrounding invading hyphae of the wild-type, but not the Δals3 mutant strain. This mutant was also unable to damage epithelial cells in vitro. Therefore, C. albicans can exploit iron from ferritin via morphology dependent binding through Als3, suggesting that this single protein has multiple virulence attributes.

Iron is an essential nutrient for all microbes. Many human pathogenic microbes have developed sophisticated strategies to acquire iron from the host as most compartments in the body contain little free iron. For example, in oral epithelial cells intracellular iron is bound to ferritin, a protein that is highly resistant to microbial attack. In fact, no microorganism has so far been shown to directly exploit ferritin as an iron source during interaction with host cells. This study demonstrates that the pathogenic fungus Candida albicans can use ferritin as the sole source of iron. Most intriguingly, C. albicans binds ferritin via a receptor that is only exposed on invasive hyphae. This receptor is Als3, which is a member of the Als-protein family. Als3 was previously demonstrated to be an adhesin with invasin-like properties. Mutants lacking Als3 failed to bind ferritin, grew poorly with ferritin as an iron source and were unable to damage epithelial cells. Strains of the baker's yeast expressing C. albicans Als3, but not two closely related proteins, Als1 or Als5, were able to bind ferritin. Therefore, C. albicans uses an additional morphology specific and unique iron uptake strategy based on ferritin while invading into host cells where ferritin is located.

Histoplasma capsulatum secreted gamma-glutamyltransferase reduces iron by generating an efficient ferric reductant

The intracellular fungal pathogen Histoplasma capsulatum (Hc) resides in mammalian macrophages and causes respiratory and systemic disease. Iron limitation is an important host antimicrobial defence, and iron acquisition is critical for microbial pathogenesis. Hc displays several iron acquisition mechanisms, including secreted glutathione-dependent ferric reductase activity (GSH-FeR). We purified this enzyme from culture supernatant and identified a novel extracellular iron reduction strategy involving gamma-glutamyltransferase (Ggt1) activity. The 320 kDa complex was composed of glycosylated protein subunits of about 50 and 37 kDa. The purified enzyme exhibited gamma-glutamyl transfer activity as well as iron reduction activity in the presence of glutathione. We cloned and manipulated expression of the encoding gene. Overexpression or RNAi silencing affected both GGT and GSH-FeR activities concurrently. Enzyme inhibition experiments showed that the activity is complex and involves two reactions. First, Ggt1 initiates enzymatic breakdown of GSH by cleavage of the gamma-glutamyl bond and release of cysteinylglycine. Second, the thiol group of the released dipeptide reduces ferric to ferrous iron. A combination of kinetic properties of both reactions resulted in efficient iron reduction over a broad pH range. Our findings provide novel insight into Hc iron acquisition strategies and reveal a unique aspect of Ggt1 function in this dimorphic mycopathogen.

Siderophores in fungal physiology and virulence

Maintaining the appropriate balance of iron between deficiency and toxicity requires fine-tuned control of systems for iron uptake and storage. Both among fungal species and within a single species, different systems for acquisition, storage, and regulation of iron are present. Here we discuss the most recent findings on the mechanisms involved in maintaining iron homeostasis with a focus on siderophores, low-molecular-mass iron chelators, employed for iron uptake and storage. Recently siderophores have been found to be crucial for pathogenicity of animal, as well as plant-pathogenic fungi and for maintenance of plant-fungal symbioses.

Identification of iron-regulated cellular proteins, Fe3+-reducing and -chelating compounds, in the white-rot fungus Perenniporia medulla-panis

In this paper, we present the responses of the white-rot fungus Perenniporia medulla-panis to iron availability with regard to alterations in growth, expression of cellular proteins, Fe3+-reducing activity, and Fe3+chelators production. Iron supplementation stimulated fungal growth but did not result in a significant increase in biomass production. Catechol and hydroxamate derivatives were produced mainly under iron deficiency, and their productions were repressed under iron supplementation conditions. Perenniporia medulla-panis showed several cellular proteins in the range of 10–90 kDa. Some of them showed negative iron-regulation. Iron-supplemented medium also repressed both cell surface and extracellular Fe3+-reducing activities; however, the highest cell surface activity was detected at the initial growth phase, whereas extracellular activity increased throughout the incubation period. No significant production of chelators and extracellular Fe3+-reducing activity were observed within the initial growth phase, suggesting that the reduction of Fe3+to Fe2+is performed by ferrireductases.

Trafficking of siderophore transporters in Saccharomyces cerevisiae and intracellular fate of ferrioxamine B conjugates

We have studied the intracellular trafficking of Sit1 [ferrioxamine B (FOB) transporter] and Enb1 (enterobactin transporter) in Saccharomyces cerevisiae using green fluorescent protein (GFP) fusion proteins. Enb1 was constitutively targeted to the plasma membrane. Sit1 was essentially targeted to the vacuolar degradation pathway when synthesized in the absence of substrate. Massive plasma membrane sorting of Sit1 was induced by various siderophore substrates of Sit1, and by coprogen, which is not a substrate of Sit1. Thus, different siderophore transporters use different regulated trafficking processes. We also studied the fate of Sit1-mediated internalized siderophores. Ferrioxamine B was recovered in isolated vacuolar fractions, where it could be detected spectrophotometrically. Ferrioxamine B coupled to an inhibitor of mitochondrial protoporphyrinogen oxidase (acifluorfen) could not reach its target unless the cells were disrupted, confirming the tight compartmentalization of siderophores within cells. Ferrioxamine B coupled to a fluorescent moiety, FOB-nitrobenz-2-oxa-1,3-diazole, used as a Sit1-dependent iron source, accumulated in the vacuolar lumen even in mutants displaying a steady-state accumulation of Sit1 at the plasma membrane or in endosomal compartments. Thus, the fates of siderophore transporters and siderophores diverge early in the trafficking process.

Glutathione-dependent extracellular ferric reductase activities in dimorphic zoopathogenic fungi

In this study, extracellular glutathione-dependent ferric reductase (GSH-FeR) activities in different dimorphic zoopathogenic fungal species were characterized. Supernatants from Blastomyces dermatitidis, Histoplasma capsulatum, Paracoccidioides brasiliensis and Sporothrix schenckii strains grown in their yeast form were able to reduce iron enzymically with glutathione as a cofactor. Some variations in the level of reduction were noted amongst the strains. This activity was stable in acidic, neutral and slightly alkaline environments and was inhibited when trivalent aluminium and gallium ions were present. Using zymography, single bands of GSH-FeRs with apparent molecular masses varying from 430 to 460 kDa were identified in all strains. The same molecular mass range was determined by size exclusion chromatography. These data demonstrate that dimorphic zoopathogenic fungi produce and secrete a family of similar GSH-FeRs that may be involved in the acquisition and utilization of iron. Siderophore production by these and other fungi has sometimes been considered to provide a full explanation of iron acquisition in these organisms. Our work reveals an additional common mechanism that may be biologically and pathogenically important. Furthermore, while some characteristics of these enzymes such as extracellular location, cofactor utilization and large size are not individually unique, when considered together and shared across a range of fungi, they represent an important novel physiological feature.

Identification of Histoplasma capsulatum transcripts induced in response to reactive nitrogen species

The pathogenic fungus Histoplasma capsulatum escapes innate immune defenses and colonizes host macrophages during infection. After the onset of adaptive immunity, the production of the antimicrobial effector nitric oxide (*NO) restricts H. capsulatum replication. However, H. capsulatum can establish persistent infections, indicating that it survives in the host despite exposure to reactive nitrogen species (RNS). To understand how H. capsulatum responds to RNS, we determined the transcriptional profile of H. capsulatum to *NO-generating compounds using a shotgun genomic microarray. We identified 695 microarray clones that were induced > or = 4-fold upon nitrosative stress. Because our microarray clones were generated from random fragments of genomic DNA, they did not necessarily correspond to H. capsulatum open reading frames. To identify induced genes, we used high-density oligonucleotide tiling arrays to determine the genomic boundaries and coding strand of 153 RNS-induced transcripts. Homologues of these genes in other organisms are involved in iron acquisition, energy production, stress response, protein folding/degradation, DNA repair, and *NO detoxification. Ectopic expression of one of these genes, a P450 nitric oxide reductase homologue, was sufficient to increase resistance of H. capsulatum to RNS in culture. We propose that H. capsulatum uses the pathways identified here to cope with RNS-induced damage during pathogenesis.

Extracellular calcium and magnesium, but not iron, are needed for optimal growth of Blastomyces dermatitidis yeast form cells in vitro

In the present study, we demonstrate that the yeast form of Blastomyces dermatitidis can proliferate for short periods of time in the absence of ferric iron but not in the absence of calcium or magnesium. The results of this study shed light on the resistance of B. dermatitidis to chelating agents, such as deferoxamine, and may explain how B. dermatitidis resists the iron-binding activity of serum transferrin.

Fungal susceptibility to zinc deprivation

Calprotectin is a neutrophil-derived antimicrobial protein that competes with microorganisms for zinc. The zinc-specific effect of calprotectin against Candida albicans appears to be related to this organism's marked susceptibility to deprivation of this metal. However, it is not known whether this susceptibility is particular to C albicans or whether it is a characteristic of pathogenic fungi in general. As a means of deciding between these 2 possibilities, we undertook the study reported here to compare the susceptibility to zinc deprivation of 6 other pathogenic fungal species in addition to that of C albicans . We tested the effect of metals in reversing growth inhibition of the 7 fungi against abscess-fluid supernatant (a source of calprotectin) and 3 chemical chelators. Data were expressed as the concentration of metal required to bring about 50% restoration of growth. Zinc was found to be much more potent than the other metals tested in reversing growth inhibition of all the organisms by human abscess fluid and all 3 chemical chelators. Copper and manganese also had some effect. In some cases, chelator stability constants were higher for other metals than for zinc; in particular, although diethylenetriaminopentaacetic acid has a stability constant for iron almost 10(10) times greater than that for zinc, zinc was more effective than iron in reversing growth inhibition by this chelator against all of the organisms. These results suggest that marked susceptibility to zinc deprivation is a general characteristic of pathogenic fungi.

Detection of 3-hydroxykynurenine in a plant pathogenic fungus

A redox-active compound has been purified from the barley powdery mildew fungus Blumeria ( Erysiphe ) graminis f. sp. hordei. A combination of spectrophotometry, MS and NMR has identified it as 3-hydroxykynurenine (3OHKyn). This compound, never previously detected in any fungus or pathogen, is best known for its role in vertebrate cataracts. It is found abundantly in developing and germinating spores and also in runner hyphae. Two roles for 3OHKyn are discussed: first, the presence of active oxygen species would enable 3OHKyn to cross-link the spore chemically with the plant. Secondly, it may be acting as an UV protectant and an antioxidant.

Characterization of the Aspergillus nidulans transporters for the siderophores enterobactin and triacetylfusarinine C

The filamentous ascomycete Aspergillus nidulans produces three major siderophores: fusigen, triacetylfusarinine C, and ferricrocin. Biosynthesis and uptake of iron from these siderophores, as well as from various heterologous siderophores, is repressed by iron and this regulation is mediated in part by the transcriptional repressor SREA. Recently we have characterized a putative siderophore-transporter-encoding gene ( mirA ). Here we present the characterization of two further SREA- and iron-regulated paralogues (mirB and mirC ), including the chromosomal localization and the complete exon/intron structure. Expression of mirA and mirB in a Saccharomyces cerevisiae strain, which lacks high affinity iron transport systems, showed that MIRA transports specifically the heterologous siderophore enterobactin and that MIRB transports exclusively the native siderophore triacetylfusarinine C. Construction and analysis of an A. nidulans mirA deletion mutant confirmed the substrate specificity of MIRA. Phylogenetic analysis of the available sequences suggests that the split of the species A. nidulans and S. cerevisiae predates the divergence of the paralogous Aspergillus siderophore transporters.

Characterization of iron-binding motifs in Candida albicans high-affinity iron permease CaFtr1p by site-directed mutagenesis

A peptide motif Glu-Xaa-Xaa-Glu has been implicated in direct binding of ferric iron in several proteins involved in iron transport, sensing or storage. However, it is not known whether the motif alone is sufficient for iron binding and whether functional replacement of the conserved residues by other amino acids with similar properties is possible. We previously identified a Candida albicans iron permease, CaFtr1p, which contains five Glu-Xaa-Xaa-Glu motifs [Ramanan and Wang (2000) Science 288, 1062-1065]. In this study, we investigated the role of each of these motifs in iron uptake by site-directed mutagenesis. Substitution of Ala for any one of the two Glu residues in Glu-Gly-Leu-Glu(158-161) abolished iron-uptake activity, while the same substitution in any of the other four motifs had little effect, indicating that only the motif at position 158-161 is required for iron transport. We then evaluated the importance of each of the residues within and immediately adjacent to this motif in iron uptake. The permease remained active when any one of the Glu residues was replaced by Asp, while it became inactive when both were replaced. We also found that the amino acid immediately in front of Glu-Gly-Leu-Glu(158-161) must be either Arg or Lys. In addition, substitution of any of the two residues in the middle with several structurally distinct amino acids had no detectable effect on iron uptake. Here we propose to extend the iron-binding motif to Arg/Lys-Glu/Asp-Xaa-Xaa-Glu or Arg/Lys-Glu-Xaa-Xaa-Glu/Asp, which may serve as a guide for the identification of potential iron-binding sites in proteins.

Siderophore uptake by Candida albicans: effect of serum treatment and comparison with Saccharomyces cerevisiae

Iron uptake systems often function as virulence factors in pathogenic organisms. Candida albicans is a fungal pathogen that infects immunocompromised hosts, such as AIDS patients or granulocytopenic bone marrow transplant recipients. Here we show that iron uptake from siderophores occurs in C. albicans and is mediated by one or more high-affinity transport systems. Iron carried on ferrioxamine B, triacethyl-fusarinine, ferrichrome, or ferricrocin was actively taken up via a high-affinity mechanism. The kinetic parameters of uptake were similar to those found in S. cerevisiae. Furthermore, for ferrichrome and ferrioxamine B, cellular uptake of fluorescent analogues was observed. In C. albicans, iron uptake from siderophores was regulated by iron availability, with iron deprivation inducing uptake. Serum exposure, which induces a morphogenic shift from yeast to filamentous forms known to be required for virulence, also resulted in induction of iron transport from ferrichrome-type siderophores. In a tup1/tup1 strain which grows constitutively in the filamentous form, iron transport was derepressed for all siderophores tested. The genes mediating uptake and utilization of iron from siderophores in C. albicans have not been identified; however, the transcript abundance for CaSIT1 was regulated in a manner consistent with the pattern of iron uptake from ferrichrome-type siderophores. Furthermore, CaSIT1 overexpression in S. cerevisiae resulted in inhibited siderophore iron uptake, suggesting that the expressed protein may interact with proteins of S. cerevisiae involved in iron uptake from siderophores. In summary, iron uptake from ferrichrome-type siderophores was induced in filamentous C. albicans, and a potential role of this iron acquisition system in pathogenicity should be considered.