urbs1, a gene regulating siderophore biosynthesis in Ustilago maydis, encodes a protein similar to the erythroid transcription factor GATA-1

SreC-dependent adaption to host iron environments regulates the transition of trophic stages and developmental processes of Curvularia lunata

Plant pathogens are challenged by host-derived iron starvation or excess during infection, but the mechanism of plant pathogens rapidly adapting to the dynamic host iron environments to assimilate iron for invasion and colonization remains largely unexplored. Here, we found that the GATA transcription factor SreC in Curvularia lunata is required for virulence and adaption to the host iron excess environment. SreC directly binds to the ATGWGATAW element in an iron-dependent manner to regulate the switch between different iron assimilation pathways, conferring adaption to host iron environments in different trophic stages of C. lunata. SreC also regulates the transition of trophic stages and developmental processes in C. lunata. SreC-dependent adaption to host iron environments is essential to the infectious growth and survival of C. lunata. We also demonstrate that CgSreA (a SreC orthologue) plays a similar role in Colletotrichum graminicola. We conclude that Sre mediates adaption to the host iron environment during infection, and the function is conserved in hemibiotrophic fungi.

Plant GATA Factors: Their Biology, Phylogeny, and Phylogenomics

GATA factors are evolutionarily conserved transcription factors that are found in animals, fungi, and plants. Compared to that of animals, the size of the plant GATA family is increased. In angiosperms, four main GATA classes and seven structural subfamilies can be defined. In recent years, knowledge about the biological role and regulation of plant GATAs has substantially improved. Individual family members have been implicated in the regulation of photomorphogenic growth, chlorophyll biosynthesis, chloroplast development, photosynthesis, and stomata formation, as well as root, leaf, and flower development. In this review, we summarize the current knowledge of plant GATA factors. Using phylogenomic analysis, we trace the evolutionary origin of the GATA classes in the green lineage and examine their relationship to animal and fungal GATAs. Finally, we speculate about a possible conservation of GATA-regulated functions across the animal, fungal, and plant kingdoms.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

Coordinated regulation of iron metabolism in Cryptococcus neoformans by GATA and CCAAT transcription factors: connections with virulence

Iron acquisition is critical for pathogenic fungi to adapt to and survive within the host environment. However, to same extent, the fungi must also avoid the detrimental effects caused by excess iron. The importance of iron has been demonstrated for the physiology and virulence of major fungal pathogens of humans including Aspergillus fumigatus, Candida albicans, and Cryptococcus neoformans. In particular, numerous studies have revealed that aspects of iron acquisition, metabolism, and homeostasis in the fungal pathogens are tightly controlled by conserved transcriptional regulators including a GATA-type iron transcription factor and the CCAAT-binding complex (CBC)/HapX orthologous protein complex. However, the specific downstream regulatory networks are slightly different in each fungus. In addition, roles have been proposed or demonstrated for other factors including monothiol glutaredoxins, BolA-like proteins, and Fe-S cluster incorporation on the GATA-type iron transcription factor and the CBC/HapX orthologous protein complex, although limited information is available. Here we focus on recent work on C. neoformans in the context of an emerging framework for fungal regulation of iron acquisition, metabolism, and homeostasis. Our specific goal is to summarize recent findings on transcriptional networks governed by the iron regulators Cir1 and HapX in C. neoformans.

Iron in plant-pathogen interactions

Iron is an essential element for most organisms. As an indispensable co-factor of many enzymes, iron is involved in various crucial metabolic processes that are required for the survival of plants and pathogens. Conversely, excessive iron produces highly active reactive oxygen species, which are toxic to the cells of plants and pathogens. Therefore, plants and pathogens have evolved sophisticated mechanisms to modulate iron status at a moderate level for maintaining their fitness. Over the past decades, many efforts have been made to reveal these mechanisms, and some progress has been made. In this review, we describe recent advances in understanding the roles of iron in plant-pathogen interactions and propose prospects for future studies.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

The Role of SreA-Mediated Iron Regulation in Maintaining Epichloë festucae-Lolium perenne Symbioses

In ascomycetes and basidiomycetes, iron-responsive GATA-type transcriptional repressors are involved in regulating iron homeostasis, notably to prevent iron toxicity through control of iron uptake. To date, it has been unknown whether this iron regulator contributes toward mutualistic endosymbiosis of microbes with plants, a system where the endophyte must function within the constraints of an in-host existence, including a dependency on the host for nutrient acquisition. Functional characterization of one such protein, SreA from Epichloë festucae, a fungal endosymbiont of cool-season grasses, indicates that regulation of iron homeostasis processes is important for symbiotic maintenance. The deletion of the sreA gene (ΔsreA) led to iron-dependent aberrant hyphal growth and the gradual loss of endophyte hyphae from perennial ryegrass. SreA negatively regulates the siderophore biosynthesis and high-affinity iron uptake systems of E. festucae, similar to other fungi, resulting in iron accumulation in mutants. Our evidence suggests that SreA is involved in the processes that moderate Epichloë iron acquisition from the plant apoplast, because overharvesting of iron in ΔsreA mutants was detected as premature chlorosis of the host using a hydroponic plant growth assay. E. festucae appears to have a tightly regulated iron management system, involving SreA that balances endophyte growth with its survival and prevents overcompetition with the host for iron in the intercellular niche, thus promoting mutualistic associations. Mutations that interfere with Epichloë iron management negatively affect iron-dependent fungal growth and destabilize mutualistic Epichloë -ryegrass associations.

The Hap Complex in Yeasts: Structure, Assembly Mode, and Gene Regulation

The CCAAT box-harboring proteins represent a family of heterotrimeric transcription factors which is highly conserved in eukaryotes. In fungi, one of the particularly important homologs of this family is the Hap complex that separates the DNA-binding domain from the activation domain and imposes essential impacts on regulation of a wide range of cellular functions. So far, a comprehensive summary of this complex has been described in filamentous fungi but not in the yeast. In this review, we summarize a number of studies related to the structure and assembly mode of the Hap complex in a list of representative yeasts. Furthermore, we emphasize recent advances in understanding the regulatory functions of this complex, with a special focus on its role in regulating respiration, production of reactive oxygen species (ROS) and iron homeostasis.

The regulation of iron homeostasis in the fungal human pathogen Candida glabrata

Iron is an essential element to most microorganisms, yet an excess of iron is toxic. Hence, living cells have to maintain a tight balance between iron uptake and iron consumption and storage. The control of intracellular iron concentrations is particularly challenging for pathogens because mammalian organisms have evolved sophisticated high-affinity systems to sequester iron from microbes and because iron availability fluctuates among the different host niches. In this review, we present the current understanding of iron homeostasis and its regulation in the fungal pathogen Candida glabrata. This yeast is an emerging pathogen which has become the second leading cause of candidemia, a life-threatening invasive mycosis. C. glabrata is relatively poorly studied compared to the closely related model yeast Saccharomyces cerevisiae or to the pathogenic yeast Candida albicans. Still, several research groups have started to identify the actors of C. glabrata iron homeostasis and its transcriptional and post-transcriptional regulation. These studies have revealed interesting particularities of C. glabrata and have shed new light on the evolution of fungal iron homeostasis.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

Dissimilar pigment regulation in Serpula lacrymans and Paxillus involutus during inter-kingdom interactions

Production of basidiomycete atromentin-derived pigments like variegatic acid (pulvinic acid-type) and involutin (diarylcyclopentenone) from the brown-rotter Serpula lacrymans and the ectomycorrhiza-forming Paxillus involutus, respectively, is induced by complex nutrition, and in the case of S. lacrymans, bacteria. Pigmentation in S. lacrymans was stimulated by 13 different bacteria and cell-wall-damaging enzymes (lytic enzymes and proteases), but not by lysozyme or mechanical damage. The use of protease inhibitors with Bacillus subtilis or heat-killed bacteria during co-culturing with S. lacrymans significantly reduced pigmentation indicating that enzymatic hyphal damage and/or released peptides, rather than mechanical injury, was the major cause of systemic pigment induction. Conversely, no significant pigmentation by bacteria was observed from P. involutus. We found additional putative transcriptional composite elements of atromentin synthetase genes in P. involutus and other ectomycorrhiza-forming species that were absent from S. lacrymans and other brown-rotters. Variegatic and its precursor xerocomic acid, but not involutin, in return inhibited swarming and colony biofilm spreading of Bacillus subtilis, but did not kill B. subtilis. We suggest that dissimilar pigment regulation by fungal lifestyle was a consequence of pigment bioactivity and additional promoter motifs. The focus on basidiomycete natural product gene induction and regulation will assist in future studies to determine global regulators, signalling pathways and associated transcription factors of basidiomycetes.

Regulation of Nitrogen Metabolism by GATA Zinc Finger Transcription Factors in Yarrowia lipolytica

Nitrogen source is commonly used to control lipid production in industrial fungi. Here we identified regulators of nitrogen catabolite repression in the oleaginous yeast Y. lipolytica to determine how the nitrogen source regulates lipid metabolism. We show that disruption of both activators and repressors of nitrogen catabolite repression leads to increased lipid accumulation via activation of carbon catabolite repression through an as yet uncharacterized method.

Fungi accumulate lipids in a manner dependent on the quantity and quality of the nitrogen source on which they are growing. In the oleaginous yeast Yarrowia lipolytica, growth on a complex source of nitrogen enables rapid growth and limited accumulation of neutral lipids, while growth on a simple nitrogen source promotes lipid accumulation in large lipid droplets. Here we examined the roles of nitrogen catabolite repression and its regulation by GATA zinc finger transcription factors on lipid metabolism in Y. lipolytica. Deletion of the GATA transcription factor genes gzf3 and gzf2 resulted in nitrogen source-specific growth defects and greater accumulation of lipids when the cells were growing on a simple nitrogen source. Deletion of gzf1, which is most similar to activators of genes repressed by nitrogen catabolite repression in filamentous ascomycetes, did not affect growth on the nitrogen sources tested. We examined gene expression of wild-type and GATA transcription factor mutants on simple and complex nitrogen sources and found that expression of enzymes involved in malate metabolism, beta-oxidation, and ammonia utilization are strongly upregulated on a simple nitrogen source. Deletion of gzf3 results in overexpression of genes with GATAA sites in their promoters, suggesting that it acts as a repressor, while gzf2 is required for expression of ammonia utilization genes but does not grossly affect the transcription level of genes predicted to be controlled by nitrogen catabolite repression. Both GATA transcription factor mutants exhibit decreased expression of genes controlled by carbon catabolite repression via the repressor mig1, including genes for beta-oxidation, highlighting the complex interplay between regulation of carbon, nitrogen, and lipid metabolism.

IMPORTANCE Nitrogen source is commonly used to control lipid production in industrial fungi. Here we identified regulators of nitrogen catabolite repression in the oleaginous yeast Y. lipolytica to determine how the nitrogen source regulates lipid metabolism. We show that disruption of both activators and repressors of nitrogen catabolite repression leads to increased lipid accumulation via activation of carbon catabolite repression through an as yet uncharacterized method.

Intracellular siderophore but not extracellular siderophore is required for full virulence in Metarhizium robertsii

Efficient iron acquisition mechanisms are fundamental for microbial survival in the environment and for pathogen virulence within their hosts. M. robertsii produces two known iron-binding natural products: metachelins, which are used to scavenge extracellular iron, and ferricrocin, which is strictly intracellular. To study the contribution of siderophore-mediated iron uptake and storage to M. robertsii fitness, we generated null mutants for each siderophore synthase gene (mrsidD and mrsidC, respectively), as well as for the iron uptake transcriptional repressor mrsreA. All of these mutants showed impaired germination speed, differential sensitivity to hydrogen peroxide, and differential ability to overcome iron chelation on growth-limiting iron concentrations. RT-qPCR data supported regulation of mrsreA, mrsidC, and mrsidD by supplied iron in vitro and during growth within the insect host, Spodoptera exigua. We also observed strong upregulation of the insect iron-binding proteins, transferrins, during infection. Insect bioassays revealed that ferricrocin is required for full virulence against S. exigua; neither the loss of metachelin production nor the deletion of the transcription factor mrsreA significantly affected M. robertsii virulence.

Perturbations in small molecule synthesis uncovers an iron-responsive secondary metabolite network in Aspergillus fumigatus

Iron plays a critical role in survival and virulence of the opportunistic pathogen Aspergillus fumigatus. Two transcription factors, the GATA-factor SreA and the bZip-factor HapX oppositely monitor iron homeostasis with HapX activating iron acquisition pathways (e.g., siderophores) and shutting down iron consumptive pathways (and SreA) during iron starvation conditions whereas SreA negatively regulates HapX and corresponding pathways during iron sufficiency. Recently the non-ribosomal peptide, hexadehydroastechrome (HAS; a tryptophan-derived iron (III)-complex), has been found important in A. fumigatus virulence. We found that HAS overproduction caused an iron starvation phenotype, from alteration of siderophore pools to regulation of iron homeostasis gene expression including sreA. Moreover, we uncovered an iron dependent secondary metabolism network where both SreA and HapX oppositely regulate multiple other secondary metabolites including HAS. This circuitry links iron-acquisition and consumption pathways with secondary metabolism-thus placing HAS as part of a metabolic feedback circuitry designed to balance iron pools in the fungus and presenting iron availability as one environmental trigger of secondary metabolism.

Fungal siderophore metabolism with a focus on Aspergillus fumigatus

Siderophores are chelators synthesized by microbes to sequester iron. This article summarizes the knowledge on the fungal siderophore metabolism with a focus on Aspergillus fumigatus. In recent years, A. fumigatus became a role model for fungal biosynthesis, uptake and degradation of siderophores as well as regulation of siderophore-mediated iron handling and the elucidation of siderophore functions. Siderophore functions comprise uptake, intracellular transport and storage of iron. This proved to be crucial not only for adaptation to iron starvation conditions but also for germination, asexual and sexual propagation, antioxidative defense, mutual interaction, microbial competition as well as virulence in plant and animal hosts. Recent studies also indicate the high potential of siderophores and its biosynthetic pathway to improve diagnosis and therapy of fungal infections.

The Sfp-type 4'-phosphopantetheinyl transferase Ppt1 of Fusarium fujikuroi controls development, secondary metabolism and pathogenicity

The heterothallic ascomycete Fusarium fujikuroi is a notorious rice pathogen causing super-elongation of plants due to the production of terpene-derived gibberellic acids (GAs) that function as natural plant hormones. Additionally, F. fujikuroi is able to produce a variety of polyketide- and non-ribosomal peptide-derived metabolites such as bikaverins, fusarubins and fusarins as well as metabolites from yet unidentified biosynthetic pathways, e.g. moniliformin. The key enzymes needed for their production belong to the family of polyketide synthases (PKSs) and non-ribosomal peptide synthases (NRPSs) that are generally known to be post-translationally modified by a Sfp-type 4′phosphopantetheinyl transferase (PPTase). In this study we provide evidence that the F. fujikuroi Sfp-type PPTase FfPpt1 is essentially involved in lysine biosynthesis and production of bikaverins, fusarubins and fusarins, but not moniliformin as shown by analytical methods. Concomitantly, targeted Ffppt1 deletion mutants reveal an enhancement of terpene-derived metabolites like GAs and volatile substances such as α-acorenol. Pathogenicity assays on rice roots using fluorescent labeled wild-type and Ffppt1 mutant strains indicate that lysine biosynthesis and iron acquisition but not PKS and NRPS metabolism is essential for establishment of primary infections of F. fujikuroi. Additionally, FfPpt1 is involved in conidiation and sexual mating recognition possibly by activating PKS- and/or NRPS-derived metabolites that could act as diffusible signals. Furthermore, the effect on iron acquisition of Ffppt1 mutants led us to identify a previously uncharacterized putative third reductive iron uptake system (FfFtr3/FfFet3) that is closely related to the FtrA/FetC system of A. fumigatus. Functional characterization provides evidence that both proteins are involved in iron acquisition and are liable to transcriptional repression of the homolog of the Aspergillus GATA-type transcription factor SreA under iron-replete conditions. Targeted deletion of the first Fusarium homolog of this GATA-type transcription factor-encoding gene, Ffsre1, strongly indicates its involvement in regulation of iron homeostasis and oxidative stress resistance.

Metabolic remodeling in iron-deficient fungi

Eukaryotic cells contain dozens, perhaps hundreds, of iron-dependent proteins, which perform critical functions in nearly every major cellular process. Nutritional iron is frequently available to cells in only limited amounts; thus, unicellular and higher eukaryotes have evolved mechanisms to cope with iron scarcity. These mechanisms have been studied at the molecular level in the model eukaryotes Saccharomyces cerevisiae and Schizosaccharomyces pombe, as well as in some pathogenic fungi. Each of these fungal species exhibits metabolic adaptations to iron deficiency that serve to reduce the cell's reliance on iron. However, the regulatory mechanisms that accomplish these adaptations differ greatly between fungal species. This article is part of a Special Issue entitled: Cell Biology of Metals.

Iron influences the abundance of the iron regulatory protein Cir1 in the fungal pathogen Cryptococcus neoformans

The GATA-type, zinc-finger protein Cir1 regulates iron uptake, iron homeostasis and virulence factor expression in the fungal pathogen Cryptococcus neoformans. The mechanisms by which Cir1 senses iron availability, although as yet undefined, are important for understanding the proliferation of the fungus in mammalian hosts. We investigated the influence of iron availability on Cir1 and found that the abundance of the protein decreases upon iron deprivation. This destabilization was influenced by reducing conditions and by inhibition of proteasome function. The combined data suggest a post-translational mechanism for the control of Cir1 abundance in response to iron and redox status.

SREB, a GATA transcription factor that directs disparate fates in Blastomyces dermatitidis including morphogenesis and siderophore biosynthesis

Blastomyces dermatitidis belongs to a group of human pathogenic fungi that exhibit thermal dimorphism. At 22 degrees C, these fungi grow as mold that produce conidia or infectious particles, whereas at 37 degrees C they convert to budding yeast. The ability to switch between these forms is essential for virulence in mammals and may enable these organisms to survive in the soil. To identify genes that regulate this phase transition, we used Agrobacterium tumefaciens to mutagenize B. dermatitidis conidia and screened transformants for defects in morphogenesis. We found that the GATA transcription factor SREB governs multiple fates in B. dermatitidis: phase transition from yeast to mold, cell growth at 22 degrees C, and biosynthesis of siderophores under iron-replete conditions. Insertional and null mutants fail to convert to mold, do not accumulate significant biomass at 22 degrees C, and are unable to suppress siderophore biosynthesis under iron-replete conditions. The defect in morphogenesis in the SREB mutant was independent of exogenous iron concentration, suggesting that SREB promotes the phase transition by altering the expression of genes that are unrelated to siderophore biosynthesis. Using bioinformatic and gene expression analyses, we identified candidate genes with upstream GATA sites whose expression is altered in the null mutant that may be direct or indirect targets of SREB and promote the phase transition. We conclude that SREB functions as a transcription factor that promotes morphogenesis and regulates siderophore biosynthesis. To our knowledge, this is the first gene identified that promotes the conversion from yeast to mold in the dimorphic fungi, and may shed light on environmental persistence of these pathogens.

Elucidation of the complete ferrichrome A biosynthetic pathway in Ustilago maydis

Iron is an important element for many essential processes in living organisms. To acquire iron, the basidiomycete Ustilago maydis synthesizes the iron-chelating siderophores ferrichrome and ferrichrome A. The chemical structures of these siderophores have been elucidated long time ago but so far only two enzymes involved in their biosynthesis have been described. Sid1, an ornithine monoxygenase, is needed for the biosynthesis of both siderophores, and Sid2, a non-ribosomal peptide synthetase (NRPS), is involved in ferrichrome generation. In this work we identified four novel enzymes, Fer3, Fer4, Fer5 and Hcs1, involved in ferrichrome A biosynthesis in U. maydis. By HPLC-MS analysis of siderophore accumulation in culture supernatants of deletion strains, we show that Fer3, an NRPS, Fer4, an enoyl-coenzyme A (CoA)-hydratase, and Fer5, an acylase, are required for ferrichrome A production. We demonstrate by conditional expression of the hydroxymethyl glutaryl (HMG)-CoA synthase Hcs1 in U. maydis that HMG-CoA is an essential precursor for ferrichrome A. In addition, we heterologously expressed and purified Hcs1, Fer4 and Fer5, and demonstrated the enzymatic activities by in vitro experiments. Thus, we describe the first complete fungal siderophore biosynthetic pathway by functionally characterizing four novel genes responsible for ferrichrome A biosynthesis in U. maydis.

SreA-mediated iron regulation in Aspergillus fumigatus

Aspergillus fumigatus, the most common airborne fungal pathogen of humans, employs two high-affinity iron uptake systems: iron uptake mediated by the extracellular siderophore triacetylfusarinine C and reductive iron assimilation. Furthermore, A. fumigatus utilizes two intracellular siderophores, ferricrocin and hydroxyferricrocin, to store iron. Siderophore biosynthesis, which is essential for virulence, is repressed by iron. Here we show that this control is mediated by the GATA factor SreA. During iron-replete conditions, SreA deficiency partially derepressed synthesis of triacetylfusarinine C and uptake of iron resulting in increased cellular accumulation of both iron and ferricrocin. Genome-wide DNA microarray analysis identified 49 genes that are repressed by iron in an SreA-dependent manner. This gene set, termed SreA regulon, includes all known genes involved in iron acquisition, putative novel siderophore biosynthetic genes, and also genes not directly linked to iron metabolism. SreA deficiency also caused upregulation of iron-dependent and antioxidative pathways, probably due to the increased iron content and iron-mediated oxidative stress. Consistently, the sreA disruption mutant displayed increased sensitivity to iron, menadion and phleomycin but retained wild-type virulence in a mouse model. As all detrimental effects of sreA disruption are restricted to iron-replete conditions these data underscore that A. fumigatus faces iron-depleted conditions during infection.

Histoplasma requires SID1, a member of an iron-regulated siderophore gene cluster, for host colonization

The macrophage is the primary host cell for the fungal pathogen Histoplasma capsulatum during mammalian infections, yet little is known about fungal genes required for intracellular replication in the host. Since the ability to scavenge iron from the host is important for the virulence of most pathogens, we investigated the role of iron acquisition in H. capsulatum pathogenesis. H. capsulatum acquires iron through the action of ferric reductases and the production of siderophores, but the genes responsible for these activities and their role in virulence have not been determined. We identified a discrete set of co-regulated genes whose transcription is induced under low iron conditions. These genes all appeared to be involved in the synthesis, secretion, and utilization of siderophores. Surprisingly, the majority of these transcriptionally co-regulated genes were found clustered adjacent to each other in the genome of the three sequenced strains of H. capsulatum, suggesting that their proximity might foster coordinate gene regulation. Additionally, we identified a consensus sequence in the promoters of all of these genes that may contribute to iron-regulated gene expression. The gene set included L-ornithine monooxygenase (SID1), the enzyme that catalyzes the first committed step in siderophore production in other fungi. Disruption of SID1 by allelic replacement resulted in poor growth under low iron conditions, as well as a loss of siderophore production. Strains deficient in SID1 showed a significant growth defect in murine bone-marrow-derived macrophages and attenuation in the mouse model of infection. These data indicated that H. capsulatum utilizes siderophores in addition to other iron acquisition mechanisms for optimal growth during infection.

Fungal infections are a growing public health threat, particularly for immunocompromised individuals such as people with AIDS, organ transplant recipients, and cancer patients. Present antifungal therapies are often highly toxic and resistance to these therapies continues to rise. Histoplasma capsulatum is a pathogenic fungus that infects humans, causing pulmonary and systemic disease. It is the most common cause of fungal respiratory infection in the world, and is endemic to the Mississippi and Ohio River valleys of the United States. H. capsulatum produces small molecules, called siderophores, to acquire iron, an essential nutrient. We have identified genes that are involved in the synthesis of siderophores in this fungus and have found that siderophore production in H. capsulatum is important for its virulence. Since siderophore production is confined to microbes and plays no role in human biology, it is an excellent target for rational drug design.

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.

Iron and siderophores in fungal-host interactions

Most fungi and bacteria express specific mechanisms for the acquisition of iron from the hosts they infect for their own survival. This is primarily because iron plays a key catalytic role in various vital cellular reactions in conjunction with the fact that iron is not freely available in these environments due to host sequestration. High-affinity iron uptake systems, such as siderophore-mediated iron uptake and reductive iron assimilation, enable fungi to acquire limited iron from animal or plant hosts. Regulating iron uptake is crucial to maintain iron homeostasis, a state necessary to avoid iron-induced toxicity from iron abundance, while simultaneously supplying iron required for biochemical demand. Siderophores play diverse roles in fungal-host interactions, many of which have been principally delineated from gene deletions in non-ribosomal peptide synthetases, enzymes required for siderophore biosynthesis. These analyses have demonstrated that siderophores are required for virulence, resistance to oxidative stress, asexual/sexual development, iron storage, and protection against iron-induced toxicity in some fungal organisms. In this review, the strategies fungi employ to obtain iron, siderophore biosynthesis, and the regulatory mechanisms governing iron homeostasis will be discussed with an emphasis on siderophore function and relevance for fungal organisms in their interactions with their hosts.

Metallosensors, the ups and downs of gene regulation

In fungal cells, transcriptional regulatory mechanisms play a central role in both the homeostatic regulation of the essential metals iron, copper and zinc and in the detoxification of heavy metal ions such as cadmium. Fungi detect changes in metal ion levels using unique metallo-regulatory factors whose activity is responsive to the cellular metal ion status. New studies have revealed that these factors not only regulate the expression of genes required for metal ion acquisition, storage or detoxification but also globally remodel metabolism to conserve metal ions or protect against metal toxicity. This review focuses on the mechanisms metallo-regulators use to up- and down-regulate gene expression.

Expression of Candida albicans Sfu1 in fission yeast complements the loss of the iron-regulatory transcription factor Fep1 and requires Tup co-repressors

The opportunistic pathogenic yeast Candida albicans contains a gene which encodes a putative member of the iron-regulatory GATA factor protein family. This protein, referred to as suppressor of ferric uptake (Sfu1), has two Cys(2)/Cys(2)-type zinc finger domains separated by a conserved Cys-rich region. In Schizosaccharomyces pombe, the GATA-type transcription factor Fe protein 1 (Fep1) represses target gene expression when iron levels exceed those needed by the cell. To ascertain the functional similarity between Sfu1 and Fep1, the C. albicans Sfu1 was expressed in Sz. pombe cells lacking the endogenous fep1(+) gene. We determined that Sfu1 is capable of suppressing iron-related phenotypes of fep1Delta mutant cells. Using a functional SFU1-GFP fusion allele, the Sfu1 protein was localized to the nucleus under both iron-replete and iron-starved conditions. Sfu1 effectively regulated the expression of genes encoding components of the reductive and non-reductive iron transport systems. Furthermore, the iron-responsive regulation mediated by Sfu1 was GATA-dependent. The N-terminal 250 amino acid segment of Sfu1 expressed in and purified from Escherichia coli specifically associated with the hexanucleotide sequence AGATAA in an iron-dependent manner. On the other hand, expression of the full-length C. albicans Sfu1 in Sz. pombe fep1Delta tup11Delta tup12Delta triple mutant cells failed to repress target gene expression under conditions of high iron concentration. Using two-hybrid analysis, we demonstrated that Tup11 and Tup12 physically interacted with Sfu1. Taken together, these results reveal a remarkable functional conservation between Sfu1 from C. albicans and Fep1 from Sz. pombe in their ability to sense excess iron and respond by repressing target gene transcription.

An iron enhancer element in the FTN-1 gene directs iron-dependent expression in Caenorhabditis elegans intestine

Ferritin is a ubiquitous protein that sequesters iron and protects cells from iron toxicity. Caenorhabditis elegans express two ferritins, FTN-1 and FTN-2, which are transcriptionally regulated by iron. To identify the cis-acting sequences and proteins required for iron-dependent regulation of ftn-1 and ftn-2 expression, we generated transcriptional GFP reporters corresponding to 5 '-upstream sequences of the ftn-1 and ftn-2 genes. We identified a conserved 63-bp sequence, the iron-dependent element (IDE), that is required for iron-dependent regulation of a ftn-1 GFP reporter in intestine. The IDE contains two GATA-binding motifs and three octameric direct repeats. Site-directed mutagenesis of the GATA sequences, singly or in combination, reduces ftn-1 GFP reporter expression in the intestine. In vitro DNA mobility shift assays show that the intestine-specific GATA protein ELT-2 binds to both GATA sequences. Inhibition of ELT-2 function by RNA interference blocks ftn-1 GFP reporter expression in vivo. Insertion of the IDE into the promoter region of a heterologous reporter activates iron-dependent transcription in intestine. These data demonstrate that the activation of ftn-1 and ftn-2 transcription by iron requires ELT-2 and that the IDE functions as an iron-dependent enhancer in intestine.

Ecology of siderophores with special reference to the fungi

Ecology of siderophores, as described in the present review, analyzes the factors that allow the production and function of siderophores under various environmental conditions. Microorganisms that excrete siderophores are able to grow in natural low-iron environments by extracting residual iron from insoluble iron hydroxides, protein-bound iron or from other iron chelates. Compared to the predominantly mobile bacteria, the fungi represent mostly immobile microorganisms that rely on local nutrient concentrations. Feeding the immobile is a general strategy of fungi and plants, which depend on the local nutrient resources. This also applies to iron nutrition, which can be improved by excretion of siderophores. Most fungi produce a variety of different siderophores, which cover a wide range of physico-chemical properties in order to overcome adverse local conditions of iron solubility. Resource zones will be temporally and spatially dynamic which eventually results in conidiospore production, transport to new places and outgrow of mycelia from conidiospores. Typically, extracellular and intracellular siderophores exist in fungi which function either in transport or storage of ferric iron. Consequently, extracellular and intracellular reduction of siderophores may occur depending on the fungal strain, although in most fungi transport of the intact siderophore iron complex has been observed. Regulation of siderophore biosynthesis is essential in fungi and allows an economic use of siderophores and metabolic resources. Finally, the chemical stability of fungal siderophores is an important aspect of microbial life in soil and in the rhizosphere. Thus, insolubility of iron in the environment is counteracted by dissolution and chelation through organic acids and siderophores by various fungi.

A ferroxidation/permeation iron uptake system is required for virulence in Ustilago maydis

In the smut fungus Ustilago maydis, a tightly regulated cAMP signaling cascade is necessary for pathogenic development. Transcriptome analysis using whole genome microarrays set up to identify putative target genes of the protein kinase A catalytic subunit Adr1 revealed nine genes with putative functions in two high-affinity iron uptake systems. These genes locate to three gene clusters on different chromosomes and include the previously identified complementing siderophore auxotroph genes sid1 and sid2 involved in siderophore biosynthesis. Transcription of all nine genes plus three additional genes associated with the gene clusters was also coregulated by iron through the Urbs1 transcription factor. Two components of a high-affinity iron uptake system were characterized in more detail: fer2, encoding a high-affinity iron permease; and fer1, encoding an iron multicopper oxidase. Fer2 localized to the plasma membrane and complemented an ftr1 mutant of Saccharomyces cerevisiae lacking a high-affinity iron permease. During pathogenic development, fer2 expression was confined to the phase of hyphal proliferation inside the plant. fer2 as well as fer1 deletion mutants were strongly affected in virulence. These data highlight the importance of the high-affinity iron uptake system via an iron permease and a multicopper oxidase for biotrophic development in the U. maydis/maize (Zea mays) pathosystem.

Iron gathering of opportunistic pathogenic fungi. A mini review

Iron is an essential nutrient for most organisms because it serves as a catalytic cofactor in oxidation-reduction reactions. Iron is rather unavailable because it occurs in its insoluble ferric form in oxides and hydroxides, while in serum of mammalian hosts is highly bound to carrier proteins such as transferrin, so the free iron concentration is extremely low insufficient for microbial growth. Therefore, many organisms have developed different iron-scavenging systems for solubilizing ferric iron and transporting it into cells across the fungal membrane. There are three major mechanisms by which fungi can obtain iron from the host: (a) utilization of a high affinity iron permease to transport iron intracellularly, (b) production and secretion of low molecular weight iron-specific chelators (siderophores), (c) utilization of a hem oxygenase to acquire iron from hemin. Patients with elevated levels of available serum iron treated with iron chelator, deferoxamine to remedy iron overload conditions have an increased susceptibility of invasive zygomycosis. Presumably deferoxamine predisposes patients to Zygomycetes infections by acting as a siderophore]. The frequency of zygomycosis is increasing in recent years and these infections respond very poorly to currently available antifungal agents, so new approaches to develop strategies to prevent and treat zygomycosis are urgently needed. Siderophores and iron-transport proteins have been suggested to function as virulence factors because the acquisition of iron is a crucial pathogenetic event. Biosynthesis and uptake of siderophores represent possible targets for antifungal therapy.

Regulatory networks affected by iron availability in Candida albicans

Iron, an essential element for almost every organism, serves as a regulatory signal for the expression of virulence determinants in many prokaryotic and eukaryotic pathogens. Using a custom Affymetrix GeneChip representing the entire Candida albicans genome, we examined the changes in genome-wide gene expression in this opportunistic pathogen as a function of alterations in environmental concentrations of iron. A total of 526 open reading frame (ORF) transcripts are more highly expressed when the levels of available iron are low, while 626 ORF transcripts are more highly expressed in high-iron conditions. The transcripts dominantly affected by iron concentration range from those associated with cell-surface properties to others which affect mitochondrial function, iron transport and virulence-related secreted hydrolases. Moreover gene expression as assayed in DNA microarrays confirms and extends reports of alterations in cell-surface antigens and drug sensitivity correlated with iron availability. To understand how these genes and pathways might be regulated, we isolated a gene designated SFU1 that encodes a homologue of the Ustilago maydis URBS1, a transcriptional repressor of siderophore uptake/biosynthesis. Comparisons between wild-type and SFU1-null mutant strains revealed 139 potential target genes of Sfu1p; many of which are iron-responsive. Together, these results not only expand our understanding of global iron regulation in C. albicans, but also provide insights into the potential role of iron availability in C. albicans virulence.

Ferrichrome in Schizosaccharomyces pombe ? an iron transport and iron storage compound

Schizosaccharomyces pombe has been assumed not to produce siderophores. Nevertheless, the genomic sequence of this fission yeast revealed the presence of siderophore biosynthetic genes for hydroxamates. Applying a bioassay based on an Aspergillus nidulans strain deficient in siderophore biosynthesis, and using reversed-phase HPLC and mass spectrometry analysis, we demonstrate that S. pombe excretes and accumulates intracellularly the hydroxamate-type siderophore ferrichrome. Under iron-limiting conditions, the cellular ferrichrome pool was present in the desferri-form, while under iron-richconditions, in the ferri-form. In contrast to S. pombe, hydroxamate-type siderophores could not be detected intwo other yeast species, Saccharomyces cerevisiae and Candida albicans.

Nitrogen metabolism and virulence of Candida albicans require the GATA-type transcriptional activator encoded by GAT1

Nitrogen acquisition and metabolism is central to microbial growth. A conserved family of zinc-finger containing transcriptional regulators known as GATA-factors ensures efficient utilization of available nitrogen sources by fungi. GATA factors activate expression of nitrogen catabolic pathways when preferred nitrogen sources are absent or limiting, a phenomenon known as nitrogen catabolite repression. GAT1 of Candida albicans encodes a GATA-factor homologous to the AREA protein of Aspergillus nidulans and related transcription factors involved in nitrogen regulation. Two observations implicated GAT1 in nitrogen regulation. The growth of mutants lacking GAT1 was reduced when isoleucine, tyrosine or tryptophan were the sole source of nitrogen. Secondly, when cultured on a secondary nitrogen source, gat1Delta mutants were unable to activate expression of GAP1, UGA4 or DAL5, which were shown to be nitrogen regulated in C. albicans. This regulatory defect did not prevent filamentation of gat1Delta mutants in nitrogen repressing or non-repressing conditions, demonstrating that nitrogen catabolite repression does not influence dimorphism. The mutants were, however, highly attenuated in a murine model of disseminated candidiasis. Attenuation was not associated with any diminution of growth in serum or ability to utilize serum amino acids. The results indicate an important role for nitrogen regulation in the virulence of C. albicans.

Iron-responsive genes of Phanerochaete chrysosporium isolated by differential display reverse transcription polymerase chain reaction

White-rot fungus Phanerochaete chrysosporium, a ligninolytic basidiomycete, was studied to identify iron-responsive genes. Using the differential display reverse transcription PCR technique (DDRT-PCR), a total of 97 differentially expressed cDNA fragments were identified by comparing band intensities among fingerprints obtained from mycelia cultivated in iron-deficient and iron-replete media. Transcripts induced under iron-starvation exhibited homologies to: a modular polyketide synthase, a TonB protein, a probable transmembrane protein, a putative ABC transporter permease and a HSP70-related heat-shock protein. Modular polyketide synthase and TonB proteins are normally expressed under iron-starvation and are known to be involved in biosynthesis and transport of siderophores respectively. Also, a deduced protein with 96% similarity to a precursor of the well-known P. chrysosporium lignin peroxidase was identified under iron-deficiency. Two DDRT-PCR products confirmed their iron-induced expression. One was homologue to the CNOT3, which is a global regulator of RNA polymerase II transcription and has been implicated in multiple roles in the control of mRNA metabolism. The other was similar to the Schizosaccharomyces pombe putative proteasome maturation factor upm1. In conclusion, the majority of iron-responsive P. chrysosporium transcripts isolated in the DDRT-PCR encode proteins involved in iron acquisition, especially members of biosynthesis and transport of iron chelators.

dffA Gene fromAspergillus oryzae encodesl-ornithineN5-oxygenase and is indispensable for deferriferrichrysin biosynthesis

We identified and analyzed the dffA gene from Aspergillus oryzae which encodes L-ornithine N5-oxygenase involved in the biosynthesis of deferriferrichrysin, a type of siderophore which is a low-molecular-weight iron chelating compound. From among more than 20,000 clones in an A. oryzae EST (expressed sequence tag) library, we found only one clone encoding a protein that exhibited homology to theUstilago maydis sid1 protein (Sid1) and Pseudomonas aeruginosa pvdA protein (PvdA), both known as the only examples of L-ornithine N5-oxygenase. The complete gene sequence shows that the dffA gene includes a 1575-bp open reading frame (ORF), one 66-bp intorn, which is a typical intorn length inA. oryzae, and encodes 502 amino acids with putative FAD-binding, NADP-binding, and 'FATGY' motifs, which are conserved inN-hydroxylating enzymes. As well as that of the U. maydis sid1 gene,dffA gene expression was induced under iron-limited conditions, and the promoter region has several GATA-type transcription regulator binding motifs. When the dffA gene was expressed under the control of the a-amylase promoter in A. oryzae, transformants revealed inducible high L-ornithine N5-oxygenase activities. In addition, a dffA gene disruptant showed no deferriferrichrysin production even under iron-limited conditions. These results clearly suggest that the dffA gene is indispensable for deferriferrichrysin biosynthesis in A. oryzae.

Arabidopsis thaliana GATA factors: organisation, expression and DNA-binding characteristics

Many light-responsive promoters contain GATA motifs and a number of nuclear proteins have been defined that interact with these elements. Type-IV zinc-finger proteins have been extensively characterised in animals and fungi and are referred to as GATA factors by virtue of their affinity for promoter elements containing this sequence. We previously identified cDNA sequences representing four Arabidopsis thaliana type-TV zinc-finger proteins. Here we define the organisation and expression of GATA-1, GATA-2, GATA-3 and GATA-4 as well as DNA-binding characteristics of their encoded proteins. Transcripts from all four genes can be detected in all tissues examined suggesting that they are not developmentally regulated at the level of transcription. In vitro binding experiments with Escherichia coli-derived recombinant proteins were performed using motifs previously defined as targets for nuclear GATA-binding proteins. These studies reveal differences in DNA binding specificity of GATA-1 as compared to the other three proteins. In vivo protein-DNA interactions monitored by yeast one-hybrid assays reveal different binding characteristics as compared to those defined with E. coli-derived recombinant protein. Trans-activation of gene expression by the four Arabidopsis proteins via some, but not all, DNA elements tested indicates that the Arabidopsis proteins can form functional interactions with previously defined promoter elements containing GATA motifs. We conclude that the Arabidopsis type-IV zinc-finger proteins may represent the previously defined family of nuclear GATA-binding proteins implicated in light-responsive transcription.

Fep1, an iron sensor regulating iron transporter gene expression in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells acquire iron under high affinity conditions through the action of a cell surface ferric reductase encoded by the frp1(+) gene and a two-component iron-transporting complex encoded by the fip1(+) and fio1(+) genes. When cells are grown in the presence of iron, transcription of all three genes is blocked. A conserved regulatory element, 5'-(A/T)GATAA-3', located upstream of the frp1(+), fip1(+), and fio1(+) genes, is necessary for iron repression. We have cloned a novel gene, termed fep1(+), which encodes an iron-sensing transcription factor. Binding studies reveal that the putative DNA binding domain of Fep1 expressed as a fusion protein in Escherichia coli specifically interacts with the 5'-(A/T)GATAA-3' sequence in an iron-dependent manner. In a fep1 Delta mutant strain, the fio1(+) gene is highly expressed and is unregulated by iron. Furthermore, the fep1 Delta mutation increases activity of the cell surface iron reductase and renders cells hypersensitive to the iron-dependent free radical generator phleomycin. Mutations in the transcriptional co-repressors tup11(+) and tup12(+) are phenocopies to fep1(+). Indeed, strains with both tup11 Delta and tup12 Delta deletions fail to sense iron. This suggests that in the presence of iron and Fep1, the Tup11 and Tup12 proteins may act as co-repressors for down-regulation of genes encoding components of the reductive iron transport machinery.

Iron-regulated proteins in Phanerochaete chrysosporium and Lentinula edodes: differential analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis and two-dimensional polyacrylamide gel electrophoresis profiles

Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and two-dimensional gel electrophoresis (2-DE) were used to identify iron-responsive proteins in the white-rot species (Phanerochaete chrysosporium and Lentinula edodes), by comparing the differential patterns of cellular and membrane proteins obtained from iron-sufficient and iron-deficient mycelia. Six cellular proteins induced by iron restriction have been observed in SDS-PAGE for P. chrysosporium and twelve for L. edodes. In 2-DE, the numbers of iron-restricted induced proteins were 12 and 9, respectively, in a resolution range of 15-60 kDa and pI 4.5-8.1. SDS-PAGE for the plasma membrane protein did not show differences, whereas the outer-membrane protein profile showed 6 and 5 proteins induced by iron depletion in P. chrysosporium and L. edodes, respectively. The results presented here are important data to unravel mechanisms of biosynthesis and/or transport of the iron-complexing agents in ligninolytic fungi and to further correlate them to the ligninolytic processes.

SREA is involved in regulation of siderophore biosynthesis, utilization and uptake in Aspergillus nidulans

Under conditions of low iron availability, most fungi excrete siderophores in order to mobilize extracellular iron. We show that lack of the GATA-type transcription factor SREA in Aspergillus nidulans not only leads to derepression of siderophore biosynthesis but also to deregulation of siderophore-bound iron uptake and ornithine esterase expression. Furthermore, SREA deficiency causes increased accumulation of ferricrocin, the siderophore responsible for intracellular iron storage. In sreA deletion strains, extracellular siderophore production is derepressed but still regulated negatively by iron availability, indicating the presence of an additional iron-regulatory mechanism. In contrast, iron affects ferricrocin accumulation in a positive way, suggesting a protective role for this siderophore in detoxification of intracellular iron excess. The harmfulness of deregulated iron uptake in this mutant is demonstrated by increased expression of genes encoding the antioxidative enzymes catalase CATB and the superoxide dismutases SODA and SODB. It is noteworthy that iron starvation was found to repress catB expression in wild-type (wt) and SREA-deficient strains, consistent with catB being subject to SREA-independent iron regulation. Differential display led to the identification of putative SREA target genes amcA and mirA. The deduced MIRA amino acid sequence displays significant similarity to recently characterized siderophore permeases of Saccharomyces cerevisiae. amcA encodes a putative mitochondrial carrier for the siderophore precursor ornithine, indicating cross-regulation of siderophore and ornithine metabolism.

The Aspergillus nidulans GATA transcription factor gene areB encodes at least three proteins and features three classes of mutation

In Aspergillus nidulans, the principal transcription factor regulating nitrogen metabolism, AREA, belongs to the GATA family of DNA-binding proteins. In seeking additional GATA factors, we have cloned areB, which was originally identified via a genetic screen for suppressors of areA loss-of-function mutations. Based on our analysis, areB is predicted to encode at least three distinct protein products. These arise from the use of two promoters, differential splicing and translation initiating at AUG and non-AUG start codons. All the putative products include a GATA domain and a putative Leu zipper. These regions show strong sequence similarity to regulatory proteins from Saccharomyces cerevisiae (Dal80p and Gzf3p), Penicillium chrysogenum (NREB) and Neurospora crassa (ASD4). We have characterized three classes of mutation in areB; the first are loss-of-function mutations that terminate the polypeptides within or before the GATA domain. The second class truncates the GATA factor either within or upstream of the putative Leu zipper but retains the GATA domain. The third class fuses novel gene sequences to areB with the potential to produce putative chimeric polypeptides. These novel gene fusions transform the putative negative-acting transcription factor into an activator that can partially replace areA.

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

Almost all iron uptake by fungi involves reduction from Fe(III) to Fe(II) in order to facilitate ligand exchange. This leads to two mechanisms: uptake before reduction, or reduction before uptake. Many fungi secrete specific hydroxamate siderophores when short of iron. The mechanism with uptake before reduction is described in the context of siderophore synthesis and usage, since it applies to many (but not all) siderophores. The hydroxamate functional group is synthesized from ornithine by N5 hydroxylation and acylation. In most fungal siderophores, two or three modified ornithines are joined together by a non-ribosomal peptide synthetase. The transcription of these genes is regulated by an iron activated repressor. There is evidence that the iron-free siderophore may be stored in intracellular vesicles until secretion is required. After loading with iron, re-entry is likely to be via a proton symport. In some fungi, siderophores are used for iron storage. The iron is liberated by an NADPH-linked reductase. The second mechanism starts with Fe(III) reduction. In yeast, this is catalysed by an NADPH-linked transmembrane reductase, which has homology with the NADPH oxidase of neutrophils. There are two closely similar reductases with overlapping roles in Fe(III) and Cu(II) reduction, while the substrates for reduction include Fe(III)-siderophores. External reductants, which may be important in certain fungi, include 3-hydroxyanthranilic acid, melanin, cellobiose dehydrogenase and 2,5-dimethylhydroquinone. In yeast, a high-affinity iron uptake pathway involves reoxidation of Fe(II) to Fe(III), probably to confer specificity for iron. This is catalysed by a copper protein which has homology with ceruloplasmin, and is closely coupled to Fe(III) transport. The transcription of these genes is regulated by an iron-inhibited activator. Because of its copper requirement, the high-affinity pathway is blocked by disruption of genes for copper metabolism. A low-affinity uptake transports Fe(II) directly and is important in anoxic growth. In many fungi, mechanisms with internal or external reduction are both important. The external reduction is applicable to almost any Fe(III) complex, while internal reduction is more efficient at low iron but requires a siderophore permease through which toxins might enter. Both mechanisms require close coupling of Fe(III) reduction and Fe(II) utilization in order to minimize production of active oxygen.