The interplay between vacuolar and siderophore-mediated iron storage in Aspergillus fumigatus.

Iron is an essential element for all eukaryotes but its excess has deleterious effects. Aspergillus fumigatus produces extracellular siderophores for iron uptake and the intracellular siderophore ferricrocin (FC) for distribution and storage of iron. Iron excess has previously been shown to increase the content of ferric FC and the expression of the putative vacuolar iron importer CccA (AFUA_4G12530), indicating a role of both the vacuole and FC in iron detoxification. In this study, we show that CccA-deficiency decreases iron resistance in particular in combination with derepressed iron uptake, while overproduction of CccA increases iron resistance. Green fluorescence protein-tagging confirmed localization of CccA in the vacuolar membrane. In contrast to CccA-deficiency, inactivation of FC biosynthesis did not affect iron resistance, which indicates that vacuolar rather than FC-mediated iron storage is the major iron detoxifying mechanism. After uptake, extracellular siderophore backbones are hydrolyzed and recycled. Lack of FC, CccA, and in particular lack of both increased the cellular content of iron chelated by siderophore breakdown products. These data indicate that the transfer of iron from extracellular siderophores to the metabolism, FC or the vacuole precedes recycling of siderophore breakdown products. Furthermore, this study indicates that CccA does not play an exclusive role in vacuolar iron storage for nutritional reuse.

The CCAAT-binding complex coordinates the oxidative stress response in eukaryotes.

The heterotrimeric CCAAT-binding complex is evolutionary conserved in eukaryotic organisms. The corresponding Aspergillus nidulans CCAAT-binding factor (AnCF) consists of the subunits HapB, HapC and HapE. All of the three subunits are necessary for DNA binding. Here, we demonstrate that AnCF senses the redox status of the cell via oxidative modification of thiol groups within the histone fold motif of HapC. Mutational and in vitro interaction analyses revealed that two of these cysteine residues are indispensable for stable HapC/HapE subcomplex formation and high-affinity DNA binding of AnCF. Oxidized HapC is unable to participate in AnCF assembly and localizes in the cytoplasm, but can be recycled by the thioredoxin system in vitro and in vivo. Furthermore, deletion of the hapC gene led to an impaired oxidative stress response. Therefore, the central transcription factor AnCF is regulated at the post-transcriptional level by the redox status of the cell serving for a coordinated activation and deactivation of antioxidative defense mechanisms including the specific transcriptional activator NapA, production of enzymes such as catalase, thioredoxin or peroxiredoxin, and maintenance of a distinct glutathione homeostasis. The underlying fine-tuned mechanism very likely represents a general feature of the CCAAT-binding complexes in eukaryotes.

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.

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.

Distinct roles for intra- and extracellular siderophores during Aspergillus fumigatus infection.

Siderophore biosynthesis by the highly lethal mould Aspergillus fumigatus is essential for virulence, but non-existent in humans, presenting a rare opportunity to strategize therapeutically against this pathogen. We have previously demonstrated that A. fumigatus excretes fusarinine C and triacetylfusarinine C to capture extracellular iron, and uses ferricrocin for hyphal iron storage. Here, we delineate pathways of intra- and extracellular siderophore biosynthesis and show that A. fumigatus synthesizes a developmentally regulated fourth siderophore, termed hydroxyferricrocin, employed for conidial iron storage. By inactivation of the nonribosomal peptide synthetase SidC, we demonstrate that the intracellular siderophores are required for germ tube formation, asexual sporulation, resistance to oxidative stress, catalase A activity, and virulence. Restoration of the conidial hydroxyferricrocin content partially rescues the virulence of the apathogenic siderophore null mutant Delta sidA, demonstrating an important role for the conidial siderophore during initiation of infection. Abrogation of extracellular siderophore biosynthesis following inactivation of the acyl transferase SidF or the nonribosomal peptide synthetase SidD leads to complete dependence upon reductive iron assimilation for growth under iron-limiting conditions, partial sensitivity to oxidative stress, and significantly reduced virulence, despite normal germ tube formation. Our findings reveal distinct cellular and disease-related roles for intra- and extracellular siderophores during mammalian Aspergillus infection.

Interaction of HapX with the CCAAT-binding complex--a novel mechanism of gene regulation by iron.

Iron homeostasis requires subtle control systems, as iron is both essential and toxic. In Aspergillus nidulans, iron represses iron acquisition via the GATA factor SreA, and induces iron-dependent pathways at the transcriptional level, by a so far unknown mechanism. Here, we demonstrate that iron-dependent pathways (e.g., heme biosynthesis) are repressed during iron-depleted conditions by physical interaction of HapX with the CCAAT-binding core complex (CBC). Proteome analysis identified putative HapX targets. Mutual transcriptional control between hapX and sreA and synthetic lethality resulting from deletion of both regulatory genes indicate a tight interplay of these control systems. Expression of genes encoding CBC subunits was not influenced by iron availability, and their deletion was deleterious during iron-depleted and iron-replete conditions. Expression of hapX was repressed by iron and its deletion was deleterious during iron-depleted conditions only. These data indicate that the CBC has a general role and that HapX function is confined to iron-depleted conditions. Remarkably, CBC-mediated regulation has an inverse impact on the expression of the same gene set in A. nidulans, compared with Saccharomyces cerevisae.

The intracellular siderophore ferricrocin is involved in iron storage, oxidative-stress resistance, germination, and sexual development in Aspergillus nidulans.

Iron is required by most organisms, but an excess of this metal is potentially toxic. Consequently, uptake and intracellular storage of iron are tightly controlled. The filamentous fungus A. nidulans lacks the iron storage compound ferritin but possesses an intracellular siderophore, which is accumulated in a highly regulated manner as iron-free desferri-ferricrocin or iron-containing ferricrocin via transcriptional regulation of the nonribosomal peptide synthetase SidC. Biosynthesis of desferri-ferricrocin was low during iron-replete conditions but up-regulated by both iron starvation and intracellular iron excess, the latter caused by either a shift from iron-depleted to high-iron conditions or deregulation of iron uptake. Consequently, ferricrocin constituted only about 5% of the total iron content under iron-replete conditions but up to 64% during conditions of intracellular excess. In contrast, during iron starvation, desferri-ferricrocin was accumulated, which appears to represent a proactive strategy to prevent iron toxicity. Accumulation of the intracellular siderophore was also up-regulated by oxidative stress, which underscores the intertwining of iron metabolism and oxidative stress. Lack of the intracellular siderophore causes pleiotropic effects, as SidC deficiency results in (i) less-efficient utilization of iron, indicated by reduced growth under iron-depleted conditions and a higher iron demand under iron-replete conditions, (ii) delayed germination under iron-depleted conditions, (iii) increased sensitivity of conidia to oxidative stress, and (iv) elimination of cleistothecia formation in homothallic conditions.

Biosynthesis and uptake of siderophores is controlled by the PacC-mediated ambient-pH Regulatory system in Aspergillus nidulans.

Biosynthesis and uptake of siderophores in Aspergillus nidulans are regulated not only by iron availability but also by ambient pH: expression of this high-affinity iron uptake system is elevated by an increase in the ambient pH. Mediation of this regulation by the transcriptional regulator PacC has been confirmed via acidity- and alkalinity-mimicking mutants.

4'-phosphopantetheinyl transferase-encoding npgA is essential for siderophore biosynthesis in Aspergillus nidulans.

Aspergillus nidulans produces two major siderophores: it excretes triacetylfusarinine C to capture iron and contains ferricrocin as an intracellular iron-storage compound. Siderophore biosynthesis involves the enzymatic activity of nonribosomal peptide synthetases (NRPS). NRPS contain 4'-phosphopantetheine as an essential prosthetic group, which is attached by 4'-phosphopantetheinyl transferases. A. nidulans appears to possess at least one gene, npgA, encoding such an enzyme. Using a strain carrying a temperature-sensitive allele, cfwA2, we showed that NpgA is essential for biosynthesis of both the peptide bond-containing ferricrocin and the ester bond-containing triacetylfusarinene C. The cfwA2 strain was found to be iron-starved at the restrictive temperature during iron-replete conditions, consistent with the siderophore system being the major iron-uptake system-as we recently demonstrated. Northern analysis indicated that, in contrast to other genes which are involved in siderophore biosynthesis and uptake, expression of npgA is not controlled by the GATA-transcription factor SreA. It was shown previously that NpgA is required for biosynthesis of penicillin, pigment, and potentially lysine via the alpha-aminoadipate pathway. Supplementation with lysine plus triacetylfusarinine C restored normal growth of the cfwA2 strain at the restrictive temperature, suggesting that the growth defect of the mutant is mainly due to impaired biosynthesis of siderophores and lysine.

The siderophore system is essential for viability of Aspergillus nidulans: functional analysis of two genes encoding l-ornithine N 5-monooxygenase (sidA) and a non-ribosomal peptide synthetase (sidC).

The filamentous ascomycete A. nidulans produces two major siderophores: it excretes triacetylfusarinine C to capture iron and contains ferricrocin intracellularly. In this study we report the characterization of two siderophore biosynthetic genes, sidA encoding l-ornithine N(5)-monooxygenase and sidC encoding a non-ribosomal peptide synthetase respectively. Disruption of sidC eliminated synthesis of ferricrocin and deletion of sidA completely blocked siderophore biosynthesis. Siderophore-deficient strains were unable to grow, unless the growth medium was supplemented with siderophores, suggesting that the siderophore system is the major iron assimilatory system of A. nidulans during both iron depleted and iron-replete conditions. Partial restoration of the growth of siderophore-deficient mutants by high concentrations of Fe(2+) (but not Fe(3+)) indicates the presence of an additional ferrous transport system and the absence of an efficient reductive iron assmilatory system. Uptake studies demonstrated that TAFC-bound iron is transferred to cellular ferricrocin whereas ferricrocin is stored after uptake. The siderophore-deficient mutant was able to synthesize ferricrocin from triacetylfusarinine C. Ferricrocin-deficiency caused an increased intracellular labile iron pool, upregulation of antioxidative enzymes and elevated sensitivity to the redox cycler paraquat. This indicates that the lack of this cellular iron storage compound causes oxidative stress. Moreover, ferricrocin biosynthesis was found to be crucial for efficient conidiation.