The ability to acquire iron in vivo is essential for most microbial pathogens. Here we show that Aspergillus fumigatus does not have specific mechanisms for the utilization of host iron sources. However, it does have functional siderophore-assisted iron mobilization and reductive iron assimilation systems, both of which are induced upon iron deprivation. Abrogation of reductive iron assimilation, by inactivation of the high affinity iron permease (FtrA), has no effect on virulence in a murine model of invasive aspergillosis. In striking contrast, A. fumigatus l-ornithine-N 5-monooxygenase (SidA), which catalyses the first committed step of hydroxamate-type siderophore biosynthesis, is absolutely essential for virulence. Thus, A. fumigatus SidA is an essential virulence attribute. Combined with the absence of a sidA ortholog—and the fungal siderophore system in general—in mammals, these data demonstrate that the siderophore biosynthetic pathway represents a promising new target for the development of antifungal therapies.

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 ΔsidA, 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.

SummaryAspergillus 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 SreAdependent 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 ironmediated 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.

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.

Aspergillus fumigatus excretes the fusarinine-type siderophore desferri-triacetylfusarinine C (DF-TafC) to mobilize iron. DF-TafC is a cyclic peptide consisting of three N 5 -cis-anhydromevalonyl-N 5 -hydroxy-N 2 -acetyl-L-ornithine residues linked by ester bonds; these linkages are in contrast to peptide linkages found for ferrichrome-type siderophores. Subsequent to the binding of iron and uptake, triacetylfusarinine C (TafC) is hydrolyzed, the cleavage products are excreted, and the iron is transferred to the metabolism or to the intracellular siderophore desferri-ferricrocin (DF-FC) for iron storage. Here we report the identification and characterization of the TafC esterase EstB, the first eukaryotic siderophore-degrading enzyme to be characterized at the molecular level. The encoding gene, estB, was found to be located in an iron-regulated gene cluster, indicating a role in iron metabolism. Deletion of estB in A. fumigatus eliminated TafC esterase activity of cellular extracts and caused increased intracellular accumulation of TafC and TafC hydrolysis products in vivo. Escherichia coli-expressed EstB displayed specific TafC esterase activity but did not hydrolyze fusarinine C, which has the same core structure as TafC but lacks three N 2 -acetyl residues. Localization of EstB via enhanced green fluorescent protein tagging suggested that TafC hydrolysis takes place in the cytoplasm. EstB abrogation reduced the intracellular transfer rate of iron from TafC to DF-FC and delayed iron sensing. Furthermore, EstB deficiency caused a decreased radial growth rate under iron-depleted but not iron-replete conditions. Taken together, these data suggest that EstB-mediated TafC hydrolysis optimizes but is not essential for TafC-mediated iron uptake in A. fumigatus.