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 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.

Iron is an essential trace element for almost all organisms. However, an excess of this metal within cells can be deleterious on account of catalysis of cell-damaging hydroxyl radicals. Therefore, the concentration of iron within cells is tightly regulated and the primary control occurs by regulating its uptake. Under conditions of low iron availability, most fungi mobilize extracellular iron by excretion of low-molecular-mass ferric iron chelators, termed siderophores. Due to the potential impact of iron metabolism on fungal pathogenicity, a better insight into siderophore-mediated iron uptake is needed. In Aspergillus nidulans, siderophore biosynthesis and uptake are negatively regulated by the GATA-type transcription factor SREA. Hence, genes involved in siderophore biosynthesis and uptake are characterized by transcriptional induction under iron limitation in wild-type strain and de-repression in an sreA-deletion strain under conditions of sufficient iron supply. Such genes have been searched for using different strategies, e.g. differential mRNA display and expression analysis of candidate genes from various A. nidulans sequence databases. The identified genes presumably encode enzymes needed for siderophore biosynthesis, and transporters involved in siderophore uptake and/or excretion. The functional characterization of these genes will help to unravel the pathways involved in siderophore biosynthesis and uptake.

In a search for iron-regulated proteins of Aspergillus nidulans and Aspergillus fumigatus a 16-kDa protein was identified which is about 5-fold upregulated during iron starvation in both species and which can be approximately 500-fold enriched by simple one-step chromatography on Amberlite XAD-16 resin. N-terminal protein sequence analysis and cloning of the respective A. nidulans cDNA identified this protein as a Cu/Znsuperoxide dismutase (SODA). Northern analysis revealed that upregulation of sodA expression occurs at the level of transcript accumulation. This seems to be a specific low iron response and not a general starvation answer since sodA transcript levels do not respond to carbon or nitrogen starvation. In contrast, copper depletion leads to transcriptional downregulation of sodA. Furthermore, sodA expression was found still to be subject to iron regulation in an A. nidulans mutant lacking SREA, a regulator of iron homeostasis, indicating that sodA expression is regulated by an SREA-independent mechanism. The data presented suggest that SODA plays a protective role under iron deplete conditions. ß