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.