Sre1, an iron-modulated GATA DNA-binding protein of iron-uptake genes in the fungal pathogen Histoplasma capsulatum

The pathogenic fungus Histoplasma capsulatum requires iron for its survival during macrophage infection. Because iron is toxic at high levels, iron acquisition in pathogenic organisms, including H. capsulatum, is a highly regulated process. In response to excess iron, H. capsulatum represses transcription of genes involved in iron uptake. We report here that SRE1, a gene encoding a GATA-type protein, bound to promoter sequences of genes involved in siderophore biosynthesis. Sre1 had sequence similarity to the fungal negative regulators of siderophore biosynthesis. Expression of SRE1 was reduced under iron-starving conditions, underscoring its role as a negative regulator of genes involved in iron uptake. Sre1p specifically bound DNA containing the 5'-(G/A)ATC(T/A)GATAA-3' sequence, and that binding was both iron- and zinc-dependent. Metal analysis indicated that a substoichiometric amount of iron, predominately Fe3+, was bound to the purified protein. About 0.5-1 equiv of Fe3+ per monomer was necessary for full DNA-binding activity. Mutations in the conserved cysteine residues in the cysteine-rich region led to a decrease in bound iron. The loss of iron led to a similar to 2.5-fold decrease in DNA-binding affinity, indicating that iron was directly involved in SRE1 regulation of iron-uptake genes.