ABSTRACT
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.
Subject(s)
Aspergillus nidulans/metabolism , Bacterial Outer Membrane Proteins/metabolism , Carrier Proteins/metabolism , Enterobactin/metabolism , Receptors, Cell Surface/metabolism , Siderophores/metabolism , Amino Acid Sequence , Aspergillus nidulans/classification , Aspergillus nidulans/genetics , Bacterial Outer Membrane Proteins/genetics , Carrier Proteins/genetics , Cloning, Molecular , Consensus Sequence , DNA Primers , Exons , Introns , Iron/metabolism , Membrane Transport Proteins/metabolism , Molecular Sequence Data , Phylogeny , Plasmids , Polymerase Chain Reaction , Receptors, Cell Surface/genetics , Recombinant Proteins/metabolism , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae Proteins/metabolism , Sequence AlignmentABSTRACT
In the filamentous fungus Aspergillus nidulans, iron homeostasis is regulated at the transcriptional level by the negative-acting GATA factor SREA. In this study the expression of a putative heme-containing metalloreductase-encoding gene, freA, was found to be upregulated by iron limitation independently of SREA, demonstrating the existence of an iron-regulatory mechanism which does not involve SREA. In contrast to freA, various other genes encoding proteins in need of iron-containing cofactors-acoA, lysF, and cycA-were downregulated in response to iron depletion. Remarkably, SREA deficiency led to increased expression of acoA, lysF, and cycA under iron-replete growth conditions.
