Regulatory cross-talk between sulphur and iron metabolism.

<p>(A) Fungal cultures were shifted after prolonged growth in medium containing methionine as sole source of sulphur [Met (22 h)] to medium depleted for this amino acid but containing sulphate, and samples for RNA preparation were taken after the indicated time points. Northern hybridisation expression analysis of several genes directly related with iron metabolism was carried out. Although there is sufficient iron in the medium (100 µM), after eight hours of incubation in media containing sulphate (constituting sulphur starving conditions for the mutant) increased transcription of genes typically expressed under iron-depleted conditions (<i>mirB</i>: siderophore transporter; <i>hapX:</i> transcriptional activator of the iron regulon; <i>sidA</i>: L-ornithine monooxygenase; <i>amcA</i>: mitochondrial ornithine exporter) is observed in the <i>metRΔ</i> strain as well as decreased expression of genes whose products participate in iron consuming processes (<i>cycA</i>: cytochrome C; <i>acoA</i>: aconitase; <i>isa1</i>: mitochondrial cluster biosynthetic protein). (B) Relative quantity of ferricrocin (FC) in the respective fungal mycelia. FC levels are already elevated before the shift, and they increase nearly fivefold under sulphur starvation conditions. (C) The wild-type strain is able to grow in the presence of 10 mM of iron, a substantially high concentration that already provokes toxic effects. When sufficient methionine (5 mM) is present, the <i>metRΔ</i> mutant behaves as the wild-type and the reconstituted strain. However, under sulphur starvation, the mutant is not able grow on 0.5 mM of iron or higher concentrations. This hypersensitivity might be a result of the lower expression of <i>cccA</i> gene (encoding a vacuolar iron transporter which participates in iron detoxification) in the <i>metRΔ</i> mutant, especially under sulphur starvation conditions.