Yeasts take up iron by three main mechanisms. In the reductive uptake mechanism, specialized flavo-hemoproteins (Fre) dissociate extracellular ferric complexes by reduction involving trans-plasma membrane electron transfer. The resulting free iron is then imported by a high-affinity permease system (Ftr), coupled to a copper-dependent oxidase (Fet), which channels iron through the plasma membrane. As a consequence, iron uptake by this mechanism is dependent on the availability of copper. In the siderophore-mediated mechanism, siderophores excreted by the cells or produced by other bacterial or fungal species are taken up without prior dissociation, via specific, copper-independent high-affinity receptors. The iron is then dissociated from the siderophores intracellularly, probably by reduction. In the heme uptake mechanism, free heme or heme bound to hemoglobin is taken up as such, probably by endocytosis. Iron is released intracellularly after hydrolysis of the porphyrin ring catalyzed by heme oxygenase. Within the cell, iron is stored in vacuoles or in siderophores.

Iron can be mobilized from vacuoles by a reductive mechanism homologous to that found at the plasma membrane. Regulation of iron uptake and iron use are mediated by transcriptional regulators acting either as activators in iron-deficient conditions or as repressors in iron-rich conditions, according to the yeast species; these regulators thus adjust the iron uptake flux to the cell's requirements. In the baker's yeast, Saccharomyces cerevisiae, a post-transcriptional mechanism is active under low iron conditions, involving the degradation of RNAs encoding inessential iron-utilizing proteins. Other fungi have mechanisms serving a similar purpose at the transcriptional level. Studies in S. cerevisiae show that mitochondria are central to regulating cellular iron homeostasis, through the synthesis of iron-sulfur clusters.

Further reading: Iron Uptake and Homeostasis in Microorganisms

Labels: fungal, fungi, Iron acquisition mechanisms, Iron transporters, Iron uptake in Yeasts, Iron uptake systems, Iron-homeostasis, Iron-uptake, mycology, Siderophores, yeast, Yeasts