Sulfur, Phosphorus, and Iron Metabolism

This chapter focuses on the varied fungal responses to limitation for iron, phosphorus, and sulfur. Common threads include the versatility and resourcefulness of fungi in the acquisition of these nutrients. Further, transcriptional upregulation of high-affinity transporters is a repeated theme that allows for the scavenging under low and growth limiting nutrient element levels. Storage and macronutrient homeostasis, including regulatory aspects, are also briefly discussed. While Aspergillus and Neurospora are considered overall in greater depth in the chapter, details of the metabolism of iron, phosphorus, and sulfur from other filamentous fungal species are included throughout. Release of cysteine and methionine from exogenous protein can also serve as a source of sulfur for many fungi. The uptake of sulfate into fungal cells is carried out by sulfate permeases, providing a primary sulfur source for the subsequent assimilation pathway. Most studies on sulfur transport in fungi have focused on sulfate permeases in Saccharomyces cerevisiae, Neurospora crassa, and A. nidulans. Among the filamentous fungi, N. crassa and A. nidulans have served as primary model systems for sulfur metabolism. Siderophore-mediated nonreductive iron uptake represents the second type of high-affinity system for iron acquisition. Siderophores, which can be classified as catecholates, phenolates, carboxylates, hydroxylates, and mixed types, are synthesized under iron limiting conditions to chelate ferric (Fe3+) iron.