Environmental responses and the control of iron homeostasis in fungal systems

Organisms need to actively respond to changes in the environment and, particularly under diverse conditions, they ought to ensure access to nutrients. Among micronutrients, iron is a key component of several enzymes and participates in a variety of cellular processes. Iron deprivation therefore poses a serious challenge to both unicellular and multicellular individuals. Nevertheless, excess of this metal is toxic, compromising cell function and viability. Thus, it is not surprising that organisms have evolved sophisticated mechanisms to tightly regulate cellular iron levels. In the last decade, major advances have been achieved in the molecular understanding of how fungi respond to changing iron concentrations. Moreover, this metal has been recognized as an important element impacting pathogenic and saprophytic fungal lifestyles. An interconnected transcriptional negative feedback loop has been described as central in the regulation of genes encoding for iron uptake and utilization components in fungi. The observation that light, oxygen, or nutrients can also impact the expression of some of these elements suggests that additional environmental inputs-besides iron levels-may as well modulate the machinery underpinning iron homeostasis. This review highlights some of the latest findings associated with iron-regulated processes in fungi and revisits the increasing transcriptional complexity involved in the control of this metal homeostasis. In addition, we present the first in silico evidence of genes encoding for putative ferritins in zygomycetes and chytrids, as well as other ferritin-like sequences widespread among fungi, which raises interesting questions relative to iron storage in this particular group of organisms.