Ferric reductase genes involved in high‐affinity iron uptake are differentially regulated in yeast and hyphae of Candida albicans

Abstract

The pathogenic yeast Candida albicans possesses a reductive iron uptake system which is active in iron-restricted conditions. The sequestration of iron by this mechanism initially requires the reduction of free iron to the soluble ferrous form, which is catalysed by ferric reductase proteins. Reduced iron is then taken up into the cell by a complex of a multicopper oxidase protein and an iron transport protein. Multicopper oxidase proteins require copper to function and so reductive iron and copper uptake are inextricably linked. It has previously been established that Fre10 is the major cell surface ferric reductase in C. albicans and that transcription of FRE10 is regulated in response to iron levels. We demonstrate here that Fre10 is also a cupric reductase and that Fre7 also makes a significant contribution to cell surface ferric and cupric reductase activity. It is also shown, for the first time, that transcription of FRE10 and FRE7 is lower in hyphae compared to yeast and that this leads to a corresponding decrease in cell surface ferric, but not cupric, reductase activity. This demonstrates that the regulation of two virulence determinants, the reductive iron uptake system and the morphological form of C. albicans, are linked.

Role of Ferric Reductases in Iron Acquisition and Virulence in the Fungal Pathogen Cryptococcus neoformans

“…In contrast, addition of copper restored melanin production in the fre4⌬ mutant, as seen in other melanin-negative or -deficient mutants of C. neoformans: e.g., mel1, mel5, mel7, vph1, and clc-a mutations (47)(48)(49)(50). Ferric reductases are also known to reduce copper at the cell surface, as reported in S. cerevisiae and C. albicans (18,22,34). It is therefore possible that Fre4 in C. neoformans is a multifunctional enzyme with the ability to reduce copper, and thus its loss resulted in a defect in copper uptake and subsequently affected laccase activity.…”

“…However, the molecular genetics of the enzymatic reduction of ferric to ferrous iron have not been investigated for this pathogen. Ferric reductases are present in all fungal species, with variable numbers predicted across fungal genomes; these enzymes have been studied extensively in S. cerevisiae and, to some extent, in C. albicans, S. pombe, and A. fumigatus (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)42). In our study, we identified eight candidate ferric reductase (FRE) genes in C. neoformans, characterized their expression and contributions to reductase activity, and investigated their roles in virulence factor production.…”

“…Given that some ferric reductases also have cupric reductase activity (18,22,34), we also tested each fre⌬ deletion strain for this activity. For cupric ion reduction, the WT and fre⌬ deletion strains were grown in defined LCM without or with 100 M CuSO 4 .…”

“…The transcription of FRE1 and FRE7 is also induced by copper depletion (18,20,21). In C. albicans, Fre10 and Fre7 are the major ferric reductases, and they also have cupric reductase activity (12,15,22). The genes encoding these and other reductases respond to a wide range of environmental conditions, including iron depletion (23,24).…”

Comparison of the hemolytic activity between C. albicans and non-albicans Candida species

“…This enzyme degrades host erythrocytes to release iron for use in growth and metabolism of these fungi in cases of systemic infections. 16 We also compared the ability of C. albicans and non-albicans Candida species isolated from the oral cavity of HIV-positive patients to produce hemolysin. C. albicans is the predominant species associated with mucosal and systemic fungal infections.…”