Genetic evidence that ferric reductase is required for iron uptake in Saccharomyces cerevisiae

Simultaneous Enhancement of iron Deficiency Tolerance and Iron Accumulation in Rice by Combining the Knockdown of OsHRZ Ubiquitin Ligases with the Introduction of Engineered Ferric-chelate Reductase

Iron is an essential micronutrient for living organisms, but its solubility is extremely low under alkaline conditions. Plants often suffer from iron deficiency chlorosis in calcareous soils, which consist of approximately 30% of the world's cultivated area, severely limiting plant productivity. Iron deficiency anemia is also a widespread problem in humans, especially in Asian and African people who take up iron mainly from staple foods containing low iron concentrations. Transgenic manipulation of genes involved in plant iron uptake, translocation, and storage has made improvements in enhancing iron deficiency tolerance or iron accumulation in edible parts, but these two properties have been characterized separately. We previously produced transgenic rice lines, with concomitant improvement of iron deficiency tolerance and grain iron accumulation by knocking-down OsHRZ ubiquitin ligases, which negatively regulate iron deficiency response and iron accumulation in rice. In the present report, we aimed to further improve the iron deficiency tolerance and grain iron accumulation of OsHRZ knockdown rice by the simultaneous introduction of the engineered ferric-chelate reductase gene Refre1/372 under the control of the OsIRT1 promoter for further enhancement of iron uptake. We obtained several transgenic rice lines with repressed OsHRZ expression and induced Refre1/372 expression. These lines showed a variable degree of iron deficiency tolerance in calcareous soils, with increased iron accumulation in brown seeds under both iron-deficient and iron-sufficient soil cultures. Selected OsHRZ knockdown plus Refre1/372 lines showed similar or better growth compared with that of singly introduced OsHRZ knockdown or Refre1/372 lines in calcareous soils under both non-submerged and submerged conditions. After submerged calcareous soil cultivation, these OsHRZ knockdown plus Refre1/372 lines accumulated 2.5-4.3 times and 17-23 times more iron concentrations than that of non-transformants in brown rice and straw, respectively, which was comparable or superior to a single OsHRZ knockdown line. Our results indicate that the combined introduction of OsHRZ knockdown and OsIRT1 promoter-Refre1/372 is highly effective in further improving the iron deficiency tolerance without compromising the iron accumulation of the OsHRZ knockdown effects.

A quick journey into the diversity of iron uptake strategies in photosynthetic organisms

Iron (Fe) is involved in multiple processes that contribute to the maintenance of the cellular homeostasis of all living beings. In photosynthetic organisms, Fe is notably required for photosynthesis. Although iron is generally abundant in the environment, it is frequently poorly bioavailable. This review focuses on the molecular strategies that photosynthetic organisms have evolved to optimize iron acquisition, using Arabidopsis thaliana, rice (Oryza sativa), and some unicellular algae as models. Non-graminaceous plants, including Arabidopsis, take up iron from the soil by an acidification-reduction-transport process (strategy I) requiring specific proteins that were recently shown to associate in a dedicated complex. On the other hand, graminaceous plants, such as rice, use the so-called strategy II to acquire iron, which relies on the uptake of Fe³⁺ chelated by phytosiderophores that are secreted by the plant into the rhizosphere. However, apart these main strategies, accessory mechanisms contribute to robust iron uptake in both Arabidopsis and rice. Unicellular algae combine reductive and non-reductive mechanisms for iron uptake and present important specificities compared to land plants. Since the majority of the molecular actors required for iron acquisition in algae are not conserved in land plants, questions arise about the evolution of the Fe uptake processes upon land colonization.

Copper metabolism in Saccharomyces cerevisiae: an update

Copper is an essential element in all forms of life. It acts as a cofactor of some enzymes and is involved in forming proper protein conformations. However, excess copper ions in cells are detrimental as they can generate free radicals or disrupt protein structures. Therefore, all life forms have evolved conserved and exquisite copper metabolic systems to maintain copper homeostasis. The yeast Saccharomyces cerevisiae has been widely used to investigate copper metabolism as it is convenient for this purpose. In this review, we summarize the mechanism of copper metabolism in Saccharomyces cerevisiae according to the latest literature. In brief, bioavailable copper ions are incorporated into yeast cells mainly via the high-affinity transporters Ctr1 and Ctr3. Then, intracellular Cu⁺ ions are delivered to different organelles or cuproproteins by different chaperones, including Ccs1, Atx1, and Cox17. Excess copper ions bind to glutathione (GSH), metallothioneins, and copper complexes are sequestered into vacuoles to avoid toxicity. Copper-sensing transcription factors Ace1 and Mac1 regulate the expression of genes involved in copper detoxification and uptake/mobilization in response to changes in intracellular copper levels. Though numerous recent breakthroughs in understanding yeast's copper metabolism have been achieved, some issues remain unresolved. Completely elucidating the mechanism of copper metabolism in yeast helps decode the corresponding system in humans and understand how copper-related diseases develop.

A Vacuolar Membrane Ferric-Chelate Reductase, OsFRO1, Alleviates Fe Toxicity in Rice (Oryza sativa L.)

Ferric reductase oxidase (FRO), the enzyme that reduced ferric iron [Fe (III)] into ferrous iron [Fe (II)], is known to play important roles in Fe absorption and homeostasis in plants that utilize a strategy I mechanism to obtain iron. Rice can use both strategies I and II for Fe uptake depending on the growth conditions. FRO is encoded by two genes in rice genome. Amino acid sequence alignment shows that OsFRO1 contains all necessary predicted motifs for a functional FRO enzyme, whereas OsFRO2 lacks a complete transmembrane domain at the N-terminal. Transient expression of OsFRO1: GFP protein fusion revealed that OsFRO1 is localized to the vacuolar membrane in rice protoplast. OsFRO1 is primarily expressed in leaves and transcript abundance was decreased under excess Fe conditions. Transgenic plants overexpressing OsFRO1 were more sensitive to Fe toxicity, in contrast RNA interference lines showed more tolerance to Fe excess stress. Furthermore, RNAi lines showed decreased Fe concentrations compared to wild type plants under Fe excess condition. Together these data show that OsFRO1 is involved in reducing ferric Fe into ferrous Fe in the vacuole, and makes the vacuolar stored Fe available to the cytoplasm through Fe (II) or chelated Fe (II) transporters. Under Fe excess condition, the downregulation of OsFRO1 in the RNAi plants reduced the amount of Fe (II) available for cytoplasm, to alleviate Fe excess toxicity. This indicates that OsFRO1 plays an important role to maintain Fe homeostasis between the cytoplasm and vacuole in rice.

The Transcription Factor VdHapX Controls Iron Homeostasis and Is Crucial for Virulence in the Vascular Pathogen Verticillium dahliae

Iron homeostasis is essential for full virulence and viability in many pathogenic fungi. Here, we showed that the bZip transcription factor VdHapX functions as a key regulator of iron homeostasis for adaptation to iron-depleted and iron-excess conditions and is required for full virulence in the vascular wilt fungus, Verticillium dahliae Deletion of VdHapX impaired mycelial growth and conidiation under both iron starvation and iron sufficiency. Furthermore, disruption of VdHapX led to decreased formation of the long-lived survival structures of V. dahliae, known as microsclerotia. Expression of genes involved in iron utilization pathways and siderophore biosynthesis was misregulated in the ΔVdHapX strain under the iron-depleted condition. Additionally, the ΔVdHapX strain exhibited increased sensitivity to high iron concentrations and H2O2, indicating that VdHapX also contributes to iron or H2O2 detoxification. The ΔVdHapX strain showed a strong reduction in virulence on smoke tree seedlings (Cotinus coggygria) and was delayed in its ability to penetrate plant epidermal tissue.IMPORTANCE This study demonstrated that VdHapX is a conserved protein that mediates adaptation to iron starvation and excesses, affects microsclerotium formation, and is crucial for virulence of V. dahliae.

Structural basis for promotion of duodenal iron absorption by enteric ferric reductase with ascorbate

Dietary iron absorption is regulated by duodenal cytochrome b (Dcytb), an integral membrane protein that catalyzes reduction of nonheme Fe³⁺ by electron transfer from ascorbate across the membrane. This step is essential to enable iron uptake by the divalent metal transporter. Here we report the crystallographic structures of human Dcytb and its complex with ascorbate and Zn²⁺. Each monomer of the homodimeric protein possesses cytoplasmic and apical heme groups, as well as cytoplasmic and apical ascorbate-binding sites located adjacent to each heme. Zn²⁺ coordinates to two hydroxyl groups of the apical ascorbate and to a histidine residue. Biochemical analysis indicates that Fe³⁺ competes with Zn²⁺ for this binding site. These results provide a structural basis for the mechanism by which Fe³⁺ uptake is promoted by reducing agents and should facilitate structure-based development of improved agents for absorption of orally administered iron.

Uptake of iron by Kluyveromyces marxianus DSM 5422 cultivated in a whey-based medium

The ability of Kluyveromyces marxianus for converting lactose into ethyl acetate offers a chance for the economical reuse of whey. Iron plays a significant role in this process as ester synthesis requires a low intracellular iron content, x_Fe . The iron content in turn is decreased by growth due to cell expansion and increased by iron uptake. Thus, the iron-uptake rate, ψ, is important for the considered process. Iron uptake by K. marxianus DSM 5422 was studied in aerobic cultivation on a whey-borne medium with varied initial iron content, in part combined with a feed of iron under intensive growth conditions. A possible precipitation of iron that would pretend iron uptake was verified not to have occurred. Regularly measured dissolved iron concentrations, C_Fe,L , allowed the x_Fe and ψ parameters to be obtained by model-based iron balancing. The achieved data were used for establishing a ψ(C_Fe,L , x_Fe ) model. Mathematical simulations based on this iron-uptake model reproduced the performed cultivation processes. The created iron-uptake model allows for a future predictive system to be developed for the optimization of biotechnological ester production.

FreB is involved in the ferric metabolism and multiple pathogenicity-related traits of Verticillium dahliae

Ferric reductases are integral membrane proteins involved in the reduction of environmental ferric iron into the biologically available ferrous iron. In the most overwhelming phytopathogenic fungus, Verticillium dahliae, these ferric reductase are not studied in details. In this study we explored the role of FreB gene (VDAG_06616) in the ferric reduction and virulence of V. dahliae by generating the knockout mutants (ΔFreB) and complementary strains (ΔFreB-C) using protoplast transformation. When cultured on media supplemented with FeSO₄, FeCl₃ and no iron, ΔFreB exhibited significantly reduced growth and spore production especially on media with no iron. Transmembrane ferric reductase activity of ΔFreB was decreased up to 50% than wild type strains (Vd-wt). The activity was fully restored in ΔFreB-C. Meanwhile, the expression levels of other related genes (Frect-4, Frect-5, Frect-6 and Met) were obviously increased in ΔFreB. Compared with the Vd-wt and ΔFreB-C, ΔFreB-1 and ΔFreB-2 were impaired in colony diameter and spore number on different carbon sources (starch, sucrose, galactose and xylose). ΔFreB-1 and ΔFreB-2 were also highly sensitive to oxidative stress as revealed by the plate diffusion assay when 100 µM H₂O₂ was applied to the fungal culture. When Nicotiana benthamiana plants were inoculated, ΔFreB exhibited less disease symptoms than Vd-wt and ΔFreB-C. In conclusion, the present findings not only indicate that FreB mediates the ferric metabolism and is required for the full virulence in V. dahliae, but would also accelerate future investigation to uncover the pathogenic mechanism of this fungus.

Genetic variation and expression changes associated with molybdate resistance from a glutathione producing wine strain of Saccharomyces cerevisiae

Glutathione (GSH) production during wine fermentation is a desirable trait as it can limit must and wine oxidation and protect various aromatic compounds. UMCC 2581 is a Saccharomyces cerevisiae wine strain with enhanced GSH content at the end of wine fermentation. This strain was previously derived by selection for molybdate resistance following a sexual cycle of UMCC 855 using an evolution-based strategy. In this study, we examined genetic and gene expression changes associated with the derivation of UMCC 2581. For genetic analysis we sporulated the diploid UMCC 855 parental strain and found four phenotype classes of segregants related to molybdate resistance, demonstrating the presence of segregating variation from the parental strain. Using bulk segregant analysis we mapped molybdate traits to two loci. By sequencing both the parental and evolved strain genomes we identified candidate mutations within the two regions as well as an extra copy of chromosome 1 in UMCC 2581. Combining the mapped loci with gene expression profiles of the evolved and parental strains we identified a number of candidate genes with genetic and/or gene expression changes that could underlie molybdate resistance and increased GSH levels. Our results provide insight into the genetic basis of GSH production relevant to winemaking and highlight the value of enhancing wine strains using existing variation present in wine strains.

The Fungal Pathogen Candida glabrata Does Not Depend on Surface Ferric Reductases for Iron Acquisition

Iron acquisition is a crucial virulence determinant for many bacteria and fungi, including the opportunistic fungal pathogens Candida albicans and C. glabrata. While the diverse strategies used by C. albicans for obtaining iron from the host are well-described, much less is known about the acquisition of this micronutrient from host sources by C. glabrata – a distant relative of C. albicans with closer evolutionary ties to Saccharomyces cerevisiae, which nonetheless causes severe clinical symptoms in humans. Here we show that C. glabrata is much more restricted than C. albicans in using host iron sources, lacking, for example, the ability to grow on transferrin and hemin/hemoglobin. Instead, C. glabrata is able to use ferritin and non-protein-bound iron (FeCl₃) as iron sources in a pH-dependent manner. As in other fungal pathogens, iron-dependent growth requires the reductive high affinity (HA) iron uptake system. Typically highly conserved, this uptake mechanism normally relies on initial ferric reduction by cell-surface ferric reductases. The C. glabrata genome contains only three such putative ferric reductases, which were found to be dispensable for iron-dependent growth. In addition and in contrast to C. albicans and S. cerevisiae, we also detected no surface ferric reductase activity in C. glabrata. Instead, extracellular ferric reduction was found in this and the two other fungal species, which was largely dependent on an excreted low-molecular weight, non-protein ferric reductant. We therefore propose an iron acquisition strategy of C. glabrata which differs from other pathogenic fungi, such as C. albicans, in that it depends on a limited set of host iron sources and that it lacks the need for surface ferric reductases. Extracellular ferric reduction by a secreted molecule possibly compensates for the loss of surface ferric reductase activity in the HA iron uptake system.

Heme acquisition in the parasitic filarial nematode Brugia malayi

Nematodes lack a heme biosynthetic pathway and must acquire heme from exogenous sources. Given the indispensable role of heme, this auxotrophy may be exploited to develop drugs that interfere with heme uptake in parasites. Although multiple heme-responsive genes (HRGs) have been characterized within the free-living nematode Caenorhabditis elegans, we have undertaken the first study of heme transport in Brugia malayi, a causative agent of lymphatic filariasis. Through functional assays in yeast, as well as heme analog, RNAi, and transcriptomic experiments, we have shown that the heme transporter B. malayi HRG-1 (BmHRG-1) is indeed functional in B. malayi In addition, BmHRG-1 localizes both to the endocytic compartments and cell membrane when expressed in yeast cells. Transcriptomic sequencing revealed that BmHRG-1, BmHRG-2, and BmMRP-5 (all orthologs of HRGs in C. elegans) are down-regulated in heme-treated B. malayi, as compared to non-heme-treated control worms. Likely because of short gene lengths, multiple exons, other HRGs in B. malayi (BmHRG-3-6) remain unidentified. Although the precise mechanisms of heme homeostasis in a nematode with the ability to acquire heme remains unknown, this study clearly demonstrates that the filarial nematode B. malayi is capable of transporting exogenous heme.-Luck, A. N., Yuan, X., Voronin, D., Slatko, B. E., Hamza, I., Foster, J. M. Heme acquisition in the parasitic filarial nematode Brugia malayi.

Identification of ferrichrome- and ferrioxamine B-mediated iron uptake by Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic fungal pathogen for immunocompromised patients, and genes involved in siderophore metabolism have been identified as virulence factors. Recently, we identified the membrane transporters sit1 and sit2, which are putative virulence factors of A. fumigatus; sit1 and sit2 are homologous to yeast Sit1, and sit1 and sit2 gene expression was up-regulated after iron depletion. When expressed heterologously in Saccharomyces cerevisiae, sit1 and sit2 were localized to the plasma membrane; sit1 efficiently complemented ferrichrome (FC) and ferrioxamine B (FOB) uptake in yeast cells, whereas sit2 complemented only FC uptake. Deletion of sit1 resulted in a decrease in FOB and FC uptake, and deletion of sit2 resulted in a decrease in FC uptake in A. fumigatus. It is of interest that a sit1 and sit2 double-deletion mutant resulted in a synergistic decrease in FC uptake activity. Both sit1 and sit2 were localized to the plasma membrane in A. fumigatus. The expression levels of the sit1 and sit2 genes were dependent on hapX under low-but not high-iron conditions. Furthermore, mirB, and sidA gene expression was up-regulated and sreA expression down-regulated when sit1 and sit2 were deleted. Although sit1 and sit2 failed to affect mouse survival rate, these genes affected conidial killing activity. Taken together, our results suggest that sit1 and sit2 are siderophore transporters and putative virulence factors localized to the plasma membrane.

Physical interaction between Sit1 and Aft1 upregulates FOB uptake activity by inhibiting protein degradation of Sit1 in Saccharomyces cerevisiae

Previously, we reported that Aft1 regulates Sit1 by modulating the ubiquitination of Sit1 in Saccharomyces cerevisiae. Here, we report the function of the physical interaction between Sit1 and Aft1 in ferrioxamine B (FOB) uptake. The interaction between Sit1 and Aft1 induced protein localization of Sit1 to the plasma membrane, and more Sit1 was detected in the plasma membrane when Sit1 and Aft1 were coexpressed compared with Sit1 expression alone. The MSN5-deletion mutant, which failed to translocate Aft1 to the cytosolic compartment, showed lower FOB uptake activity than the wild type. However, higher free iron uptake activity was detected in the MSN5-deletion mutant. Furthermore, the strain transformed with AFT1-1(up) plasmid, which failed to regulate Aft1 via iron concentration and accumulated Aft1 in the nucleus, showed lower FOB uptake activity. The Aft1 Y179F mutant, which contained a tyrosine residue that was changed to phenylalanine, failed to interact physically with Sit1 and showed more degradation of the Sit1 and, ultimately, lower FOB uptake activity. Additionally, we found that MG132 and PMSF, which are inhibitors of proteasomes and serine proteases, respectively, increased the Sit1 protein level. Taken together, these results suggest that the protein-protein interaction between Sit1 and Aft1 is an important factor in the FOB uptake activity of Sit1.

Intracellular siderophore but not extracellular siderophore is required for full virulence in Metarhizium robertsii

Efficient iron acquisition mechanisms are fundamental for microbial survival in the environment and for pathogen virulence within their hosts. M. robertsii produces two known iron-binding natural products: metachelins, which are used to scavenge extracellular iron, and ferricrocin, which is strictly intracellular. To study the contribution of siderophore-mediated iron uptake and storage to M. robertsii fitness, we generated null mutants for each siderophore synthase gene (mrsidD and mrsidC, respectively), as well as for the iron uptake transcriptional repressor mrsreA. All of these mutants showed impaired germination speed, differential sensitivity to hydrogen peroxide, and differential ability to overcome iron chelation on growth-limiting iron concentrations. RT-qPCR data supported regulation of mrsreA, mrsidC, and mrsidD by supplied iron in vitro and during growth within the insect host, Spodoptera exigua. We also observed strong upregulation of the insect iron-binding proteins, transferrins, during infection. Insect bioassays revealed that ferricrocin is required for full virulence against S. exigua; neither the loss of metachelin production nor the deletion of the transcription factor mrsreA significantly affected M. robertsii virulence.

Fob1 and Fob2 Proteins Are Virulence Determinants of Rhizopus oryzae via Facilitating Iron Uptake from Ferrioxamine

Dialysis patients with chronic renal failure receiving deferoxamine for treating iron overload are uniquely predisposed for mucormycosis, which is most often caused by Rhizopus oryzae. Although the deferoxamine siderophore is not secreted by Mucorales, previous studies established that Rhizopus species utilize iron from ferrioxamine (iron-rich form of deferoxamine). Here we determined that the CBS domain proteins of Fob1 and Fob2 act as receptors on the cell surface of R. oryzae during iron uptake from ferrioxamine. Fob1 and Fob2 cell surface expression was induced in the presence of ferrioxamine and bound radiolabeled ferrioxamine. A R. oryzae strain with targeted reduced Fob1/Fob2 expression was impaired for iron uptake, germinating, and growing on medium with ferrioxamine as the sole source of iron. This strain also exhibited reduced virulence in a deferoxamine-treated, but not the diabetic ketoacidotic (DKA), mouse model of mucormycosis. The mechanism by which R. oryzae obtains iron from ferrioxamine involves the reductase/permease uptake system since the growth on ferrioxamine supplemented medium is associated with elevated reductase activity and the use of the ferrous chelator bathophenanthroline disulfonate abrogates iron uptake and growth on medium supplemented with ferrioxamine as a sole source of iron. Finally, R. oryzae mutants with reduced copies of the high affinity iron permease (FTR1) or with decreased FTR1 expression had an impaired iron uptake from ferrioxamine in vitro and reduced virulence in the deferoxamine-treated mouse model of mucormycosis. These two receptors appear to be conserved in Mucorales, and can be the subject of future novel therapy to maintain the use of deferoxamine for treating iron-overload.

Deferoxamine is an iron-chelating agent often used to treat patients with acute iron poisoning, such as seen in dialysis patients with chronic renal failure. These patients are uniquely predisposed to a deadly fungal infection, called mucormycosis, because deferoxamine supplies iron that supports growth of fungi causing this infection. Apart from the important basic knowledge in delineating iron uptake mechanisms in cells, understanding how organisms causing mucormycosis obtain iron from ferrioxamine (deferoxamine bound with iron) is likely to develop strategies to treat mucormycosis infections in patients treated with deferoxamine. In this study we identified two cell surface receptors that bind ferrioxamine and facilitate iron uptake in Rhizopus oryzae, the most causative fungus of mucormycosis. These receptors are required for full virulence of R. oryzae in mice treated with deferoxamine. From genetic and biochemical studies it appears that the fungus binds ferrioxamine via these two receptors then liberates iron through a chemical modification step prior to transporting into the fungal cell without the internalization of deferoxamine.

Iron uptake mechanism in the chrysophyte microalga Dinobryon

The mechanism of iron uptake in the chrysophyte microalga Dinobryon was studied. Previous studies have shown that iron is the dominant limiting elements for growth of Dinobryon in the Eshkol reservoir in northern Israel, which control its burst of bloom. It is demonstrated that Dinobryon has a light-stimulated ferrireductase activity, which is sensitive to the photosynthetic electron transport inhibitor DCMU and to the uncoupler CCCP. Iron uptake is also light-dependent, is inhibited by DCMU and by CCCP and also by the ferrous iron chelator BPDS. These results suggest that ferric iron reduction by ferrireductase is involved in iron uptake in Dinobryon and that photosynthesis provides the major reducing power to energize iron acquisition. Iron deprivation does not enhance but rather inhibits iron uptake contrary to observations in other algae.

Control of metazoan heme homeostasis by a conserved multidrug resistance protein

Several lines of evidence predict that specific pathways must exist in metazoans for the escorted movement of heme, an essential but cytotoxic iron-containing organic ring, within and between cells and tissues, but these pathways remain obscure. In Caenorhabditis elegans, embryonic development is inextricably dependent on both maternally derived heme and environmentally acquired heme. Here, we show that the multidrug resistance protein MRP-5/ABCC5 likely acts as a heme exporter, and targeted depletion of mrp-5 in the intestine causes embryonic lethality. Transient knockdown of mrp5 in zebrafish leads to morphological defects and failure to hemoglobinize red blood cells. MRP5 resides on the plasma membrane and endosomal compartments and regulates export of cytosolic heme. Together, our genetic studies in worms, yeast, zebrafish, and mammalian cells identify a conserved, physiological role for a multidrug resistance protein in regulating systemic heme homeostasis. We envision other MRP family members may play similar unanticipated physiological roles in animal development.

Brain iron homeostasis: from molecular mechanisms to clinical significance and therapeutic opportunities

Iron has emerged as a significant cause of neurotoxicity in several neurodegenerative conditions, including Alzheimer's disease (AD), Parkinson's disease (PD), sporadic Creutzfeldt-Jakob disease (sCJD), and others. In some cases, the underlying cause of iron mis-metabolism is known, while in others, our understanding is, at best, incomplete. Recent evidence implicating key proteins involved in the pathogenesis of AD, PD, and sCJD in cellular iron metabolism suggests that imbalance of brain iron homeostasis associated with these disorders is a direct consequence of disease pathogenesis. A complete understanding of the molecular events leading to this phenotype is lacking partly because of the complex regulation of iron homeostasis within the brain. Since systemic organs and the brain share several iron regulatory mechanisms and iron-modulating proteins, dysfunction of a specific pathway or selective absence of iron-modulating protein(s) in systemic organs has provided important insights into the maintenance of iron homeostasis within the brain. Here, we review recent information on the regulation of iron uptake and utilization in systemic organs and within the complex environment of the brain, with particular emphasis on the underlying mechanisms leading to brain iron mis-metabolism in specific neurodegenerative conditions. Mouse models that have been instrumental in understanding systemic and brain disorders associated with iron mis-metabolism are also described, followed by current therapeutic strategies which are aimed at restoring brain iron homeostasis in different neurodegenerative conditions. We conclude by highlighting important gaps in our understanding of brain iron metabolism and mis-metabolism, particularly in the context of neurodegenerative disorders.

Yeast flavohemoglobin protects against nitrosative stress and controls ferric reductase activity

The role of Saccharomyces cerevisiae flavohemoglobin (Yhb1) is controversial and far from understood. This study compares the effects of nitrosative and oxidative challenge on the yeast mutant lacking the YHB1 gene. Growth of the mutant was impaired by nitrosoglutathione and peroxynitrite, whereas increased sensitivity to reactive oxygen species was not observed. Increased levels of intracellular NO(*) after incubation with NO(*) donors were found in the mutants cells as compared to the wild-type cells. Deletion of the YHB1 gene was found to augment the reduction of Fe(3+) by yeast cells which suggests that flavohemoglobin participates in regulation of the activity of plasma membrane ferric reductase(s).

Yeast protective response to arsenate involves the repression of the high affinity iron uptake system

Arsenic is a double-edge sword. On the one hand it is powerful carcinogen and on the other it is used therapeutically to treat acute promyelocytic leukemia. Here we report that arsenic activates the iron responsive transcription factor, Aft1, as a consequence of a defective high-affinity iron uptake mediated by Fet3 and Ftr1, whose mRNAs are drastically decreased upon arsenic exposure. Moreover, arsenic causes the internalization and degradation of Fet3. Most importantly, fet3ftr1 mutant exhibits increased arsenic resistance and decreased arsenic accumulation over the wild-type suggesting that Fet3 plays a role in arsenic toxicity. Finally we provide data suggesting that arsenic also disrupts iron uptake in mammals and the link between Fet3, arsenic and iron, can be relevant to clinical applications.

Insight into the evolution of the iron oxidation pathways

Iron is a ubiquitous element in the universe. Ferrous iron (Fe(II)) was abundant in the primordial ocean until the oxygenation of the Earth's atmosphere led to its widespread oxidation and precipitation. This change of iron bioavailability likely put selective pressure on the evolution of life. This element is essential to most extant life forms and is an important cofactor in many redox-active proteins involved in a number of vital pathways. In addition, iron plays a central role in many environments as an energy source for some microorganisms. This review is focused on Fe(II) oxidation. The fact that the ability to oxidize Fe(II) is widely distributed in Bacteria and Archaea and in a number of quite different biotopes suggests that the dissimilatory Fe(II) oxidation is an ancient energy metabolism. Based on what is known today about Fe(II) oxidation pathways, we propose that they arose independently more than once in evolution and evolved convergently. The iron paleochemistry, the phylogeny, the physiology of the iron oxidizers, and the nature of the cofactors of the redox proteins involved in these pathways suggest a possible scenario for the timescale in which each type of Fe(II) oxidation pathways evolved. The nitrate dependent anoxic iron oxidizers are likely the most ancient iron oxidizers. We suggest that the phototrophic anoxic iron oxidizers arose in surface waters after the Archaea/Bacteria-split but before the Great Oxidation Event. The neutrophilic oxic iron oxidizers possibly appeared in microaerobic marine environments prior to the Great Oxidation Event while the acidophilic ones emerged likely after the advent of atmospheric O(2). This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

The ins and outs of algal metal transport

Metal transporters are a central component in the interaction of algae with their environment. They represent the first line of defense to cellular perturbations in metal concentration, and by analyzing algal metal transporter repertoires, we gain insight into a fundamental aspect of algal biology. The ability of individual algae to thrive in environments with unique geochemistry, compared to non-algal species commonly used as reference organisms for metal homeostasis, provides an opportunity to broaden our understanding of biological metal requirements, preferences and trafficking. Chlamydomonas reinhardtii is the best developed reference organism for the study of algal biology, especially with respect to metal metabolism; however, the diversity of algal niches necessitates a comparative genomic analysis of all sequenced algal genomes. A comparison between known and putative proteins in animals, plants, fungi and algae using protein similarity networks has revealed the presence of novel metal metabolism components in Chlamydomonas including new iron and copper transporters. This analysis also supports the concept that, in terms of metal metabolism, algae from similar niches are more related to one another than to algae from the same phylogenetic clade. This article is part of a Special Issue entitled: Cell Biology of Metals.

Iron and neurodegeneration: from cellular homeostasis to disease

Accumulation of iron (Fe) is often detected in the brains of people suffering from neurodegenerative diseases. High Fe concentrations have been consistently observed in Parkinson's, Alzheimer's, and Huntington's diseases; however, it is not clear whether this Fe contributes to the progression of these diseases. Other conditions, such as Friedreich's ataxia or neuroferritinopathy are associated with genetic factors that cause Fe misregulation. Consequently, excessive intracellular Fe increases oxidative stress, which leads to neuronal dysfunction and death. The characterization of the mechanisms involved in the misregulation of Fe in the brain is crucial to understand the pathology of the neurodegenerative disorders and develop new therapeutic strategies. Saccharomyces cerevisiae, as the best understood eukaryotic organism, has already begun to play a role in the neurological disorders; thus it could perhaps become a valuable tool also to study the metalloneurobiology.

Non-transferrin bound iron: a key role in iron overload and iron toxicity

BACKGROUND

Besides transferrin iron, which represents the normal form of circulating iron, non-transferrin bound iron (NTBI) has been identified in the plasma of patients with various pathological conditions in which transferrin saturation is significantly elevated.

SCOPE OF THE REVIEW

To show that: i) NTBI is present not only during chronic iron overload disorders (hemochromatosis, transfusional iron overload) but also in miscellaneous diseases which are not primarily iron overloaded conditions; ii) this iron species represents a potentially toxic iron form due to its high propensity to induce reactive oxygen species and is responsible for cellular damage not only at the plasma membrane level but also towards different intracellular organelles; iii) the NTBI concept may be expanded to include intracytosolic iron forms which are not linked to ferritin, the major storage protein which exerts, at the cellular level, the same type of protective effect towards the intracellular environment as transferrin in the plasma.

MAJOR CONCLUSIONS

Plasma NTBI and especially labile plasma iron determinations represent a new important biological tool since elimination of this toxic iron species is a major therapeutic goal.

GENERAL SIGNIFICANCE

The NTBI approach represents an important mechanistic concept for explaining cellular iron excess and toxicity and provides new important biochemical diagnostic tools. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Increased iron supplied through Fet3p results in replicative life span extension of Saccharomyces cerevisiae under conditions requiring respiratory metabolism

We have previously shown that copper supplementation extends the replicative life span of Saccharomyces cerevisiae when grown under conditions forcing cells to respire. We now show that copper's effect on life span is through Fet3p, a copper containing enzyme responsible for high affinity transport of iron into yeast cells. Life span extensions can also be obtained by supplementing the growth medium with 1mM ferric chloride. Extension by high iron levels is still dependent on the presence of Fet3p. Life span extension by iron or copper requires growth on media containing glycerol as the sole carbon source, which forces yeast to respire. Yeast grown on glucose containing media supplemented with iron show no extension of life span. The iron associated with cells grown in media supplemented with copper or iron is 1.4-1.8 times that of cells grown without copper or iron supplementation. As with copper supplementation, iron supplementation partially rescues the life span of superoxide dismutase mutants. Cells grown with copper supplementation display decreased production of superoxide as measured by dihydroethidium staining.

Biosynthetic and iron metabolism is regulated by thiol proteome changes dependent on glutaredoxin-2 and mitochondrial peroxiredoxin-1 in Saccharomyces cerevisiae

Redoxins are involved in maintenance of thiol redox homeostasis, but their exact sites of action are only partly known. We have applied a combined redox proteomics and transcriptomics experimental strategy to discover specific functions of two interacting redoxins: dually localized glutaredoxin 2 (Grx2p) and mitochondrial peroxiredoxin 1 (Prx1p). We have identified 139 proteins showing differential postranslational thiol redox modifications when the cells do not express Grx2p, Prx1p, or both and have mapped the precise cysteines involved in each case. Some of these modifications constitute functional switches that affect metabolic and signaling pathways as the primary effect, leading to gene transcription remodeling as the secondary adaptive effect as demonstrated by a parallel high throughput gene expression analysis. The results suggest that in the absence of Grx2p, the metabolic flow toward nucleotide and aromatic amino acid biosynthesis is slowed down by redox modification of the key enzymes Rpe1p (D-ribulose-5-phosphate 3-epimerase), Tkl1p (transketolase) and Aro4p (3-deoxy-D-arabino-heptulosonate-7-phosphate synthase). The glycolytic mainstream is then diverted toward carbohydrate storage by induction of trehalose and glycogen biosynthesis genes. Porphyrin biosynthesis may also be compromised by inactivation of the redox-sensitive cytosolic enzymes Hem12p (uroporphyrinogen decarboxylase) and Sam1p (S-adenosyl methionine synthetase) and a battery of respiratory genes sensitive to low heme levels are induced. Genes of the Aft1p-dependent iron regulon were induced specifically in the absence of Prx1p despite optimal mitochondrial Fe-S biogenesis, suggesting dysfunction of the mitochondria to the cytosol signaling pathway. Strikingly, requirement of Grx2p for these events places dithiolic Grx2 in the framework of iron metabolism.

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.

Accelerating glycolytic flux of Torulopsis glabrata CCTCC M202019 at high oxidoreduction potential created using potassium ferricyanide

This study aimed to increase the glycolytic flux of the multivitamin auxotrophic yeast Torulopsis glabrata by redirecting NADH oxidation from oxidative phosphorylation to membrane-bound ferric reductase. We added potassium ferricyanide as electron acceptor to T. glabrata culture broth at 20% dissolved oxygen (DO) concentration, which resulted in: (1) decreases in the NADH content, NADH/NAD(+) ratio, and ATP level of 45.3%, 60.3%, and 15.2%, respectively; (2) high activities of the key glycolytic enzymes hexokinase, phosphofructokinase, and pyruvate kinase, as well as high expression levels of the genes encoding these enzymes; and (3) increases in the specific glucose consumption rate and pyruvate yield of T. glabrata was by 45.5% and 23.1%, respectively. Our results showed that membrane-bound ferric reductase offers an alternative and efficient NADH oxidation pathway at lower DO concentration, which increases the glycolytic flux of T. glabrata.

Effects of liming on potential oxalate secretion and iron chelation of beech ectomycorrhizal root tips

Liming is used to counteract forest decline induced by soil acidification. It consists of Ca and Mg input to forest soil and not only restores tree mineral nutrition but also modifies the availability of nutrients in soil. Ectomycorrhizal (ECM) fungi are involved in mineral nutrient uptake by trees and can recover them through dissolution of mineral surface. Oxalate and siderophore secretion are considered as the main agents of mineral weathering by ECMs. Here, we studied the effects of liming on the potential oxalate secretion and iron complexation by individual beech ECM root tips. Results show that freshly excised Lactarius subdulcis root tips from limed plots presented a high potential oxalate exudation of 177 μM tip(-1) h(-1). As this ECM species distribution is very dense, it is likely that, in the field, oxalate concentrations in the vicinity of its clusters could be very high. This points out that not only extraradical mycelium but also ECM root tips of certain species can contribute significantly to mineral weathering. Nonmetric multidimensional scaling (NMDS) separated potential oxalate production by ECM root tips in limed and untreated plots, and this activity was mainly driven by L. subdulcis ECMs, but NMDS on potential activity of iron mobilization by ECM root tips did not show a difference between limed and untreated plots. As the mean oxalate secretion did not significantly correlated with the mean iron mobilization by ECM morphotype, we conclude that iron complexation was due to either other organic acids or to siderophores.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

A novel function of Aft1 in regulating ferrioxamine B uptake: Aft1 modulates Arn3 ubiquitination in Saccharomyces cerevisiae

Aft1 is a transcriptional activator in Saccharomyces cerevisiae that responds to iron availability and regulates the expression of genes in the iron regulon, such as FET3, FTR1 and the ARN family. Using a two-hybrid screen, we found that Aft1 physically interacts with the FOB (ferrioxamine B) transporter Arn3. This interaction modulates the ability of Arn3 to take up FOB. The interaction between Arn3 and Aft1 was confirmed by beta-galactosidase, co-immunoprecipitation and SPR (surface plasmon resonance) assays. Truncated Aft1 had a stronger interaction with Arn3 and caused a higher FOB-uptake activity than full-length Aft1. Interestingly, only full-length Aft1 induced the correct localization of Arn3 in response to FOB. Furthermore, we found Aft1 affected Arn3 ubiquitination. These results suggest that Aft1 interacts with Arn3 and may regulate the ubiquitination of Arn3 in the cytosolic compartment.

Down-regulation of a manganese transporter in the face of metal toxicity

The yeast Smf1p Nramp manganese transporter is posttranslationally regulated by environmental manganese. Smf1p is stabilized at the cell surface with manganese starvation, but is largely degraded in the vacuole with physiological manganese through a mechanism involving the Rsp5p adaptor complex Bsd2p/Tre1p/Tre2p. We now describe an additional level of Smf1p regulation that occurs with toxicity from manganese, but not other essential metals. This regulation is largely Smf1p-specific. As with physiological manganese, toxic manganese triggers vacuolar degradation of Smf1p by trafficking through the multivesicular body. However, regulation by toxic manganese does not involve Bsd2p/Tre1p/Tre2p. Toxic manganese triggers both endocytosis of cell surface Smf1p and vacuolar targeting of intracellular Smf1p through the exocytic pathway. Notably, the kinetics of vacuolar targeting for Smf1p are relatively slow with toxic manganese and require prolonged exposures to the metal. Down-regulation of Smf1p by toxic manganese does not require transport activity of Smf1p, whereas such transport activity is needed for Smf1p regulation by manganese starvation. Furthermore, the responses to manganese starvation and manganese toxicity involve separate cellular compartments. We provide evidence that manganese starvation is sensed within the lumen of the secretory pathway, whereas manganese toxicity is sensed within an extra-Golgi/cytosolic compartment of the cell.

Ferric reductase A is essential for effective iron acquisition in Paracoccus denitrificans

Based on N-terminal sequences obtained from the purified cytoplasmic ferric reductases FerA and FerB, their corresponding genes were identified in the published genome sequence of Paracoccus denitrificans Pd1222. The ferA and ferB genes were cloned and individually inactivated by insertion of a kanamycin resistance marker, and then returned to P. denitrificans for exchange with their wild-type copies. The resulting ferA and ferB mutant strains showed normal growth in brain heart infusion broth. Unlike the ferB mutant, the strain lacking FerA did not grow on succinate minimal medium with ferric 2,3-dihydroxybenzoate as the iron source, and grew only poorly in the presence of ferric sulfate, chloride, citrate, NTA, EDTA and EGTA. Moreover, the ferA mutant strain was unable to produce catechols, which are normally detectable in supernatants from iron-limited wild-type cultures. Complementation of the ferA mutation using a derivative of the conjugative broad-host-range plasmid pEG400 that contained the whole ferA gene and its putative promoter region largely restored the wild-type phenotype. Partial, though significant, restoration could also be achieved with 1 mM chorismate added to the growth medium. The purified FerA protein acted as an NADH : FMN oxidoreductase and catalysed the FMN-mediated reductive release of iron from the ferric complex of parabactin, the major catecholate siderophore of P. denitrificans. The deduced amino acid sequence of the FerA protein has closest similarity to flavin reductases that form part of the flavin-dependent two-component monooxygenases. Taken together, our results demonstrate an essential role of reduced flavins in the utilization of exogenous ferric iron. These flavins not only provide the electrons for Fe(III) reduction but most probably also affect the rate of siderophore production.

The role of Dcytb in iron metabolism: an update

Dcytb (duodenal cytochrome b) is an iron-regulated ferric reductase highly expressed in duodenal enterocytes. Its location and strong regulation by iron has indicated it plays an important role in iron absorption. Expression of Dcytb in cells (Caco-2 and MDCK) was found to increase both ferric reductase activity and stimulate uptake of 59Fe. An additional increase in cupric reductase activity was found in MDCK (Madin–Darby canine kidney) cells expressing Dcytb. Expression and purification of Dcytb in insect cells reveals that Dcytb is a di-haem protein and that the haems are reducible by ascorbate, indicating that ascorbate is the likely intracelluar electron donor. Studies underway in Dcytb-knockout mice reveal that Dcytb is the only iron-regulated ferric reductase in the duodenal mucosa and that loss of Dcytb affects iron absorption.

Opposing activities of the Snx3-retromer complex and ESCRT proteins mediate regulated cargo sorting at a common endosome

Endocytosed proteins are either delivered to the lysosome to be degraded or are exported from the endosomal system and delivered to other organelles. Sorting of the Saccharomyces cerevisiae reductive iron transporter, composed of the Fet3 and Ftr1 proteins, in the endosomal system is regulated by available iron; in iron-starved cells, Fet3-Ftr1 is sorted by Snx3/Grd19 and retromer into a recycling pathway that delivers it back to the plasma membrane, but when starved cells are exposed to iron, Fet3-Ftr1 is targeted to the lysosome-like vacuole and is degraded. We report that iron-induced endocytosis of Fet3-Ftr1 is independent of Fet3-Ftr1 ubiquitylation, and after endocytosis, degradation of Fet3-Ftr1 is mediated by the multivesicular body (MVB) sorting pathway. In mutant cells lacking any component of the ESCRT protein-dependent MVB sorting machinery, the Rsp5 ubiquitin ligase, or in wild-type cells expressing Fet3-Ftr1 lacking cytosolic lysyl ubiquitin acceptor sites, Fet3-Ftr1 is constitutively sorted into the recycling pathway independent of iron status. In the presence and absence of iron, Fet3-Ftr1 transits an endosomal compartment where a subunit of the MVB sorting receptor (Vps27), Snx3/Grd19, and retromer proteins colocalize. We propose that this endosome is where Rsp5 ubiquitylates Fet3-Ftr1 and where the recycling and degradative pathways diverge.

Genes differentially expressed in conidia and hyphae of Aspergillus fumigatus upon exposure to human neutrophils

BACKGROUND

Aspergillus fumigatus is the most common etiologic agent of invasive aspergillosis in immunocompromised patients. Several studies have addressed the mechanism involved in host defense but only few have investigated the pathogen's response to attack by the host cells. To our knowledge, this is the first study that investigates the genes differentially expressed in conidia vs hyphae of A. fumigatus in response to neutrophils from healthy donors as well as from those with chronic granulomatous disease (CGD) which are defective in the production of reactive oxygen species.

METHODOLOGY/PRINCIPAL FINDINGS

Transcriptional profiles of conidia and hyphae exposed to neutrophils, either from normal donors or from CGD patients, were obtained by using the genome-wide microarray. Upon exposure to either normal or CGD neutrophils, 244 genes were up-regulated in conidia but not in hyphae. Several of these genes are involved in the degradation of fatty acids, peroxisome function and the glyoxylate cycle which suggests that conidia exposed to neutrophils reprogram their metabolism to adjust to the host environment. In addition, the mRNA levels of four genes encoding proteins putatively involved in iron/copper assimilation were found to be higher in conidia and hyphae exposed to normal neutrophils compared to those exposed to CGD neutrophils. Deletants in several of the differentially expressed genes showed phenotypes related to the proposed functions, i.e. deletants of genes involved in fatty acid catabolism showed defective growth on fatty acids and the deletants of iron/copper assimilation showed higher sensitivity to the oxidative agent menadione. None of these deletants, however, showed reduced resistance to neutrophil attack.

CONCLUSION

This work reveals the complex response of the fungus to leukocytes, one of the major host factors involved in antifungal defense, and identifies fungal genes that may be involved in establishing or prolonging infections in humans.

Duodenal cytochrome B expression stimulates iron uptake by human intestinal epithelial cells

Duodenal cytochrome B (Dcytb) is localized principally in the apical membrane of the enterocyte. It is thought to act as a ferric reductase that furnishes Fe(II), the specific and selective iron species transported by divalent metal transporter 1 (DMT1) in the duodenal enterocytes. Expression of both genes is strongly iron regulated and is thought to be required for transcellular iron trafficking in concert in response to physiological requirements. We tested this hypothesis by expressing Dcytb in Caco-2 cells, a human cell line model often used to mimic intestinal enterocytes. Iron uptake (59Fe) was significantly higher in Dcytb-transfected Caco-2 cells than in cells transfected with empty vector as a control. Fe(III) reductase activity of Dcytb was measured with ferrozine, a strong chelator of Fe(II) species. Cells expressing Dcytb exhibited enhanced ferric reductase activity as well as increased 59Fe uptake compared with cells transfected with empty vector as a control. Ferrozine blocked iron uptake and preincubation of cells with dehydroascorbate (to increase cellular ascorbate levels) stimulated iron uptake. Cotransfection of Dcytb and DMT1 resulted in an additive increase in iron uptake by the cells. The results confirm Dcytb can act as a ferric reductase that stimulates iron uptake in Caco-2 cells.

Siderophores in fungal physiology and virulence

Maintaining the appropriate balance of iron between deficiency and toxicity requires fine-tuned control of systems for iron uptake and storage. Both among fungal species and within a single species, different systems for acquisition, storage, and regulation of iron are present. Here we discuss the most recent findings on the mechanisms involved in maintaining iron homeostasis with a focus on siderophores, low-molecular-mass iron chelators, employed for iron uptake and storage. Recently siderophores have been found to be crucial for pathogenicity of animal, as well as plant-pathogenic fungi and for maintenance of plant-fungal symbioses.

Partial conservation of functions between eukaryotic frataxin and the Escherichia coli frataxin homolog CyaY

Frataxin is a mitochondrial protein structurally conserved from bacteria to humans. Eukaryotic frataxins are known to be involved in the maintenance of mitochondrial iron balance via roles in iron delivery and iron detoxification. The prokaryotic frataxin homolog, CyaY, has been shown to bind and donate iron for the assembly of [2Fe-2S] clusters in vitro. However, in contrast to the severe phenotypes associated with the partial or complete loss of frataxin in humans and other eukaryotes, deletion of the cyaY gene does not cause any obvious alteration of iron balance in bacterial cells, an effect that probably reflects functional redundancy between CyaY and other bacterial proteins. To study CyaY function in a nonredundant setting, we have expressed a mitochondria-targeted form of CyaY in a Saccharomyces cerevisiae strain depleted of the endogenous yeast frataxin protein (yfh1Delta). We show that in this strain CyaY complements to a large extent the loss of iron-sulfur cluster enzyme activities and heme synthesis, and thereby maintains a nearly normal respiratory growth. In addition, CyaY effectively protects yfh1Delta from oxidative damage during treatment with hydrogen peroxide but is less efficient in detoxifying excess labile iron during aerobic growth.