Frataxin Is Localized to Both the Chloroplast and Mitochondrion and Is Involved in Chloroplast Fe-S Protein Function in Arabidopsis

Characterization of Arabidopsis thaliana FRATAXIN HOMOLOG in heme catabolism

Frataxin plays vital roles in various iron related processes. Arabidopsis thaliana FRATAXIN HOMOLOG (AtFH) is the first identified plant frataxin and has been found to regulate the last step of heme biosynthesis. Here, we report the involvement of AtFH in heme catabolism by regulating the activity of heme oxygenase. AtFH forms a homodimer, and its crystal structure shows the dimeric interactions. A structural comparison with known frataxin structures suggests the iron binding sites, and the site for heme oxygenase activity is possibly located in a region containing Glu78. The results indicate a previously uncharacterized role of plant frataxin in heme catabolism.

Molecular investigation of how drought stress affects chlorophyll metabolism and photosynthesis in leaves of C3 and C4 plant species: A transcriptome meta-analysis

Drought stress has a significant impact on photosynthesis in plants, leading to reduced photosynthesis rates and affecting plant growth and yield. Understanding the effects of drought stress on photosynthetic pathways, particularly in C3 and C4 plants, is crucial for maximizing agricultural productivity and maintaining food security. In this study, we analyzed RNA-seq data from leaves of common wheat (Triticum aestivum) and sorghum (Sorghum bicolor), as representatives of C3 and C4, using a meta-analysis approach to investigate the photosynthesis-related genes involved in the response to drought stress. We identified specific genes and components of the photosynthesis pathway that are affected by drought stress. The findings suggest that wheat and sorghum respond differently to drought stress, with sorghum showing a more effective defense system against photoinhibition and damage to photosystems. On the other hand, it seems that in wheat, in order to deal with oxidative stress, the expression of homologous genes of C4 enzyme and genes involved in heme and siroheme synthesis pathway has increased under stress. This is probably due to the higher photoinhibition in C3 photosynthetic system compared to C4. Furthermore, drought stress affected chlorophyll biosynthesis and degradation pathways in both wheat and sorghum, but compared with sorghum, drought stress had a greater inhibitory effect on chlorophyll biosynthesis in wheat, which indicates the difference in their ability to cope with photoinhibition.

Metal Homeostasis in Land Plants: A Perpetual Balancing Act Beyond the Fulfilment of Metalloproteome Cofactor Demands

One of life's decisive innovations was to harness the catalytic power of metals for cellular chemistry. With life's expansion, global atmospheric and biogeochemical cycles underwent dramatic changes. Although initially harmful, they permitted the evolution of multicellularity and the colonization of land. In land plants as primary producers, metal homeostasis faces heightened demands, in part because soil is a challenging environment for nutrient balancing. To avoid both nutrient metal limitation and metal toxicity, plants must maintain the homeostasis of metals within tighter limits than the homeostasis of other minerals. This review describes the present model of protein metalation and sketches its transfer from unicellular organisms to land plants as complex multicellular organisms. The inseparable connection between metal and redox homeostasis increasingly draws our attention to more general regulatory roles of metals. Mineral co-option, the use of nutrient or other metals for functions other than nutrition, is an emerging concept beyond that of nutritional immunity.

Genome-Wide Identification, Characterization and Expression Profiling of Potato (Solanum tuberosum) Frataxin (FH) Gene

Frataxin (FH) plays a crucial role in the biogenesis of mitochondria and the regulation of iron in the cells of various organisms. However, there has been very little research on FH in plants. In this study, the potato FH gene (StFH) was identified and characterized using a genome-wide approach, and its sequence was compared to those of FH genes from Arabidopsis, rice, and maize. The FH genes were found to have a lineage-specific distribution and were more conserved in monocots than in dicots. While multiple copies of FH genes have been reported in some species, including plants, only one isoform of FH was found in potato. The expression of StFH in leaves and roots was analyzed under two different abiotic stress conditions, and the results showed that StFH was upregulated more in leaves and that its expression levels increased with the severity of the stress. This is the first study to examine the expression of an FH gene under abiotic stress conditions.

Advances in Iron Retrograde Signaling Mechanisms and Uptake Regulation in Photosynthetic Organisms

Iron (Fe) is an essential metal for the growth and development of different organisms, including plants and algae. This metal participates in different biological processes, among which are cellular respiration and photosynthesis. Fe is found associated with heme groups and as part of inorganic Fe-S groups as cofactors of numerous cellular proteins. Although Fe is abundant in soils, it is often not bioavailable due to soil pH. For this reason, photosynthetic organisms have developed different strategies for the uptake, the sensing of Fe intracellular levels but also different mechanisms that maintain and regulate adequate concentrations of this metal in response to physiological needs. This work focuses on discussing recent advances in the characterization of the mechanisms of Fe homeostasis and Fe retrograde signaling in photosynthetic organisms.

Structural and Functional Characterization of CreFH1, the Frataxin Homolog from Chlamydomonas reinhardtii

Frataxin plays a key role in cellular iron homeostasis of different organisms. It has been implicated in iron storage, detoxification, delivery for Fe-S cluster assembly and heme biosynthesis. However, its specific role in iron metabolism remains unclear, especially in photosynthetic organisms. To gain insight into the role and properties of frataxin in algae, we identified the gene CreFH1, which codes for the frataxin homolog from Chlamydomonas reinhardtii. We performed the cloning, expression and biochemical characterization of CreFH1. This protein has a predicted mitochondrial transit peptide and a significant structural similarity to other members of the frataxin family. In addition, CreFH1 was able to form a dimer in vitro, and this effect was increased by the addition of Cu²⁺ and also attenuated the Fenton reaction in the presence of a mixture of Fe²⁺ and H₂O₂. Bacterial cells with overexpression of CreFH1 showed increased growth in the presence of different metals, such as Fe, Cu, Zn and Ni and H₂O₂. Thus, results indicated that CreFH1 is a functional protein that shows some distinctive features compared to its more well-known counterparts, and would play an important role in response to oxidative stress in C. reinhardtii.

Iron in leaves: chemical forms, signalling, and in-cell distribution

Iron (Fe) is an essential transition metal. Based on its redox-active nature under biological conditions, various Fe compounds serve as cofactors in redox enzymes. In plants, the photosynthetic machinery has the highest demand for Fe. In consequence, the delivery and incorporation of Fe into cofactors of the photosynthetic apparatus is the focus of Fe metabolism in leaves. Disturbance of foliar Fe homeostasis leads to impaired biosynthesis of chlorophylls and composition of the photosynthetic machinery. Nevertheless, mitochondrial function also has a significant demand for Fe. The proper incorporation of Fe into proteins and cofactors as well as a balanced intracellular Fe status in leaf cells require the ability to sense Fe, but may also rely on indirect signals that report on the physiological processes connected to Fe homeostasis. Although multiple pieces of information have been gained on Fe signalling in roots, the regulation of Fe status in leaves has not yet been clarified in detail. In this review, we give an overview on current knowledge of foliar Fe homeostasis, from the chemical forms to the allocation and sensing of Fe in leaves.

Heme oxygenase-nitric oxide crosstalk-mediated iron homeostasis in plants under oxidative stress

Plant growth under abiotic stress conditions significantly enhances intracellular generation of reactive oxygen species (ROS). Oxidative status of plant cells is directly affected by the modulation of iron homeostasis. Among mammals and plants, heme oxygenase-1 (HO-1) is a well-known antioxidant enzyme. It catalyzes oxygenation of heme, thereby producing Fe²⁺, CO and biliverdin as byproducts. The antioxidant potential of HO-1 is primarily due to its catalytic reaction byproducts. Biliverdin and bilirubin possess conjugated π-electrons which escalate the ability of these biomolecules to scavenge free radicals. CO also enhances the ROS scavenging ability of plants cells by upregulating catalase and peroxidase activity. Enhanced expression of HO-1 in plants under oxidative stress accompanies sequestration of iron in specialized iron storage proteins localized in plastids and mitochondria, namely ferritin for Fe³⁺ storage and frataxin for storage of Fe-S clusters, respectively. Nitric oxide (NO) crosstalks with HO-1 at multiple levels, more so in plants under oxidative stress, in order to maintain intracellular iron status. Formation of dinitrosyl-iron complexes (DNICs) significantly prevents Fenton reaction during oxidative stress. DNICs also release NO upon dissociation in target cells over long distance in plants. They also function as antioxidants against superoxide anions and lipidic free radicals. A number of NO-modulated transcription factors also facilitate iron homeostasis in plant cells. Plants facing oxidative stress exhibit modulation of lateral root formation by HO-1 through NO and auxin-dependent pathways. The present review provides an in-depth analysis of the structure-function relationship of HO-1 in plants and mammals, correlating them with their adaptive mechanisms of survival under stress.

Occurrence, Evolution and Specificities of Iron-Sulfur Proteins and Maturation Factors in Chloroplasts from Algae

Iron-containing proteins, including iron-sulfur (Fe-S) proteins, are essential for numerous electron transfer and metabolic reactions. They are present in most subcellular compartments. In plastids, in addition to sustaining the linear and cyclic photosynthetic electron transfer chains, Fe-S proteins participate in carbon, nitrogen, and sulfur assimilation, tetrapyrrole and isoprenoid metabolism, and lipoic acid and thiamine synthesis. The synthesis of Fe-S clusters, their trafficking, and their insertion into chloroplastic proteins necessitate the so-called sulfur mobilization (SUF) protein machinery. In the first part, we describe the molecular mechanisms that allow Fe-S cluster synthesis and insertion into acceptor proteins by the SUF machinery and analyze the occurrence of the SUF components in microalgae, focusing in particular on the green alga Chlamydomonas reinhardtii. In the second part, we describe chloroplastic Fe-S protein-dependent pathways that are specific to Chlamydomonas or for which Chlamydomonas presents specificities compared to terrestrial plants, putting notable emphasis on the contribution of Fe-S proteins to chlorophyll synthesis in the dark and to the fermentative metabolism. The occurrence and evolutionary conservation of these enzymes and pathways have been analyzed in all supergroups of microalgae performing oxygenic photosynthesis.

PAP/SAL1 retrograde signaling pathway modulates iron deficiency response in alkaline soils

Iron (Fe) is an essential micronutrient for plants and is present abundantly in the Earth's crust. However, Fe bioavailability in alkaline soils is low due to the decreased solubility of the ferric ions. Previously, we have demonstrated the relationship between the PAP/SAL1 retrograde signaling pathway, the activity of Strategy I Fe uptake genes (FIT, FRO2, IRT1), and ethylene signaling. In this work, we have characterized mutant lines that are deficient in this retrograde signaling pathway and their ability to grow in alkaline soils. This adverse growth condition caused less impact on mutant plants, which showed less reduced rosette area, and higher carotenoid, chlorophyll and Fe content than wild-type plants. Several genes involved in the biosynthesis and excretion of secondary metabolites derived from the phenylpropanoid pathway, which improve Fe uptake, were elevated in mutant plants. Finally, we observed an increase in excreted fluorescent phenolic compounds in mutant lines compared to wild-type plants. In this way, PAP/SAL1 mutants showed alterations in the biosynthesis of metabolites that mobilize Fe, which ultimately improved these plants ability to grow in alkaline soils. Results agree with the existence of a link between the PAP/SAL1 retrograde signaling pathway and the regulation of Fe deficiency responses in Arabidopsis.

Fe-S Protein Synthesis in Green Algae Mitochondria

Iron and sulfur are two essential elements for all organisms. These elements form the Fe-S clusters that are present as cofactors in numerous proteins and protein complexes related to key processes in cells, such as respiration and photosynthesis, and participate in numerous enzymatic reactions. In photosynthetic organisms, the ISC and SUF Fe-S cluster synthesis pathways are located in organelles, mitochondria, and chloroplasts, respectively. There is also a third biosynthetic machinery in the cytosol (CIA) that is dependent on the mitochondria for its function. The genes and proteins that participate in these assembly pathways have been described mainly in bacteria, yeasts, humans, and recently in higher plants. However, little is known about the proteins that participate in these processes in algae. This review work is mainly focused on releasing the information on the existence of genes and proteins of green algae (chlorophytes) that could participate in the assembly process of Fe-S groups, especially in the mitochondrial ISC and CIA pathways.

Iron-Sulfur Cluster Protein NITROGEN FIXATION S-LIKE1 and Its Interactor FRATAXIN Function in Plant Immunity

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are present in all kingdoms of life as part of a large number of proteins involved in several cellular processes, including DNA replication and metabolism. In this work, we demonstrate an additional role for two Fe-S cluster genes in biotic stress responses in plants. Eleven Fe-S cluster genes, including the NITROGEN FIXATION S-LIKE1 (NFS1) and its interactor FRATAXIN (FH), when silenced in Nicotiana benthamiana, compromised nonhost resistance to Pseudomonas syringae pv. tomato T1. NbNFS1 expression was induced by pathogens and salicylic acid. Arabidopsis (Arabidopsis thaliana) atnfs and atfh mutants, with reduced AtNFS1 or AtFH gene expression, respectively, showed increased susceptibility to both host and nonhost pathogen infection. Arabidopsis AtNFS1 and AtFH overexpressor lines displayed decreased susceptibility to infection by host pathogen P syringae pv. tomato DC3000. The AtNFS1 overexpression line exhibited constitutive upregulation of several defense-related genes and enrichment of gene ontology terms related to immunity and salicylic acid responses. Our results demonstrate that NFS1 and its interactor FH are involved not only in nonhost resistance but also in basal resistance, suggesting a new role of the Fe-S cluster pathway in plant immunity.

Iron-Sulfur Cluster Complex Assembly in the Mitochondria of Arabidopsis thaliana

In plants, the cysteine desulfurase (AtNFS1) and frataxin (AtFH) are involved in the formation of Fe-S groups in mitochondria, specifically, in Fe and sulfur loading onto scaffold proteins, and the subsequent formation of the mature Fe-S cluster. We found that the small mitochondrial chaperone, AtISD11, and AtFH are positive regulators for AtNFS1 activity in Arabidopsis. Moreover, when the three proteins were incubated together, a stronger attenuation of the Fenton reaction was observed compared to that observed with AtFH alone. Using pull-down assays, we found that these three proteins physically interact, and sequence alignment and docking studies showed that several amino acid residues reported as critical for the interaction of their human homologous are conserved. Our results suggest that AtFH, AtNFS1 and AtISD11 form a multiprotein complex that could be involved in different stages of the iron-sulfur cluster (ISC) pathway in plant mitochondria.

Chloroplast Transition Metal Regulation for Efficient Photosynthesis

Plants require sunlight, water, CO₂, and essential nutrients to drive photosynthesis and fulfill their life cycle. The photosynthetic apparatus resides in chloroplasts and fundamentally relies on transition metals as catalysts and cofactors. Accordingly, chloroplasts are particularly rich in iron (Fe), manganese (Mn), and copper (Cu). Owing to their redox properties, those metals need to be carefully balanced within the cell. However, the regulation of transition metal homeostasis in chloroplasts is poorly understood. With the availability of the arabidopsis genome information and membrane protein databases, a wider catalogue for searching chloroplast metal transporters has considerably advanced the study of transition metal regulation. This review provides an updated overview of the chloroplast transition metal requirements and the transporters involved for efficient photosynthesis in higher plants.

The PAP/SAL1 retrograde signaling pathway is involved in iron homeostasis

There is a link between PAP/SAL retrograde pathway, ethylene signaling and Fe metabolism in Arabidopsis. Nuclear gene expression is regulated by a diversity of retrograde signals that travel from organelles to the nucleus in a lineal or classical model. One such signal molecule is 3'-phosphoadenisine-5'-phosphate (PAP) and it's in vivo levels are regulated by SAL1/FRY1, a phosphatase enzyme located in chloroplast and mitochondria. This metabolite inhibits the action of a group of exorribonucleases which participate in post-transcriptional gene expression regulation. Transcriptome analysis of Arabidopsis thaliana mutant plants in PAP-SAL1 pathway revealed that the ferritin genes AtFER1, AtFER3, and AtFER4 are up-regulated. In this work we studied Fe metabolism in three different mutants of the PAP/SAL1 retrograde pathway. Mutant plants showed increased Fe accumulation in roots, shoots and seeds when grown in Fe-sufficient condition, and a constitutive activation of the Strategy I Fe uptake genes. As a consequence, they grew more vigorously than wild type plants in Fe-deficient medium. However, when mutant plants grown in Fe-deficient conditions were sprayed with Fe in their leaves, they were unable to deactivate root Fe uptake. Ethylene synthesis inhibition revert the constitutive Fe uptake phenotype. We propose that there is a link between PAP/SAL pathway, ethylene signaling and Fe metabolism.

Essential and Detrimental - an Update on Intracellular Iron Trafficking and Homeostasis

Chloroplasts, mitochondria and vacuoles represent characteristic organelles of the plant cell, with a predominant function in cellular metabolism. Chloroplasts are the site of photosynthesis and therefore basic and essential for photoautotrophic growth of plants. Mitochondria produce energy during respiration and vacuoles act as internal waste and storage compartments. Moreover, chloroplasts and mitochondria are sites for the biosynthesis of various compounds of primary and secondary metabolism. For photosynthesis and energy generation, the internal membranes of chloroplasts and mitochondria are equipped with electron transport chains. To perform proper electron transfer and several biosynthetic functions, both organelles contain transition metals and here iron is by far the most abundant. Although iron is thus essential for plant growth and development, it becomes toxic when present in excess and/or in its free, ionic form. The harmful effect of the latter is caused by the generation of oxidative stress. As a consequence, iron transport and homeostasis have to be tightly controlled during plant growth and development. In addition to the corresponding transport and homeostasis proteins, the vacuole plays an important role as an intracellular iron storage and release compartment at certain developmental stages. In this review, we will summarize current knowledge on iron transport and homeostasis in chloroplasts, mitochondria and vacuoles. In addition, we aim to integrate the physiological impact of intracellular iron homeostasis on cellular and developmental processes.

The Role of Iron in Friedreich's Ataxia: Insights From Studies in Human Tissues and Cellular and Animal Models

Friedreich's ataxia (FRDA) is a rare early-onset degenerative disease that affects both the central and peripheral nervous systems, and other extraneural tissues, mainly the heart and endocrine pancreas. This disorder progresses as a mixed sensory and cerebellar ataxia, primarily disturbing the proprioceptive pathways in the spinal cord, peripheral nerves and nuclei of the cerebellum. FRDA is an inherited disease with an autosomal recessive pattern caused by an insufficient amount of the nuclear-encoded mitochondrial protein frataxin, which is an essential and highly evolutionary conserved protein whose deficit results in iron metabolism dysregulation and mitochondrial dysfunction. The first experimental evidence connecting frataxin with iron homeostasis came from Saccharomyces cerevisiae; iron accumulates in the mitochondria of yeast with deletion of the frataxin ortholog gene. This finding was soon linked to previous observations of iron deposits in the hearts of FRDA patients and was later reported in animal models of the disease. Despite advances made in the understanding of FRDA pathophysiology, the role of iron in this disease has not yet been completely clarified. Some of the questions still unresolved include the molecular mechanisms responsible for the iron accumulation and iron-mediated toxicity. Here, we review the contribution of the cellular and animal models of FRDA and relevance of the studies using FRDA patient samples to gain knowledge about these issues. Mechanisms of mitochondrial iron overload are discussed considering the potential roles of frataxin in the major mitochondrial metabolic pathways that use iron. We also analyzed the effect of iron toxicity on neuronal degeneration in FRDA by reactive oxygen species (ROS)-dependent and ROS-independent mechanisms. Finally, therapeutic strategies based on the control of iron toxicity are considered.

Cucumber metal tolerance protein 7 (CsMTP7) is involved in the accumulation of Fe in mitochondria under Fe excess

The plant metal tolerance protein family (MTP) includes 12 members that have been classified into three phylogenetically different subgroups - Zn-cation diffusion facilitator (CDF), Fe/Zn-CDF and Mn-CDF - based on their putative metal specificity. To date, only members belonging to the Zn-CDF or Mn-CDF group have been characterized functionally. The plant Fe/Zn-CDF subgroup includes two proteins, MTP6 and MTP7, but their function and metal specificity have not been confirmed. In this study we showed that cucumber CsMTP7 is a highly specific mitochondrial Fe importer that is able to confer yeast tolerance to Fe excess through increased accumulation of Fe in the mitochondria. We also demonstrated that CsMTP7 contributes to the increased accumulation of Fe in the mitochondria of Arabidopsis thaliana protoplasts. The transcripts and mitochondrial levels of CsMTP7 and ferritin - the iron-storing protein - are significantly increased in cucumber roots in response to Fe excess. This finding suggests that CsMTP7 and ferritin work in concert to accumulate Fe in plant mitochondria. As genes that encode orthologous proteins have been identified in phylogenetically distant organisms, including Archaea, cyanobacteria, humans and plants, but not in yeast, we concluded that the MTP7-mediated mitochondrial Fe accumulation may be conserved in the species, and express mitochondrial ferritin for mitochondrial Fe storage.

Over-expression of SINAL7 increases biomass and drought tolerance, and also delays senescence in Arabidopsis

The seven in absentia like 7 gene (At5g37890, SINAL7) from Arabidopsis thaliana encodes a RING finger protein belonging to the SINA superfamily that possesses E3 ubiquitin-ligase activity. SINAL7 has the ability to self-ubiquitinate and to mono-ubiquitinate glyceraldehyde-3-P dehydrogenase 1 (GAPC1), suggesting a role for both proteins in a hypothetical signaling pathway in Arabidopsis. In this study, the in vivo effects of SINAL7 on plant physiology were examined by over-expressing SINAL7 in transgenic Arabidopsis plants. Phenotypic and gene expression analyses suggest the involvement of SINAL7 in the regulation of several vegetative parameters, essentially those that affect the aerial parts of the plants. Over-expression of SINAL7 resulted in an increase in the concentrations of hexoses and sucrose, with a concommitant increase in plant biomass, particularly in the number of rosette leaves and stem thickness. Interestingly, using the CAB1 (chlorophyll ab binding protein 1) gene as a marker revealed a delay in the onset of senescence. Transgenic plants also displayed a remarkable level of drought resistance, indicating the complexity of the response to SINAL7 over-expression.

Roles and maturation of iron-sulfur proteins in plastids

One reason why iron is an essential element for most organisms is its presence in prosthetic groups such as hemes or iron–sulfur (Fe–S) clusters, which are notably required for electron transfer reactions. As an organelle with an intense metabolism in plants, chloroplast relies on many Fe–S proteins. This includes those present in the electron transfer chain which will be, in fact, essential for most other metabolic processes occurring in chloroplasts, e.g., carbon fixation, nitrogen and sulfur assimilation, pigment, amino acid, and vitamin biosynthetic pathways to cite only a few examples. The maturation of these Fe–S proteins requires a complex and specific machinery named SUF (sulfur mobilisation). The assembly process can be split in two major steps, (1) the de novo assembly on scaffold proteins which requires ATP, iron and sulfur atoms, electrons, and thus the concerted action of several proteins forming early acting assembly complexes, and (2) the transfer of the preformed Fe–S cluster to client proteins using a set of late-acting maturation factors. Similar machineries, having in common these basic principles, are present in the cytosol and in mitochondria. This review focuses on the currently known molecular details concerning the assembly and roles of Fe–S proteins in plastids.

Differential Diel Translation of Transcripts With Roles in the Transfer and Utilization of Iron-Sulfur Clusters in Arabidopsis

Iron-sulfur (Fe-S) clusters are evolutionarily ancient ubiquitous protein cofactors which have mostly catalytic functions but can also have structural roles. In Arabidopsis thaliana, we presently know a total of 124 Fe-S metalloproteins that are encoded in the genome. Fe-S clusters are highly sensitive to oxidation. Therefore, we hypothesized that Fe-S cluster protein biogenesis is adjusted following the daily rhythms in metabolism driven by photosynthesis at the whole-plant, organ, cellular and sub-cellular levels. It had been concluded previously that little such regulation occurs at the transcript level among the genes functioning in Fe-S cluster assembly. As an initial step toward testing our hypothesis, we thus addressed the diel time course of the translation state of relevant transcripts based on publicly available genome-wide microarray data. This analysis can answer whether the translation of the pool of transcripts of a given gene is temporarily either enhanced or suppressed, and when during the day. Thirty-three percent of the transcripts with functions in Fe-S cluster assembly exhibited significant changes in translation state over a diurnal time course, compared to 26% of all detected transcripts. These transcripts comprised functions in all three steps of cluster assembly including persulfide formation, Fe-S cluster formation and Fe-S cluster transfer to target apoproteins. The number of Fe-S cluster carrier/transfer functions contributed more than half of these transcripts, which reached maxima in translation state either during the night or the end of the night. Similarly, translation state of mitochondrial frataxin and ferredoxin, which are thought to contribute Fe and electrons during cluster formation, peaked during the night. By contrast, translation state of chloroplast SUFE2 in persulfide formation and cytosolic Fe-S cluster formation scaffold protein NBP35 reached maxima in translation state during the day. Among the transcripts encoding target Fe-S cluster-utilizing proteins, 19% exhibited diurnal variation in translation state. Day-time maxima of translation state were most common among these transcripts, with none of the maxima during the night (ZT18). We conclude that diurnal regulation of translation state is important in metalloprotein biogenesis. Future models of Fe-S protein biogenesis require more comprehensive data and will have to accommodate diurnal dynamics.

Plant Frataxin in Metal Metabolism

Frataxin is a highly conserved protein from prokaryotes to eukaryotes. Several functions related to iron metabolism have been postulated for this protein, including Fe-S cluster and heme synthesis, response to oxidative damage and oxidative phosphorylation. In plants, the presence of one or two isoforms of this protein with dual localization in mitochondria and chloroplasts has been reported. Frataxin deficiency affects iron metabolism in both organelles, leading to an impairment of mitochondrial respiration, and chlorophyll and photosynthetic electron transport deficiency in chloroplasts. In addition, plant frataxins can react with Cu²⁺ ions and dimerize, which causes the reduction of free Cu ions. This could provide an additional defense mechanism against the oxidation of Fe-S groups by Cu ions. While there is a consensus on the involvement of frataxin in iron homeostasis in most organisms, the interaction of plant frataxins with Cu ions, the presence of different isoforms, and/or the localization in two plant organelles suggest that this protein might have additional functions in vegetal tissues.

Identification of two frataxin isoforms in Zea mays: Structural and functional studies

Frataxin is a ubiquitous protein that plays a role in Fe-S cluster biosynthesis and iron and heme metabolism, although its molecular functions are not entirely clear. In non-photosynthetic eukaryotes, frataxin is encoded by a single gene, and the protein localizes to mitochondria. Here we report the presence of two functional frataxin isoforms in Zea mays, ZmFH-1 and ZmFH-2. We confirmed our previous findings regarding plant frataxins: both proteins have dual localization in mitochondria and chloroplasts. Physiological, biochemical and biophysical studies show some differences in the expression pattern, protection against oxidants and in the aggregation state of both isoforms, suggesting that the two frataxin homologs would play similar but not identical roles in plant cell metabolism. In addition, two specific features of plant frataxins were evidenced: their ability to form dimers and their tendency to undergo conformational change under oxygen exposure.