A Multicopper Ferroxidase Involved in Iron Binding to Transferrins in Dunaliella salina Plasma Membranes

In vivo localization of iron starvation induced proteins under variable iron supplementation regimes in Phaeodactylum tricornutum

The model pennate diatom Phaeodactylum tricornutum is able to assimilate a range of iron sources. It therefore provides a platform to study different mechanisms of iron processing concomitantly in the same cell. In this study, we follow the localization of three iron starvation induced proteins (ISIPs) in vivo, driven by their native promoters and tagged by fluorophores in an engineered line of P. tricornutum. We find that the localization patterns of ISIPs are dynamic and variable depending on the overall iron status of the cell and the source of iron it is exposed to. Notwithstanding, a shared destination of the three ISIPs both under ferric iron and siderophore-bound iron supplementation is a globular compartment in the vicinity of the chloroplast. In a proteomic analysis, we identify that the cell engages endocytosis machinery involved in the vesicular trafficking as a response to siderophore molecules, even when these are not bound to iron. Our results suggest that there may be a direct vesicle traffic connection between the diatom cell membrane and the periplastidial compartment (PPC) that co-opts clathrin-mediated endocytosis and the "cytoplasm to vacuole" (Cvt) pathway, for proteins involved in iron assimilation. Proteomics data are available via ProteomeXchange with identifier PXD021172.

Highlight

The marine diatom P. tricornutum engages a vesicular network to traffic siderophores and phytotransferrin from the cell membrane directly to a putative iron processing site in the vicinity of the chloroplast.

Coordinated homeostasis of essential mineral nutrients: a focus on iron

In plants, iron (Fe) transport and homeostasis are highly regulated processes. Fe deficiency or excess dramatically limits plant and algal productivity. Interestingly, complex and unexpected interconnections between Fe and various macro- and micronutrient homeostatic networks, supposedly maintaining general ionic equilibrium and balanced nutrition, are currently being uncovered. Although these interactions have profound consequences for our understanding of Fe homeostasis and its regulation, their molecular bases and biological significance remain poorly understood. Here, we review recent knowledge gained on how Fe interacts with micronutrient (e.g. zinc, manganese) and macronutrient (e.g. sulfur, phosphate) homeostasis, and on how these interactions affect Fe uptake and trafficking. Finally, we highlight the importance of developing an improved model of how Fe signaling pathways are integrated into functional networks to control plant growth and development in response to fluctuating environments.

Iron metabolism strategies in diatoms

Diatoms are one of the most successful group of photosynthetic eukaryotes in the contemporary ocean. They are ubiquitously distributed and are the most abundant primary producers in polar waters. Equally remarkable is their ability to tolerate iron deprivation and respond to periodic iron fertilization. Despite their relatively large cell sizes, diatoms tolerate iron limitation and frequently dominate iron-stimulated phytoplankton blooms, both natural and artificial. Here, we review the main iron use strategies of diatoms, including their ability to assimilate and store a range of iron sources, and the adaptations of their photosynthetic machinery and architecture to iron deprivation. Our synthesis relies on published literature and is complemented by a search of 82 diatom transcriptomes, including information collected from seven representatives of the most abundant diatom genera in the world's oceans.

Iron Uptake Mechanisms in Marine Phytoplankton

Oceanic phytoplankton species have highly efficient mechanisms of iron acquisition, as they can take up iron from environments in which it is present at subnanomolar concentrations. In eukaryotes, three main models were proposed for iron transport into the cells by first studying the kinetics of iron uptake in different algal species and then, more recently, by using modern biological techniques on the model diatom Phaeodactylum tricornutum. In the first model, the rate of uptake is dependent on the concentration of unchelated Fe species, and is thus limited thermodynamically. Iron is transported by endocytosis after carbonate-dependent binding of Fe(III)' (inorganic soluble ferric species) to phytotransferrin at the cell surface. In this strategy the cells are able to take up iron from very low iron concentration. In an alternative model, kinetically limited for iron acquisition, the extracellular reduction of all iron species (including Fe') is a prerequisite for iron acquisition. This strategy allows the cells to take up iron from a great variety of ferric species. In a third model, hydroxamate siderophores can be transported by endocytosis (dependent on ISIP1) after binding to the FBP1 protein, and iron is released from the siderophores by FRE2-dependent reduction. In prokaryotes, one mechanism of iron uptake is based on the use of siderophores excreted by the cells. Iron-loaded siderophores are transported across the cell outer membrane via a TonB-dependent transporter (TBDT), and are then transported into the cells by an ABC transporter. Open ocean cyanobacteria do not excrete siderophores but can probably use siderophores produced by other organisms. In an alternative model, inorganic ferric species are transported through the outer membrane by TBDT or by porins, and are taken up by the ABC transporter system FutABC. Alternatively, ferric iron of the periplasmic space can be reduced by the alternative respiratory terminal oxidase (ARTO) and the ferrous ions can be transported by divalent metal transporters (FeoB or ZIP). After reoxidation, iron can be taken up by the high-affinity permease Ftr1.

Comparative study of Cu uptake and early transcriptome responses in the green microalga Chlamydomonas reinhardtii and the macrophyte Elodea nuttallii

Microalgae are widely used as representative primary producers in ecotoxicology, while macrophytes are much less studied. Here we compared the bioavailability and cellular toxicity pathways of 2 h-exposure to 10^-6 mol L^-1 Cu in the macrophyte Elodea nuttallii and the green microalga Chlamydomonas reinhardtii. Uptake rate was similar but faster in the algae than in the macrophyte, while RNA-Sequencing revealed a similar number of regulated genes. Early-regulated genes were congruent with expected adverse outcome pathways for Cu with Gene Ontology terms including gene regulation, energy metabolism, transport, cell processes, stress, antioxidant metabolism and development. However, the gene regulation level was higher in E. nuttallii than in C. reinhardtii and several categories were more represented in the macrophyte than in the microalga. Moreover, several categories including oxidative pentose phosphate pathway (OPP), nitrate metabolism and metal handling were only found for E. nuttallii, whereas categories such as cell motility, polyamine metabolism, mitochondrial electron transport and tricarboxylic acid cycle (TCA) were unique to C. reinhardtii. These differences were attributed to morphological and metabolic differences and highlighted dissimilarities between a sessile and a mobile species. Our results highlight the efficiency of transcriptomics to assess early molecular responses in biota, and the importance of studying more aquatic plants for a better understanding on the impact and fate of environmental contaminants.

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.

Gene regulation of iron-deficiency responses is associated with carbon monoxide and heme oxydase 1 in Chlamydomonas reinhardtii

Carbon monoxide (CO) as an endogenous gaseous molecule regulates a variety of biological processes in animals. However, CO regulating nutrient stress responses in green alga is largely unknown. On the other hand, heme oxydase (HO1 as a rate-limiting enzyme of the first step for heme degration and to catalyze heme into biliverdin (BV), which is concomitant with releasing of CO and ferrous ions, probably participates in the process of CO-regulating response to nutrient stress in green alga. In this paper, we described an observation that CO could regulate iron-homeostasis in iron-starving Chlamydomonas reinhardtii. Exogenous CO at 8 µM was able to prevent the iron deficient-inducing chlorosis and improve chlorophyll accumulation. Expression pattern of FOX1, FTR1 and ferredoxin was up-regulated by CO exposure in iron-deficient mediam. treatment with external CO increasing iron accumulation in iron-deficient C. reinhardtii. Moreover, to get insights into the regulatory role of HO1, we constructed a transgenic alga overexpressing HO1 and HO1 knock-out mutants. The results show that there was no significant influence on chlorosis with HO1 overexpression of C. reinhardtii under iron-deficiency and the chlorophyll accumulation, and gene expression associated with iron deficiency of mutant were greatly improved. Otherwise, those results from HO1 knock-out mutants were opposite to HO1 overexpression mutants. Finally, CO exposure induced NO accumulation in cells. However, such an action could be blocked by NO scavenger cPTIO. These results indicate that CO/HO1 may play an important role in improving green algae adaptation to iron deficiency or cross-talking with NO under the iron deficiency.

A comparative study of iron uptake mechanisms in marine microalgae: iron binding at the cell surface is a critical step

We investigated iron uptake mechanisms in five marine microalgae from different ecologically important phyla: the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana, the prasinophyceae Ostreococcus tauri and Micromonas pusilla, and the coccolithophore Emiliania huxleyi. Among these species, only the two diatoms were clearly able to reduce iron, via an inducible (P. tricornutum) or constitutive (T. pseudonana) ferrireductase system displaying characteristics similar to the yeast (Saccharomyces cerevisiae) flavohemoproteins proteins. Iron uptake mechanisms probably involve very different components according to the species, but the species we studied shared common features. Regardless of the presence and/or induction of a ferrireductase system, all the species were able to take up both ferric and ferrous iron, and iron reduction was not a prerequisite for uptake. Iron uptake decreased with increasing the affinity constants of iron-ligand complexes and with increasing ligand-iron ratios. Therefore, at least one step of the iron uptake mechanism involves a thermodynamically controlled process. Another step escapes to simple thermodynamic rules and involves specific and strong binding of ferric as well as ferrous iron at the cell surface before uptake of iron. Binding was paradoxically increased in iron-rich conditions, whereas uptake per se was induced in all species only after prolonged iron deprivation. We sought cell proteins loaded with iron following iron uptake. One such protein in O. tauri may be ferritin, and in P. tricornutum, Isip1 may be involved. We conclude that the species we studied have uptake systems for both ferric and ferrous iron, both involving specific iron binding at the cell surface.

Genome and low-iron response of an oceanic diatom adapted to chronic iron limitation

BACKGROUND

Biogeochemical elemental cycling is driven by primary production of biomass via phototrophic phytoplankton growth, with 40% of marine productivity being assigned to diatoms. Phytoplankton growth is widely limited by the availability of iron, an essential component of the photosynthetic apparatus. The oceanic diatom Thalassiosira oceanica shows a remarkable tolerance to low-iron conditions and was chosen as a model for deciphering the cellular response upon shortage of this essential micronutrient.

RESULTS

The combined efforts in genomics, transcriptomics and proteomics reveal an unexpected metabolic flexibility in response to iron availability for T. oceanica CCMP1005. The complex response comprises cellular retrenchment as well as remodeling of bioenergetic pathways, where the abundance of iron-rich photosynthetic proteins is lowered, whereas iron-rich mitochondrial proteins are preserved. As a consequence of iron deprivation, the photosynthetic machinery undergoes a remodeling to adjust the light energy utilization with the overall decrease in photosynthetic electron transfer complexes.

CONCLUSIONS

Beneficial adaptations to low-iron environments include strategies to lower the cellular iron requirements and to enhance iron uptake. A novel contribution enhancing iron economy of phototrophic growth is observed with the iron-regulated substitution of three metal-containing fructose-bisphosphate aldolases involved in metabolic conversion of carbohydrates for enzymes that do not contain metals. Further, our data identify candidate components of a high-affinity iron-uptake system, with several of the involved genes and domains originating from duplication events. A high genomic plasticity, as seen from the fraction of genes acquired through horizontal gene transfer, provides the platform for these complex adaptations to a low-iron world.

Genome and low-iron response of an oceanic diatom adapted to chronic iron limitation

BACKGROUND

Biogeochemical elemental cycling is driven by primary production of biomass via phototrophic phytoplankton growth, with 40% of marine productivity being assigned to diatoms. Phytoplankton growth is widely limited by the availability of iron, an essential component of the photosynthetic apparatus. The oceanic diatom Thalassiosira oceanica shows a remarkable tolerance to low-iron conditions and was chosen as a model for deciphering the cellular response upon shortage of this essential micronutrient.

RESULTS

The combined efforts in genomics, transcriptomics and proteomics reveal an unexpected metabolic flexibility in response to iron availability for T. oceanica CCMP1005. The complex response comprises cellular retrenchment as well as remodeling of bioenergetic pathways, where the abundance of iron-rich photosynthetic proteins is lowered, whereas iron-rich mitochondrial proteins are preserved. As a consequence of iron deprivation, the photosynthetic machinery undergoes a remodeling to adjust the light energy utilization with the overall decrease in photosynthetic electron transfer complexes.

CONCLUSIONS

Beneficial adaptations to low-iron environments include strategies to lower the cellular iron requirements and to enhance iron uptake. A novel contribution enhancing iron economy of phototrophic growth is observed with the iron-regulated substitution of three metal-containing fructose-bisphosphate aldolases involved in metabolic conversion of carbohydrates for enzymes that do not contain metals. Further, our data identify candidate components of a high-affinity iron-uptake system, with several of the involved genes and domains originating from duplication events. A high genomic plasticity, as seen from the fraction of genes acquired through horizontal gene transfer, provides the platform for these complex adaptations to a low-iron world.

Disassembling iron availability to phytoplankton

The bioavailability of iron to microorganisms and its underlying mechanisms have far reaching repercussions to many natural systems and diverse fields of research, including ocean biogeochemistry, carbon cycling and climate, harmful algal blooms, soil and plant research, bioremediation, pathogenesis, and medicine. Within the framework of ocean sciences, short supply and restricted bioavailability of Fe to phytoplankton is thought to limit primary production and curtail atmospheric CO₂ drawdown in vast ocean regions. Yet a clear-cut definition of bioavailability remains elusive, with elements of iron speciation and kinetics, phytoplankton physiology, light, temperature, and microbial interactions, to name a few, all intricately intertwined into this concept. Here, in a synthesis of published and new data, we attempt to disassemble the complex concept of iron bioavailability to phytoplankton by individually exploring some of its facets. We distinguish between the fundamentals of bioavailability – the acquisition of Fe-substrate by phytoplankton – and added levels of complexity involving interactions among organisms, iron, and ecosystem processes. We first examine how phytoplankton acquire free and organically bound iron, drawing attention to the pervasiveness of the reductive uptake pathway in both prokaryotic and eukaryotic autotrophs. Turning to acquisition rates, we propose to view the availability of various Fe-substrates to phytoplankton as a spectrum rather than an absolute “all or nothing.” We then demonstrate the use of uptake rate constants to make comparisons across different studies, organisms, Fe-compounds, and environments, and for gaging the contribution of various Fe-substrates to phytoplankton growth in situ. Last, we describe the influence of aquatic microorganisms on iron chemistry and fate by way of organic complexation and bio-mediated redox transformations and examine the bioavailability of these bio-modified Fe species.

Iron utilization in marine cyanobacteria and eukaryotic algae

Iron is essential for aerobic organisms. Additionally, photosynthetic organisms must maintain the iron-rich photosynthetic electron transport chain, which likely evolved in the iron-replete Proterozoic ocean. The subsequent rise in oxygen since those times has drastically decreased the levels of bioavailable iron, indicating that adaptations have been made to maintain sufficient cellular iron levels in the midst of scarcity. In combination with physiological studies, the recent sequencing of marine microorganism genomes and transcriptomes has begun to reveal the mechanisms of iron acquisition and utilization that allow marine microalgae to persist in iron limited environments.

Molecular evolution of the transferrin family and associated receptors

BACKGROUND

In vertebrates, serum transferrins are essential iron transporters that have bind and release Fe(III) in response to receptor binding and changes in pH. Some family members such as lactoferrin and melanotransferrin can also bind iron while others have lost this ability and have gained other functions, e.g., inhibitor of carbonic anhydrase (mammals), saxiphilin (frogs) and otolith matrix protein 1 (fish).

SCOPE OF REVIEW

This article provides an overview of the known transferrin family members and their associated receptors and interacting partners.

MAJOR CONCLUSIONS

The number of transferrin genes has proliferated as a result of multiple duplication events, and the resulting paralogs have developed a wide array of new functions. Some homologs in the most primitive metazoan groups resemble both serum and melanotransferrins, but the major yolk proteins show considerable divergence from the rest of the family. Among the transferrin receptors, the lack of TFR2 in birds and reptiles, and the lack of any TFR homologs among the insects draw attention to the differences in iron transport and regulation in those groups.

GENERAL SIGNIFICANCE

The transferrin family members are important because of their clinical significance, interesting biochemical properties, and evolutionary history. More work is needed to better understand the functions and evolution of the non-vertebrate family members. This article is part of a Special Issue entitled Molecular Mechanisms of Iron Transport and Disorders.

Nonreductive iron uptake mechanism in the marine alveolate Chromera velia

Chromera velia is a newly cultured photosynthetic marine alveolate. This microalga has a high iron requirement for respiration and photosynthesis, although its natural environment contains less than 1 nm of this metal. We found that this organism uses a novel mechanism of iron uptake, differing from the classic reductive and siderophore-mediated iron uptake systems characterized in the model yeast Saccharomyces cerevisiae and present in most yeasts and terrestrial plants. C. velia has no trans-plasma membrane electron transfer system, and thus cannot reduce extracellular ferric chelates. It is also unable to use hydroxamate siderophores as iron sources. Iron uptake from ferric citrate by C. velia is not inhibited by a ferrous chelator, but the rate of uptake is strongly decreased by increasing the ferric ligand (citrate) concentration. The cell wall contains a large number of iron binding sites, allowing the cells to concentrate iron in the vicinity of the transport sites. We describe a model of iron uptake in which aqueous ferric ions are first concentrated in the cell wall before being taken up by the cells without prior reduction. We discuss our results in relation to the strategies used by the phytoplankton to take up iron in the oceans.

Analysis of the high-affinity iron uptake system at the Chlamydomonas reinhardtii plasma membrane

Multicopper ferroxidases play a vital role in iron metabolism in bacteria, fungi, algae, and mammals. Saccharomyces cerevisiae utilizes a channeling mechanism to couple the ferroxidase activity of Fet3p to Fe(3+) transport into the cell by Ftr1p. In contrast, the mechanisms by which mammals couple the ferroxidase reaction to iron trafficking is unclear. The human ferroxidases ceruloplasmin and hephaestin are twice the size of Fet3p and interact with proteins that are not expressed in fungi. Chlamydomonas FOX1 is a homolog of the human ferroxidases but likely supports iron uptake in a manner similar to that of yeast, since Chlamydomonas reinhardtii expresses a ferric iron permease homolog, FTR1. The results presented support this hypothesis. We show that FOX1 is trafficked to the plasma membrane and is oriented with its multicopper oxidase/ferroxidase domain in the exocytoplasmic space. Our analysis of FTR1 indicates its topology is similar to that of S. cerevisiae Ftr1p, with a potential exocytoplasmic iron channeling motif and two potential iron permeation motifs in membrane-spanning regions. We demonstrate that high-affinity iron uptake is dependent on FOX1 and the copper status of the cell. Kinetic inhibition of high-affinity iron uptake by a ferric iron chelator does not reflect the strength of the chelator, supporting a ferric iron channeling mechanism for high-affinity iron uptake in Chlamydomonas. Last, recombinant FOX1 (rFOX1) has been isolated in a partially holo form that exhibits the UV-visible absorbance spectrum of a multicopper oxidase and the catalytic activity of a ferroxidase.

Multicopper oxidases: a workshop on copper coordination chemistry, electron transfer, and metallophysiology

Multicopper oxidases (MCOs) are unique among copper proteins in that they contain at least one each of the three types of biologic copper sites, type 1, type 2, and the binuclear type 3. MCOs are descended from the family of small blue copper proteins (cupredoxins) that likely arose as a complement to the heme-iron-based cytochromes involved in electron transport; this event corresponded to the aerobiosis of the biosphere that resulted in the conversion of Fe(II) to Fe(III) as the predominant redox state of this essential metal and the solubilization of copper from Cu(2)S to Cu(H(2)O)( n ) (2+). MCOs are encoded in genomes in all three kingdoms and play essential roles in the physiology of essentially all aerobes. With four redox-active copper centers, MCOs share with terminal copper-heme oxidases the ability to catalyze the four-electron reduction of O(2) to two molecules of water. The electron transfers associated with this reaction are both outer and inner sphere in nature and their mechanisms have been fairly well established. A subset of MCO proteins exhibit specificity for Fe(2+), Cu(+), and/or Mn(2+) as reducing substrates and have been designated as metallooxidases. These enzymes, in particular the ferroxidases found in all fungi and metazoans, play critical roles in the metal metabolism of the expressing organism.

The Fox1 ferroxidase of Chlamydomonas reinhardtii: a new multicopper oxidase structural paradigm

Multicopper oxidases (MCO) contain at least four copper atoms arrayed in three distinct ligand fields supported by two canonical structural features: (1) multiples of the cupredoxin fold and (2) four unique sequence elements that include the ten histidine and one cysteine ligands to the four copper atoms. Ferroxidases are a subfamily of MCO proteins that contain residues supporting a specific reactivity towards ferrous iron; these MCOs play a vital role in iron metabolism in bacteria, algae, fungi, and mammals. In contrast to the fungal ferroxidases, e.g., Fet3p from Saccharomyces cerevisiae, the mammalian ceruloplasmin (Cp) is twice as large (six vs. three cupredoxin domains) and contains three type 1, or "blue," copper sites. Chlamydomonas reinhardtii expresses a putative ferroxidase, Fox1, which has sequence similarity to human Cp (hCp). Eschewing the standard sequence-based modeling paradigm, we have constructed a function-based model of the Fox1 protein which replicates hCp's six copper-site ligand arrays with an overall root mean square deviation of 1.4 A. Analysis of this model has led also to assignment of motifs in Fox1 that are unique to ferroxidases, the strongest evidence to date that the well-characterized fungal high-affinity iron uptake system is essential to iron homeostasis in green algae. The model of Fox1 also establishes a subfamily of MCO proteins with a noncanonical copper-ligand organization. These diverse structures suggest alternative mechanisms for intramolecular electron transfer and require a new trajectory for the evolution of the MCO superfamily.

Whole-cell response of the pennate diatom Phaeodactylum tricornutum to iron starvation

Marine primary productivity is iron (Fe)-limited in vast regions of the contemporary oceans, most notably the high nutrient low chlorophyll (HNLC) regions. Diatoms often form large blooms upon the relief of Fe limitation in HNLC regions despite their prebloom low cell density. Although Fe plays an important role in controlling diatom distribution, the mechanisms of Fe uptake and adaptation to low iron availability are largely unknown. Through a combination of nontargeted transcriptomic and metabolomic approaches, we have explored the biochemical strategies preferred by Phaeo dactylum tricornutum at growth-limiting levels of dissolved Fe. Processes carried out by components rich in Fe, such as photosynthesis, mitochondrial electron transport, and nitrate assimilation, were down-regulated. Our results show that this retrenchment is compensated by nitrogen (N) and carbon (C) reallocation from protein and carbohydrate degradation, adaptations to chlorophyll biosynthesis and pigment metabolism, removal of excess electrons by mitochondrial alternative oxidase (AOX) and non-photochemical quenching (NPQ), and augmented Fe-independent oxidative stress responses. Iron limitation leads to the elevated expression of at least three gene clusters absent from the Thalassiosira pseudonana genome that encode for components of iron capture and uptake mechanisms.

FEA1, FEA2, and FRE1, encoding two homologous secreted proteins and a candidate ferrireductase, are expressed coordinately with FOX1 and FTR1 in iron-deficient Chlamydomonas reinhardtii

Previously, we had identified FOX1 and FTR1 as iron deficiency-inducible components of a high-affinity copper-dependent iron uptake pathway in Chlamydomonas. In this work, we survey the version 3.0 draft genome to identify a ferrireductase, FRE1, and two ZIP family proteins, IRT1 and IRT2, as candidate ferrous transporters based on their increased expression in iron-deficient versus iron-replete cells. In a parallel proteomic approach, we identified FEA1 and FEA2 as the major proteins secreted by iron-deficient Chlamydomonas reinhardtii. The recovery of FEA1 and FEA2 from the medium of Chlamydomonas strain CC425 cultures is strictly correlated with iron nutrition status, and the accumulation of the corresponding mRNAs parallels that of the Chlamydomonas FOX1 and FTR1 mRNAs, although the magnitude of regulation is more dramatic for the FEA genes. Like the FOX1 and FTR1 genes, the FEA genes do not respond to copper, zinc, or manganese deficiency. The 5' flanking untranscribed sequences from the FEA1, FTR1, and FOX1 genes confer iron deficiency-dependent expression of ARS2, suggesting that the iron assimilation pathway is under transcriptional control by iron nutrition. Genetic analysis suggests that the secreted proteins FEA1 and FEA2 facilitate high-affinity iron uptake, perhaps by concentrating iron in the vicinity of the cell. Homologues of FEA1 and FRE1 were identified previously as high-CO(2)-responsive genes, HCR1 and HCR2, in Chlorococcum littorale, suggesting that components of the iron assimilation pathway are responsive to carbon nutrition. These iron response components are placed in a proposed iron assimilation pathway for Chlamydomonas.

Effects of iron deficiency on iron binding and internalization into acidic vacuoles in Dunaliella salina

Uptake of iron in the halotolerant alga Dunaliella salina is mediated by a transferrin-like protein (TTf), which binds and internalizes Fe(3+) ions. Recently, we found that iron deficiency induces a large enhancement of iron binding, which is associated with accumulation of three other plasma membrane proteins that associate with TTf. In this study, we characterized the kinetic properties of iron binding and internalization and identified the site of iron internalization. Iron deficiency induces a 4-fold increase in Fe binding, but only 50% enhancement in the rate of iron uptake and also increases the affinity for iron and bicarbonate, a coligand for iron binding. These results indicate that iron deprivation leads to accumulation and modification of iron-binding sites. Iron uptake in iron-sufficient cells is preceded by an apparent time lag, resulting from prebound iron, which can be eliminated by unloading iron-binding sites. Iron is tightly bound to surface-exposed sites and hardly exchanges with medium iron. All bound iron is subsequently internalized. Accumulation of iron inhibits further iron binding and internalization. The vacuolar inhibitor bafilomycin inhibits iron uptake and internalization. Internalized iron was localized by electron microscopy within vacuolar structures that were identified as acidic vacuoles. Iron internalization is accompanied by endocytosis of surface proteins into these acidic vacuoles. A novel kinetic mechanism for iron uptake is proposed, which includes two pools of bound/compartmentalized iron separated by a rate-limiting internalization stage. The major parameter that is modulated by iron deficiency is the iron-binding capacity. We propose that excessive iron binding in iron-deficient cells serves as a temporary reservoir for iron that is subsequently internalized. This mechanism is particularly suitable for organisms that are exposed to large fluctuations in iron availability.

Transition metal transport

Transition metal transporters are of central importance in the plant metal homeostasis network which maintains internal metal concentrations within physiological limits. An overview is given of the functions of known transition metal transporters in the context of the unique chemical properties of their substrates. The modifications of the metal homeostasis network associated with the adaptation to an extreme metalliferous environment are illustrated in two Brassicaceae metal hyperaccumulator model plants based on cross-species transcriptomics studies. In a comparison between higher plants and unicellular algae, hypotheses are generated for evolutionary changes in metal transporter complements associated with the transition to multicellularity.