Complementary DNA cloning and characterization of ferredoxin localized in bundle-sheath cells of maize leaves

Characterization of a Unique Pair of Ferredoxin and Ferredoxin NADP+ Reductase Isoforms That Operates in Non-Photosynthetic Glandular Trichomes

Our recent investigations indicated that isoforms of ferredoxin (Fd) and ferredoxin NADP+ reductase (FNR) play essential roles for the reductive steps of the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway of terpenoid biosynthesis in peppermint glandular trichomes (GTs). Based on an analysis of several transcriptome data sets, we demonstrated the presence of transcripts for a leaf-type FNR (L-FNR), a leaf-type Fd (Fd I), a root-type FNR (R-FNR), and two root-type Fds (Fd II and Fd III) in several members of the mint family (Lamiaceae). The present study reports on the biochemical characterization of all Fd and FNR isoforms of peppermint (Mentha × piperita L.). The redox potentials of Fd and FNR isoforms were determined using photoreduction methods. Based on a diaphorase assay, peppermint R-FNR had a substantially higher specificity constant (kcat/Km) for NADPH than L-FNR. Similar results were obtained with ferricyanide as an electron acceptor. When assayed for NADPH-cytochrome c reductase activity, the specificity constant with the Fd II and Fd III isoforms (when compared to Fd I) was slightly higher for L-FNR and substantially higher for R-FNR. Based on real-time quantitative PCR assays with samples representing various peppermint organs and cell types, the Fd II gene was expressed very highly in metabolically active GTs (but also present at lower levels in roots), whereas Fd III was expressed at low levels in both roots and GTs. Our data provide evidence that high transcript levels of Fd II, and not differences in the biochemical properties of the encoded enzyme when compared to those of Fd III, are likely to support the formation of copious amounts of monoterpene via the MEP pathway in peppermint GTs. This work has laid the foundation for follow-up studies to further investigate the roles of a unique R-FNR-Fd II pair in non-photosynthetic GTs of the Lamiaceae.

Inter-domain interaction of ferredoxin-NADP+ reductase important for the negative cooperativity by ferredoxin and NADP(H)

Ferredoxin-NADP+ reductase (FNR) in plants receives electrons from ferredoxin (Fd) and converts NADP+ to NADPH. The affinity between FNR and Fd is weakened by the allosteric binding of NADP(H) on FNR, which is considered as a part of negative cooperativity. We have been investigating the molecular mechanism of this phenomenon and proposed that the NADP(H)-binding signal is transferred to the Fd-binding region across the two domains of FNR, NADP(H)-binding domain and FAD-binding domain. In this study, we analyzed the effect of altering the inter-domain interaction of FNR on the negative cooperativity. Four site-directed FNR mutants at the inter-domain region were prepared, and their NADPH-dependent changes in the Km for Fd and physical binding ability to Fd were investigated. Two mutants, in which an inter-domain hydrogen bond was changed to a disulfide bond (FNR D52C/S208C) and an inter-domain salt bridge was lost (FNR D104N), were shown to suppress the negative cooperativity by using kinetic analysis and Fd-affinity chromatography. These results showed that the inter-domain interaction of FNR is important for the negative cooperativity, suggesting that the allosteric NADP(H)-binding signal is transferred to Fd-binging region by conformational changes involving inter-domain interactions of FNR.

RNA-sequencing transcriptome analysis of Avicennia marina (Forsk.) Vierh. leaf epidermis defines tissue-specific transcriptional response to salinity treatment

Avicennia marina (Forsk.) Vierh. is a typical mangrove plant. Its epidermis contains salt glands, which can secrete excess salts onto the leaf surfaces, improving the salt tolerance of the plants. However, knowledge on the epidermis-specific transcriptional responses of A. marina to salinity treatment is lacking. Thus, physiological and transcriptomic techniques were applied to unravel the salt tolerance mechanism of A. marina. Our results showed that 400 mM NaCl significantly reduced the plant height, leaf area, leaf biomass and photosynthesis of A. marina. In addition, 1565 differentially expressed genes were identified, of which 634 and 931 were up- and down-regulated. Based on Kyoto Encyclopedia of Genes and Genomes metabolic pathway enrichment analysis, we demonstrated that decreased gene expression, especially that of OEE1, PQL2, FDX3, ATPC, GAPDH, PRK, FBP and RPE, could explain the inhibited photosynthesis caused by salt treatment. Furthermore, the ability of A. marina to cope with 400 mM NaCl treatment was dependent on appropriate hormone signalling and potential sulfur-containing metabolites, such as hydrogen sulfide and cysteine biosynthesis. Overall, the present study provides a theoretical basis for the adaption of A. marina to saline habitats and a reference for studying the salt tolerance mechanism of other mangrove plants.

Exploiting photosynthesis-driven P450 activity to produce indican in tobacco chloroplasts

Photosynthetic organelles offer attractive features for engineering small molecule bioproduction by their ability to convert solar energy into chemical energy required for metabolism. The possibility to couple biochemical production directly to photosynthetic assimilation as a source of energy and substrates has intrigued metabolic engineers. Specifically, the chemical diversity found in plants often relies on cytochrome P450-mediated hydroxylations that depend on reductant supply for catalysis and which often lead to metabolic bottlenecks for heterologous production of complex molecules. By directing P450 enzymes to plant chloroplasts one can elegantly deal with such redox prerequisites. In this study, we explore the capacity of the plant photosynthetic machinery to drive P450-dependent formation of the indigo precursor indoxyl-β-D-glucoside (indican) by targeting an engineered indican biosynthetic pathway to tobacco (Nicotiana benthamiana) chloroplasts. We show that both native and engineered variants belonging to the human CYP2 family are catalytically active in chloroplasts when driven by photosynthetic reducing power and optimize construct designs to improve productivity. However, while increasing supply of tryptophan leads to an increase in indole accumulation, it does not improve indican productivity, suggesting that P450 activity limits overall productivity. Co-expression of different redox partners also does not improve productivity, indicating that supply of reducing power is not a bottleneck. Finally, in vitro kinetic measurements showed that the different redox partners were efficiently reduced by photosystem I but plant ferredoxin provided the highest light-dependent P450 activity. This study demonstrates the inherent ability of photosynthesis to support P450-dependent metabolic pathways. Plants and photosynthetic microbes are therefore uniquely suited for engineering P450-dependent metabolic pathways regardless of enzyme origin. Our findings have implications for metabolic engineering in photosynthetic hosts for production of high-value chemicals or drug metabolites for pharmacological studies.

Role of Histidine 78 of leaf ferredoxin in the interaction with ferredoxin-NADP+ reductase: regulation of pH dependency and negative cooperativity with NADP(H)

In chloroplast stroma, dynamic pH change occurs in response to fluctuating light conditions. We investigated the pH-dependent electron transfer activity between ferredoxin-NADP+ reductase (FNR) and ferredoxin (Fd) isoproteins from maize leaves. By increasing pH (from 5.5 to 8.5), the electron transfer activity from FNR to photosynthetic-type Fd (Fd1) significantly increased while the activity to nonphotosynthetic type Fd (Fd3) decreased, which was mainly due to their differences in the pH dependency of Km for Fd. Mutation of His78 of Fd1 to Val, corresponding amino acid residue in Fd3, lost the pH dependency, indicating a regulatory role of the His78 in the interaction with FNR. We previously showed that the interaction between FNR and Fd was weakened by the allosteric binding of NADP(H) on FNR. His78Val Fd1 mutant largely suppressed this negative cooperativity. These results indicate the involvement of Fd1 His78 in pH dependency and negative cooperativity in the interaction with FNR.

Functional analyses of plasmodium ferredoxin Asp97Tyr mutant related to artemisinin resistance of human malaria parasites

Mutation of Asp97Tyr in the C-terminal region of ferredoxin (PfFd) in the apicoplast of malaria parasites was recently reported to be strongly related to the parasite's resistance to the frontline antimalarial drug, artemisinin. We previously showed that the aromatic amino acid in the C-terminal region of PfFd is important for the interaction with its electron transfer partner, Fd-NADP+ reductase (PfFNR). Here, the importance of the aromatic-aromatic interaction between PfFd and PfFNR was shown using the kinetic analysis of the electron transfer reaction of site-directed mutants of PfFNR with PfFd. Mutation of Asp97Tyr of PfFd was further shown to increase the affinity with PfFNR by the measurements of the dissociation constant (Kd) using tryptophan fluorescence titration and the Michaelis constant (Km) in the kinetic analysis with PfFNRs. Diaphorase activity of PfFNR was inhibited by D97Y PfFd at lower concentration as compared to wild-type PfFd. Ascorbate radical scavenging activity of PfFd and electron transfer activity to a heterogeneous Fd-dependent enzyme was lower with D97Y PfFd than that of wild-type PfFd. These results showed that D97Y mutant of PfFd binds to PfFNR tighter than wild-type PfFd, and thus may suppress the function of PfFNR which could be associated with the action of artemisinin.

Barley Viridis-k links an evolutionarily conserved C-type ferredoxin to chlorophyll biosynthesis

Ferredoxins are single-electron carrier proteins involved in various cellular reactions. In chloroplasts, the most abundant ferredoxin accepts electrons from photosystem I and shuttles electrons via ferredoxin NADP+ oxidoreductase to generate NADPH or directly to ferredoxin dependent enzymes. In addition, plants contain other isoforms of ferredoxins. Two of these, named FdC1 and FdC2 in Arabidopsis thaliana, have C-terminal extensions and functions that are poorly understood. Here we identified disruption of the orthologous FdC2 gene in barley (Hordeum vulgare L.) mutants at the Viridis-k locus; these mutants are deficient in the aerobic cyclase reaction of chlorophyll biosynthesis. The magnesium-protoporphyrin IX monomethyl ester cyclase is one of the least characterized enzymes of the chlorophyll biosynthetic pathway and its electron donor has long been sought. Agroinfiltrations showed that the viridis-k phenotype could be complemented in vivo by Viridis-k but not by canonical ferredoxin. VirK could drive the cyclase reaction in vitro and analysis of cyclase mutants showed that in vivo accumulation of VirK is dependent on cyclase enzyme levels. The chlorophyll deficient phenotype of viridis-k mutants suggests that VirK plays an essential role in chlorophyll biosynthesis that cannot be replaced by other ferredoxins, thus assigning a specific function to this isoform of C-type ferredoxins.

Molecular mechanism of negative cooperativity of ferredoxin-NADP+ reductase by ferredoxin and NADP(H): involvement of a salt bridge between Asp60 of ferredoxin and Lys33 of FNR

Ferredoxin-NADP+ reductase (FNR) in plants receives electrons from ferredoxin (Fd) and converts NADP+ to NADPH at the end of the photosynthetic electron transfer chain. We previously showed that the interaction between FNR and Fd was weakened by the allosteric binding of NADP(H) on FNR, which was considered as a part of negative cooperativity. In this study, we investigated the molecular mechanism of this phenomenon using maize FNR and Fd, as the three-dimensional structure of this Fd:FNR complex is available. NMR chemical shift perturbation analysis identified a site (Asp60) on Fd molecule which was selectively affected by NADP(H) binding on FNR. Asp60 of Fd forms a salt bridge with Lys33 of FNR in the complex. Site-specific mutants of FdD60 and FNRK33 suppressed the negative cooperativity (downregulation of the interaction between FNR and Fd by NADPH), indicating that a salt bridge between FdD60 and FNRK33 is involved in this negative cooperativity.

Upregulation of bundle sheath electron transport capacity under limiting light in C4 Setaria viridis

C₄ photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO₂ concentration at the site of CO₂ fixation. C₄ plants benefit from high irradiance but their efficiency decreases under shade, causing a loss of productivity in crop canopies. We investigated shade acclimation responses of Setaria viridis, a model monocot of NADP-dependent malic enzyme subtype, focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO₂ assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b₆ f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C₄ plants and will support the development of strategies for crop improvement, including the engineering of C₄ photosynthesis into C₃ plants.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

Light-Independent Nitrogen Assimilation in Plant Leaves: Nitrate Incorporation into Glutamine, Glutamate, Aspartate, and Asparagine Traced by 15N

Although the nitrate assimilation into amino acids in photosynthetic leaf tissues is active under the light, the studies during 1950s and 1970s in the dark nitrate assimilation provided fragmental and variable activities, and the mechanism of reductant supply to nitrate assimilation in darkness remained unclear. ¹⁵N tracing experiments unraveled the assimilatory mechanism of nitrogen from nitrate into amino acids in the light and in darkness by the reactions of nitrate and nitrite reductases, glutamine synthetase, glutamate synthase, aspartate aminotransferase, and asparagine synthetase. Nitrogen assimilation in illuminated leaves and non-photosynthetic roots occurs either in the redundant way or in the specific manner regarding the isoforms of nitrogen assimilatory enzymes in their cellular compartments. The electron supplying systems necessary to the enzymatic reactions share in part a similar electron donor system at the expense of carbohydrates in both leaves and roots, but also distinct reducing systems regarding the reactions of Fd-nitrite reductase and Fd-glutamate synthase in the photosynthetic and non-photosynthetic organs.

C-terminal aromatic residue of Plasmodium ferredoxin important for the interaction with ferredoxin: NADP(H) oxidoreductase: possible involvement for artemisinin resistance of human malaria parasites

The malaria parasite (Plasmodium sp.) contains a plastid-derived organelle called the apicoplast, which is essential for the growth of the parasite. In this organelle, a redox system comprising plant-type ferredoxin (Fd) and Fd: NADP(H) oxidoreductase (FNR) supplies reducing power for the crucial metabolic pathways. Electron transfer between Plasmodium falciparum Fd (PfFd) and FNR (PfFNR) is performed with higher affinity and specificity than those of plant Fd and FNR. We investigated the structural basis for such superior protein–protein interaction by focussing on the Plasumodium-specific regions of PfFd. Significant contribution of the C-terminal region of PfFd for the electron transfer with PfFNR was revealed by exchanging the C-terminal three residues between plant Fd and PfFd. Further site-directed mutagenesis of the PfFd C-terminal residues indicated that the presence of aromatic residue at Positions 96 and 97 contributes to the lower Km for PfFNR. Physical binding analyses using fluorescence and calorimetric measurements supported the results. A mutation from Asp to Tyr at position 97 of PfFd was recently reported to be strongly associated with P. falciparum resistance to artemisinin, the front line anti-malarial drug. Thus, the enhanced interaction of PfFd D97Y protein with PfFNR could be involved in artemisinin resistance of human malaria parasites.

NADP(H) allosterically regulates the interaction between ferredoxin and ferredoxin-NADP+ reductase

Ferredoxin‐NADP⁺ reductase (FNR) in plants receives electrons from ferredoxin (Fd) at the end of the photosynthetic electron transfer chain and converts NADP⁺ to NADPH. The interaction between Fd and FNR in plants was previously shown to be attenuated by NADP(H). Here, we investigated the molecular mechanism of this phenomenon using maize FNR and Fd, as the three‐dimensional structure of this complex is available. NADPH, NADP⁺, and 2′5′‐ADP differentially affected the interaction, as revealed through kinetic and physical binding analyses. Site‐directed mutations of FNR which change the affinity for NADPH altered the affinity for Fd in the opposite direction to that for NADPH. We propose that the binding of NADP(H) causes a conformational change of FNR which is transferred to the Fd‐binding region through different domains of FNR, resulting in allosteric changes in the affinity for Fd.

The interaction between ferredoxin (Fd) and Fd‐NADP⁺ reductase (FNR) in plants is attenuated by NADP(H). Site‐directed mutations of FNR which change the affinity for NADPH altered the affinity for Fd in the opposite direction. We propose that the binding of NADP(H) leads to conformational changes of FNR, resulting in allosteric changes in the affinity for Fd.

Structural basis for the isotype-specific interactions of ferredoxin and ferredoxin: NADP+ oxidoreductase: an evolutionary switch between photosynthetic and heterotrophic assimilation

In higher plants, ferredoxin (Fd) and ferredoxin-NADP⁺ reductase (FNR) are each present as distinct isoproteins of photosynthetic type (leaf type) and non-photosynthetic type (root type). Root-type Fd and FNR are considered to facilitate the electron transfer from NADPH to Fd in the direction opposite to that occurring in the photosynthetic processes. We previously reported the crystal structure of the electron transfer complex between maize leaf FNR and Fd (leaf FNR:Fd complex), providing insights into the molecular interactions of the two proteins. Here we show the 2.49 Å crystal structure of the maize root FNR:Fd complex, which reveals that the orientation of FNR and Fd remarkably varies from that of the leaf FNR:Fd complex, giving a structural basis for reversing the redox path. Root FNR was previously shown to interact preferentially with root Fd over leaf Fd, while leaf FNR retains similar affinity for these two types of Fds. The structural basis for such differential interaction was investigated using site-directed mutagenesis of the isotype-specific amino acid residues on the interface of Fd and FNR, based on the crystal structures of the FNR:Fd complexes from maize leaves and roots. Kinetic and physical binding analyses of the resulting mutants lead to the conclusion that the rearrangement of the charged amino acid residues on the Fd-binding surface of FNR confers isotype-specific interaction with Fd, which brings about the evolutional switch between photosynthetic and heterotrophic redox cascades.

Photosynthetic fuel for heterologous enzymes: the role of electron carrier proteins

Plants, cyanobacteria, and algae generate a surplus of redox power through photosynthesis, which makes them attractive for biotechnological exploitations. While central metabolism consumes most of the energy, pathways introduced through metabolic engineering can also tap into this source of reducing power. Recent work on the metabolic engineering of photosynthetic organisms has shown that the electron carriers such as ferredoxin and flavodoxin can be used to couple heterologous enzymes to photosynthetic reducing power. Because these proteins have a plethora of interaction partners and rely on electrostatically steered complex formation, they form productive electron transfer complexes with non-native enzymes. A handful of examples demonstrate channeling of photosynthetic electrons to drive the activity of heterologous enzymes, and these focus mainly on hydrogenases and cytochrome P450s. However, competition from native pathways and inefficient electron transfer rates present major obstacles, which limit the productivity of heterologous reactions coupled to photosynthesis. We discuss specific approaches to address these bottlenecks and ensure high productivity of such enzymes in a photosynthetic context.

Modular electron-transport chains from eukaryotic organelles function to support nitrogenase activity

A large number of genes are necessary for the biosynthesis and activity of the enzyme nitrogenase to carry out the process of biological nitrogen fixation (BNF), which requires large amounts of ATP and reducing power. The multiplicity of the genes involved, the oxygen sensitivity of nitrogenase, plus the demand for energy and reducing power, are thought to be major obstacles to engineering BNF into cereal crops. Genes required for nitrogen fixation can be considered as three functional modules encoding electron-transport components (ETCs), proteins required for metal cluster biosynthesis, and the "core" nitrogenase apoenzyme, respectively. Among these modules, the ETC is important for the supply of reducing power. In this work, we have used Escherichia coli as a chassis to study the compatibility between molybdenum and the iron-only nitrogenases with ETC modules from target plant organelles, including chloroplasts, root plastids, and mitochondria. We have replaced an ETC module present in diazotrophic bacteria with genes encoding ferredoxin-NADPH oxidoreductases (FNRs) and their cognate ferredoxin counterparts from plant organelles. We observe that the FNR-ferredoxin module from chloroplasts and root plastids can support the activities of both types of nitrogenase. In contrast, an analogous ETC module from mitochondria could not function in electron transfer to nitrogenase. However, this incompatibility could be overcome with hybrid modules comprising mitochondrial NADPH-dependent adrenodoxin oxidoreductase and the Anabaena ferredoxins FdxH or FdxB. We pinpoint endogenous ETCs from plant organelles as power supplies to support nitrogenase for future engineering of diazotrophy in cereal crops.

Accumulation of the components of cyclic electron flow around photosystem I in C4 plants, with respect to the requirements for ATP

By concentrating CO2, C4 photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. To concentrate CO2, extra ATP is required, which would also require a change in photosynthetic electron transport in C4 photosynthesis from that in C3 photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes (ΔpH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C4 plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C4 photosynthesis. However, in C4 plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5-PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C4 photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C4 plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C4 photosynthesis, a reverse genetic approach to generate C4 transformants with respect to CEF will be necessary.

Enhancement of naphthalene tolerance in transgenic Arabidopsis plants overexpressing the ferredoxin-like protein (ADI1) from rice

The ADI1 Arabidopsis plants enhanced tolerance and degradation efficiency to naphthalene and had great potential for phytoremediation of naphthalene in the plant material before composting or harvesting and removal. Naphthalene is a global environmental concern, because this substance is assumed to contribute considerably to human cancer risk. Cleaning up naphthalene contamination in the environment is crucial. Phytoremediation is an efficient technology to clean up contaminants. However, no gene that can efficiently degrade exogenous recalcitrant naphthalene in plants has yet been discovered. Ferredoxin (Fd) is a key player of biological electron transfer reaction in the PAH degradation process. The biochemical pathway for bacterial degradation of naphthalene has been well investigated. In this study, a rice gene, ADI1, which codes for a putative photosynthetic-type Fd, has been transformed into Arabidopsis thaliana. The transgenic Arabidopsis plants enhanced tolerance and degradation efficiency of naphthalene. Compared with wild-type plants, transgenic plants assimilated naphthalene from the culture media faster and removed more of this substance. When taken together, our findings suggest that breeding plants with overexpressed ADI1 gene is an effective strategy to degrade naphthalene in the environment.

Mutation of FdC2 gene encoding a ferredoxin-like protein with C-terminal extension causes yellow-green leaf phenotype in rice

Ferredoxins (Fds) are small iron-sulfur proteins that mediate electron transfer in a wide range of metabolic reactions. Besides Fds, there is a type of Fd-like proteins designated as FdC, which have conserved elements of Fds, but contain a significant C-terminal extension. So far, only two FdC genes of Arabidopsis (Arabidopsis thaliana) have been identified in higher plants and thus the functions of FdC proteins remain largely unknown. In this study, we isolated a yellow-green leaf mutant, 501ys, in rice (Oryza sativa). The mutant exhibited yellow-green leaf phenotype and reduced chlorophyll level. The phenotype of 501ys was caused by mutation of a gene on rice chromosome 3. Map-based cloning of this mutant resulted in identification of OsFdC2 gene (LOC_Os03g48040) showing high identity with Arabidopsis FdC2 gene (AT1G32550). OsFdC2 was expressed most abundantly in leaves and its encoded protein was targeted to the chloroplast. In 501ys mutant, a missense mutation was detected in DNA sequence of the gene, resulting in an amino acid change in the encoded protein. The mutant phenotype was rescued by introduction of the wild-type gene. Therefore, we successfully identified FdC2 gene via map-based cloning approach, and demonstrated that mutation of this gene caused yellow-green leaf phenotype in rice.

Physicochemical nature of interfaces controlling ferredoxin NADP(+) reductase activity through its interprotein interactions with ferredoxin

Although acidic residues of ferredoxin (Fd) are known to be essential for activities of various Fd-dependent enzymes, including ferredoxin NADP(+) reductase (FNR) and sulfite reductase (SiR), through electrostatic interactions with basic residues of partner enzymes, non-electrostatic contributions such as hydrophobic forces remain largely unknown. We herein demonstrated that intermolecular hydrophobic and charge-charge interactions between Fd and enzymes were both critical for enzymatic activity. Systematic site-directed mutagenesis, which altered physicochemical properties of residues on the interfaces of Fd for FNR /SiR, revealed various changes in activities of both enzymes. The replacement of serine 43 of Fd to a hydrophobic residue (S43W) and charged residue (S43D) increased and decreased FNR activity, respectively, while S43W showed significantly lower SiR activity without affecting SiR activity by S43D, suggesting that hydrophobic and electrostatic interprotein forces affected FNR activity. Enzyme kinetics revealed that changes in FNR activity by mutating Fd correlated with Km, but not with kcat or activation energy, indicating that interprotein interactions determined FNR activity. Calorimetry-based binding thermodynamics between Fd and FNR showed different binding modes of FNR to wild-type, S43W, or S43D, which were controlled by enthalpy and entropy, as shown by the driving force plot. Residue-based NMR spectroscopy of (15)N FNR with Fds also revealed distinct binding modes of each complex based on different directions of NMR peak shifts with similar overall chemical shift differences. We proposed that subtle adjustments in both hydrophobic and electrostatic forces were critical for enzymatic activity, and these results may be applicable to protein-based electron transfer systems.

Ectopically expressed sweet pepper ferredoxin PFLP enhances disease resistance to Pectobacterium carotovorum subsp. carotovorum affected by harpin and protease-mediated hypersensitive response in Arabidopsis

Plant ferredoxin-like protein (PFLP) is a photosynthesis-type ferredoxin (Fd) found in sweet pepper. It contains an iron-sulphur cluster that receives and delivers electrons between enzymes involved in many fundamental metabolic processes. It has been demonstrated that transgenic plants overexpressing PFLP show a high resistance to many bacterial pathogens, although the mechanism remains unclear. In this investigation, the PFLP gene was transferred into Arabidopsis and its defective derivatives, such as npr1 (nonexpresser of pathogenesis-related gene 1) and eds1 (enhanced disease susceptibility 1) mutants and NAHG-transgenic plants. These transgenic plants were then infected with the soft-rot bacterial pathogen Pectobacterium carotovorum subsp. carotovorum (Erwinia carotovora ssp. carotovora, ECC) to investigate the mechanism behind PFLP-mediated resistance. The results revealed that, instead of showing soft-rot symptoms, ECC activated hypersensitive response (HR)-associated events, such as the accumulation of hydrogen peroxide (H2 O2 ), electrical conductivity leakage and expression of the HR marker genes (ATHSR2 and ATHSR3) in PFLP-transgenic Arabidopsis. This PFLP-mediated resistance could be abolished by inhibitors, such as diphenylene iodonium (DPI), 1-l-trans-epoxysuccinyl-leucylamido-(4-guanidino)-butane (E64) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk), but not by myriocin and fumonisin. The PFLP-transgenic plants were resistant to ECC, but not to its harpin mutant strain ECCAC5082. In the npr1 mutant and NAHG-transgenic Arabidopsis, but not in the eds1 mutant, overexpression of the PFLP gene increased resistance to ECC. Based on these results, we suggest that transgenic Arabidopsis contains high levels of ectopic PFLP; this may lead to the recognition of the harpin and to the activation of the HR and other resistance mechanisms, and is dependent on the protease-mediated pathway.

Plant type ferredoxins and ferredoxin-dependent metabolism

Ferredoxin (Fd) is a small [2Fe-2S] cluster-containing protein found in all organisms performing oxygenic photosynthesis. Fd is the first soluble acceptor of electrons on the stromal side of the chloroplast electron transport chain, and as such is pivotal to determining the distribution of these electrons to different metabolic reactions. In chloroplasts, the principle sink for electrons is in the production of NADPH, which is mostly consumed during the assimilation of CO2 . In addition to this primary function in photosynthesis, Fds are also involved in a number of other essential metabolic reactions, including biosynthesis of chlorophyll, phytochrome and fatty acids, several steps in the assimilation of sulphur and nitrogen, as well as redox signalling and maintenance of redox balance via the thioredoxin system and Halliwell-Asada cycle. This makes Fds crucial determinants of the electron transfer between the thylakoid membrane and a variety of soluble enzymes dependent on these electrons. In this article, we will first describe the current knowledge on the structure and function of the various Fd isoforms present in chloroplasts of higher plants and then discuss the processes involved in oxidation of Fd, introducing the corresponding enzymes and discussing what is known about their relative interaction with Fd.

Alteration of gene expression profile in maize infected with a double-stranded RNA fijivirus associated with symptom development

Maize rough dwarf disease caused by Rice black-streaked dwarf virus (RBSDV) is a major viral disease in China. It has been suggested that the viral infection of plants might cause distinct disease symptoms through the inhibition or activation of host gene transcription. We scanned the gene expression profile of RBSDV-infected maize through oligomer-based microarrays to reveal possible expression changes associated with symptom development. Our results demonstrate that various resistance-related maize genes and cell wall- and development-related genes, such as those for cellulose synthesis, are among the genes whose expression is dramatically altered. These results could aid in research into new strategies to protect cereal crops against viruses, and reveal the molecular mechanisms of development of specific symptoms in rough dwarf-related diseases.

A new structural insight into differential interaction of cyanobacterial and plant ferredoxins with nitrite reductase as revealed by NMR and X-ray crystallographic studies

Ferredoxin (Fd), which plays a pivotal role in photosynthesis as an electron carrier, forms a transient complex with various Fd-dependent enzymes, such as nitrite reductase (NiR), to achieve efficient intermolecular electron transfer. We studied the protein-protein interaction of Fd and NiR by NMR spectroscopy and determined three acidic regions of Fd to be major sites for the interaction with NiR, indicating that the complex is stabilized through electrostatic interaction. During this study, we found Fds from higher plant and cyanobacterium, in spite of their high structural similarities including the above acidic regions, differ remarkably in the interaction with cyanobacterial NiR. In activity assay of NiR, K(m) value for maize Fd (74.6 µM) was 9.6 times larger than that for Leptolyngbya boryana Fd (7.8 µM). The two Fds also showed a similar difference in binding assay to NiR-immobilized resin. Comparative site-specific mutagenesis of two Fds revealed that their discriminative ability for the interaction with NiR is attributed mainly to non-charged residues in the peripheral region of [2Fe-2S] cluster. These non-charged residues are conserved separately between Fds of plant and cyanobacterial origins. Our data highlight that intermolecular force(s) other than electrostatic attraction is(are) also crucial for the molecular interaction between Fd and partner enzyme.

Crystallization and preliminary X-ray studies of an electron-transfer complex of ferredoxin and ferredoxin-dependent glutamate synthase from the cyanobacterium Leptolyngbya boryana

Ferredoxin (Fd) dependent glutamate synthase (Fd-GOGAT) is a key enzyme involved in nitrogen assimilation that catalyzes the two-electron reductive conversion of Gln and 2-oxoglutarate to two molecules of Glu. Fd serves as an electron donor for Fd-GOGAT and the two proteins form a transient electron-transfer complex. In this study, these two proteins were cocrystallized using the hanging-drop vapour-diffusion method. Diffraction data were collected and processed at 2.65 Å resolution. The crystals belonged to space group P4(3), with unit-cell parameters a = b = 84.95, c = 476.31 Å.

Identification of N-terminal regions of wheat leaf ferredoxin NADP+ oxidoreductase important for interactions with ferredoxin

Wheat leaves contain two isoproteins of the photosynthetic ferredoxin:NADP(+) reductase (pFNRI and pFNRII). Truncated forms of both enzymes have been detected in vivo, but only pFNRII displays N-terminal length-dependent changes in activity. To investigate the impact of N-terminal truncation on interaction with ferredoxin (Fd), recombinant pFNRII proteins, differing by deletions of up to 25 amino acids, were generated. During purification of the isoproteins found in vivo, the longer forms of pFNRII bound more strongly to a Fd affinity column than did the shorter forms, pFNRII(ISKK) and pFNRII[N-2](KKQD). Further truncation of the N-termini resulted in a pFNRII protein which failed to bind to a Fd column. Similar k(cat) values (104-140 s(-1)) for cytochrome c reduction were measured for all but the most truncated pFNRII[N-5](DEGV), which had a k(cat) of 38 s(-1). Stopped-flow kinetic studies, examining the impact of truncation on electron flow between mutant pFNRII proteins and Fd, showed there was a variation in k(obs) from 76 to 265 s(-1) dependent on the pFNRII partner. To analyze the sites which contribute to Fd binding at the pFNRII N-terminal, three mutants were generated, in which a single or double lysine residue was changed to glutamine within the in vivo N-terminal truncation region. The mutations affected binding of pFNRII to the Fd column. Based on activity measurements, the double lysine residue change resulted in a pFNRII enzyme with decreased Fd affinity. The results highlight the importance of this flexible N-terminal region of the pFNRII protein in binding the Fd partner.

Electron transfer of site-specifically cross-linked complexes between ferredoxin and ferredoxin-NADP(+) reductase

Ferredoxin (Fd) and Fd-NADP(+) reductase (FNR) are redox partners responsible for the conversion between NADP(+) and NADPH in the plastids of photosynthetic organisms. Introduction of specific disulfide bonds between Fd and FNR by engineering cysteines into the two proteins resulted in 13 different Fd-FNR cross-linked complexes displaying a broad range of activity to catalyze the NADPH-dependent cytochrome c reduction. This variability in activity was thought to be mainly due to different levels of intramolecular electron transfer activity between the FNR and Fd domains. Stopped-flow analysis revealed such differences in the rate of electron transfer from the FNR to Fd domains in some of the cross-linked complexes. A group of the cross-linked complexes with high cytochrome c reduction activity comparable to dissociable wild-type Fd/FNR was shown to assume a similar Fd-FNR interaction mode as in the native Fd:FNR complex by analyses of NMR chemical shift perturbation and absorption spectroscopy. However, the intermolecular electron transfer of these cross-linked complexes with two Fd-binding proteins, nitrite reductase and photosystem I, was largely inhibited, most probably due to steric hindrance by the FNR moiety linked near the redox center of the Fd domain. In contrast, another group of the cross-linked complexes with low cytochrome c reduction activity tends to mediate higher intermolecular electron transfer activity. Therefore, reciprocal relationship of intramolecular and intermolecular electron transfer abilities was conferred by the linkage of Fd and FNR, which may explain the physiological significance of the separate forms of Fd and FNR in chloroplasts.

FdC1, a novel ferredoxin protein capable of alternative electron partitioning, increases in conditions of acceptor limitation at photosystem I

In higher plants, [2Fe-2S] ferredoxin (Fd) proteins are the unique electron acceptors from photosystem I (PSI). Fds are soluble, and distribute electrons to many enzymes, including Fd:NADP(H) reductase (FNR), for the photoreduction of NADP(+). In addition to well studied [2Fe-2S] Fd proteins, higher plants also possess genes for significantly different, as yet uncharacterized Fd proteins, with extended C termini (FdCs). Whether these FdC proteins function as photosynthetic electron transfer proteins is not known. We examined whether these proteins play a role as alternative electron acceptors at PSI, using quantitative RT-PCR to follow how their expression changes in response to acceptor limitation at PSI, in mutant Arabidopsis plants lacking 90-95% of photosynthetic [2Fe-2S] Fd. Expression of the gene encoding one FdC protein, FdC1, was identified as being strongly up-regulated. We confirmed that this protein was chloroplast localized and increased in abundance on PSI acceptor limitation. We purified the recombinant FdC1 protein, which exhibited a UV-visible spectrum consistent with a [2Fe-2S] cluster, confirmed by EPR analysis. Measurements of electron transfer show that FdC1 is capable of accepting electrons from PSI, but cannot support photoreduction of NADP(+). Whereas FdC1 was capable of electron transfer with FNR, redox potentiometry showed that it had a more positive redox potential than photosynthetic Fds by around 220 mV. These results indicate that FdC1 electron donation to FNR is prevented because it is thermodynamically unfavorable. Based on our data, we speculate that FdC1 has a specific function in conditions of acceptor limitation at PSI, and channels electrons away from NADP(+) photoreduction.

HC-Pro protein of sugar cane mosaic virus interacts specifically with maize ferredoxin-5 in vitro and in planta

Symptom development of a plant viral disease is a result of molecular interactions between the virus and its host plant; thus, the elucidation of specific interactions is a prerequisite to reveal the mechanism of viral pathogenesis. Here, we show that the chloroplast precursor of ferredoxin-5 (Fd V) from maize (Zea mays) interacts with the multifunctional HC-Pro protein of sugar cane mosaic virus (SCMV) in yeast, Nicotiana benthamiana cells and maize protoplasts. Our results demonstrate that the transit peptide rather than the mature protein of Fd V precursor could interact with both N-terminal (residues 1-100) and C-terminal (residues 301-460) fragments, but not the middle part (residues 101-300), of HC-Pro. In addition, SCMV HC-Pro interacted only with Fd V, and not with the other two photosynthetic ferredoxin isoproteins (Fd I and Fd II) from maize plants. SCMV infection significantly downregulated the level of Fd V mRNA in maize plants; however, no obvious changes were observed in levels of Fd I and Fd II mRNA. These results suggest that SCMV HC-Pro interacts specifically with maize Fd V and that this interaction may disturb the post-translational import of Fd V into maize bundle-sheath cell chloroplasts, which could lead to the perturbation of chloroplast structure and function.

Variable photosynthetic roles of two leaf-type ferredoxins in arabidopsis, as revealed by RNA interference

Ferredoxin (Fd) is the soluble protein that accepts electrons from photosystem I (PSI) and makes them available to stromal enzymes in higher plant chloroplasts. In linear electron flow, Fd mainly donates electrons to Fd:NADPH reductase (FNR) which generates NADPH for use in the Calvin cycle, but Fd may also return electrons to the thylakoid plastoquinone pool, forming a cyclic electron flow. Many higher plants contain two different photosynthetic Fd proteins, but there are no conserved sequence differences that allow their division into evolutionary groups. In the model C3 photosynthesizing dicot, Arabidopsis thaliana, there are two such photosynthetic Fds, and we have exploited RNA interference (RNAi) techniques to specifically decrease transcript abundance of different Fds in this plant. Surprisingly, the perturbation of photosynthesis, as measured by cholorophyll fluorescence, in RNAi lines of the two different photosynthetic Fds shows opposite trends. Linear electron flow is retarded in lines with lower Fd2 (the most abundant Fd species) levels and under certain circumstances enhanced in lines with lower Fd1 (the minor isoprotein) levels. These data are evidences for at least partially differentiated roles of Fd1 and Fd2 in photosynthetic electron transfer, possibly in the partition of electrons into linear and cyclic electron flow.

Disease resistance to bacterial pathogens affected by the amount of ferredoxin-I protein in plants

SUMMARY Ferredoxin-I (Fd-I) is a fundamental protein that is involved in several metabolic pathways. The amount of Fd-I found in plants is generally regulated by environmental stress, including biotic and abiotic events. In this study, the correlation between quantity of Fd-I and plant disease resistance was investigated. Fd-I levels were increased by inoculation with Pseudomonas syringae pv. syringae but were reduced by Erwinia carotovora ssp. carotovora. Transgenic tobacco over-expressing Fd-I with the sense sweet pepper Fd-I gene (pflp) was resistant to E. carotovora ssp. carotovora and the saprophytic bacterium P. fluorescens. By contrast, transgenic tobacco with reduced total Fd-I and the antisense pflp gene was susceptible to E. carotovora ssp. carotovora and P. fluorescens. Both of these transgenic tobaccos were resistant to P. syringae pv. syringae. By contrast, the mutated E. carotovora ssp. carotovora, with a defective harpin protein, was able to invade the sense-pflp transgenic tobacco as well as the non-transgenic tobacco. An in vitro kinase assay revealed that harpin could activate unidentified kinases to phosphorylate PFLP. These results demonstrate that Fd-I plays an important role in the disease defence mechanism.

NMR study of the electron transfer complex of plant ferredoxin and sulfite reductase: mapping the interaction sites of ferredoxin

Plant ferredoxin serves as the physiological electron donor for sulfite reductase, which catalyzes the reduction of sulfite to sulfide. Ferredoxin and sulfite reductase form an electrostatically stabilized 1:1 complex for the intermolecular electron transfer. The protein-protein interaction between these proteins from maize leaves was analyzed by nuclear magnetic resonance spectroscopy. Chemical shift perturbation and cross-saturation experiments successfully mapped the location of two major interaction sites of ferredoxin: region 1 including Glu-29, Glu-30, and Asp-34 and region 2 including Glu-92, Glu-93, and Glu-94. The importance of these two acidic patches for interaction with sulfite reductase was confirmed by site-specific mutation of acidic ferredoxin residues in regions 1 and 2, separately and in combination, by which the ability of mutant ferredoxins to transfer electrons and bind to sulfite reductase was additively lowered. Taken together, this study gives a clear illustration of the molecular interaction between ferredoxin and sulfite reductase. We also present data showing that this interaction surface of ferredoxin significantly differs from that when ferredoxin-NADP(+) reductase is the interaction partner.

The role of 5'-adenylylsulfate reductase in controlling sulfate reduction in plants

Cysteine is the first organic product of sulfate assimilation and as such is the precursor of all molecules containing reduced sulfur including methionine, glutathione, and their many metabolites. In plants, 5'-adenylylsulfate (APS) reductase is hypothesized to be a key regulatory point in sulfate assimilation and reduction. APS reductase catalyzes the two-electron reduction of APS to sulfite using glutathione as an electron donor. This paper reviews the experimental basis for this hypothesis. In addition, the results of an experiment designed to test the hypothesis by bypassing the endogenous APS reductase and its regulatory mechanisms are described. Two different bacterial assimilatory reductases were expressed in transgenic Zea mays, the thioredoxin-dependent APS reductase from Pseudomonas aeruginosa and the thioredoxin-dependent 3'-phosphoadenylylsulfate reductase from Escherichia coli. Each of them was placed under transcriptional control of the ubiquitin promoter and the protein products were targeted to chloroplasts. The leaves of transgenic Z. mays lines showed significant accumulation of reduced organic thiol compounds including cysteine, gamma-glutamylcysteine, and glutathione; and reduced inorganic forms of sulfur including sulfite and thiosulfate. Both bacterial enzymes appeared to be equally capable of deregulating the assimilative sulfate reduction pathway. The reduced sulfur compounds accumulated to such high levels that the transgenic plants showed evidence of toxicity. The results provide additional evidence that APS reductase is a major control point for sulfate reduction in Z. mays.

Sulfate assimilation and glutathione synthesis in C4 plants

Sulfate assimilation and glutathione synthesis were traditionally believed to be differentially compartmentalised in C4 plants with the synthesis of cysteine and glutathione restricted to bundle sheath and mesophyll cells, respectively. Recent studies, however, showed that although ATP sulfurylase and adenosine 5' phosphosulfate reductase, the key enzymes of sulfate assimilation, are localised exclusively in bundle sheath in maize and other C4 monocot species, this is not true for the dicot C4 species of Flaveria. On the other hand, enzymes of glutathione biosynthesis were demonstrated to be active in both types of maize cells. Therefore, in this review the recent findings on compartmentation of sulfate assimilation and glutathione metabolism in C4 plants will be summarised and the consequences for our understanding of sulfate metabolism and C4 photosynthesis will be discussed.

Functional differentiation of bundle sheath and mesophyll maize chloroplasts determined by comparative proteomics

Chloroplasts of maize (Zea mays) leaves differentiate into specific bundle sheath (BS) and mesophyll (M) types to accommodate C4 photosynthesis. Consequences for other plastid functions are not well understood but are addressed here through a quantitative comparative proteome analysis of purified M and BS chloroplast stroma. Three independent techniques were used, including cleavable stable isotope coded affinity tags. Enzymes involved in lipid biosynthesis, nitrogen import, and tetrapyrrole and isoprenoid biosynthesis are preferentially located in the M chloroplasts. By contrast, enzymes involved in starch synthesis and sulfur import preferentially accumulate in BS chloroplasts. The different soluble antioxidative systems, in particular peroxiredoxins, accumulate at higher levels in M chloroplasts. We also observed differential accumulation of proteins involved in expression of plastid-encoded proteins (e.g., EF-Tu, EF-G, and mRNA binding proteins) and thylakoid formation (VIPP1), whereas others were equally distributed. Enzymes related to the C4 shuttle, the carboxylation and regeneration phase of the Calvin cycle, and several regulators (e.g., CP12) distributed as expected. However, enzymes involved in triose phosphate reduction and triose phosphate isomerase are primarily located in the M chloroplasts, indicating that the M-localized triose phosphate shuttle should be viewed as part of the BS-localized Calvin cycle, rather than a parallel pathway.

Cyanobacterial non-mevalonate pathway: (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase interacts with ferredoxin in Thermosynechococcus elongatus BP-1

(E)-4-Hydroxy-3-methylbut-2-enyl diphosphate synthase (GcpE), which catalyzes the conversion of 2-C-methyl-D-erythritol cyclodiphosphate (MEcPP) into (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP), is an essential enzyme of the non-mevalonate (2-C-methyl-D-erythritol-4-phosphate (MEP)) pathway for isoprenoid biosynthesis. The terminal steps of the MEP pathway are still not fully understood, although this pathway is necessary for survival in various organisms such as cyanobacteria, plastids of algae and higher plants, and the apicoplast of human malaria parasites. To determine the efficient redox partner for thermophilic cyanobacterial GcpE, We have expressed the gcpE and petF genes in Escherichia coli and studied the protein-protein interaction of GcpE protein with ferredoxin I (PetF) from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1. Recombinant GcpE protein was purified by an N-terminal His(6) tag and reconstituted as a [4Fe-4S](2+) metalloprotein. GcpE was shown to interact strongly with PetF via the bacterial two-hybrid system designed to detect protein-protein interactions. Moreover, a direct protein-protein interaction between PetF and GcpE was confirmed in an in vitro glutathione S-transferase (GST) pull-down assay. To investigate electron transfer activity from PetF to GcpE, we also constructed a NADPH-dependent reducing shuttle system with purified recombinant ferredoxin-NADP(+) oxidoreductase (PetH) and PetF. The result demonstrated that PetF has the ability to transfer electrons to GcpE. Thus, the combined data provide the first evidence that GcpE is a ferredoxin-dependent enzyme in T. elongatus BP-1.

A structural basis of Equisetum arvense ferredoxin isoform II producing an alternative electron transfer with ferredoxin-NADP+ reductase

We have determined the crystal structure, at 1.2-A resolution, of Equisetum arvense ferredoxin isoform II (FdII), which lacks residues equivalent to Arg(39) and Glu(28) highly conserved among other ferredoxins (Fds). In other Fds these residues form an intramolecular salt bridge crucial for stabilization of the [2Fe-2S] cluster, which is disrupted upon complex formation with Fd-NADP(+) oxidoreductase (FNR) to form two intermolecular salt bridges. The overall structure of FdII resembles the known backbone structures of E. arvense isoform I (FdI) and other plant-type Fds. Dramatically, in the FdII structure a unique, alternative salt bridge is formed between Arg(22) and Glu(58). This results in a different relative orientation of the alpha-helix formed by Leu(23)-Glu(29) and eliminates the possibility of forming three of the five intermolecular salt bridges identified on formation of a complex between maize FdI and maize FNR. Mutation of FdII, informed by structural differences with FdI, showed that the alternative salt bridge and the absence of an otherwise conserved Tyr residue are important for the alternative stabilization of the FdII [2Fe-2S] cluster. We also investigated FdI and FdII electron transfer to FNR on chloroplast thylakoid membranes. The K(m) and V(max) values of FdII are similar to those of FdI, contrary to previous measurements of the reverse reaction, from FNR to Fd. The affinity between reduced FdI and oxidized FNR is much greater than that between oxidized FdI and reduced FNR, whereas this is not the case with FdII. The pH dependence of electron transfer by FdI, FdII, and an FdII mutant with FdI features was measured and further indicated that the binding mode to FNR differs between FdI and FdII. Based on this evidence, we hypothesize that binding modes with other Fd-dependent reductases may also vary between FdI and FdII. The structural differences between FdI and FdII therefore result in functional differences that may influence partitioning of electrons into different redox metabolic pathways.

Transient complexes of redox proteins: structural and dynamic details from NMR studies

Redox proteins participate in many metabolic routes, in particular those related to energy conversion. Protein-protein complexes of redox proteins are characterized by a weak affinity and a short lifetime. Two-dimensional NMR spectroscopy has been applied to many redox protein complexes, providing a wealth of information about the process of complex formation, the nature of the interface and the dynamic properties of the complex. These studies have shown that some complexes are non-specific and exist as a dynamic ensemble of orientations while in other complexes the proteins assume a single orientation. The binding interface in these complexes consists of a small hydrophobic patch for specificity, surrounded by polar, uncharged residues that may enhance dissociation, and, in most complexes, a ring or patch of charged residues that enhances the association by electrostatic interactions. The entry and exit port of the electrons is located within the hydrophobic interaction site, ensuring rapid electron transfer from one redox centre to the next.

A post genomic characterization of Arabidopsis ferredoxins

In higher plant plastids, ferredoxin (Fd) is the unique soluble electron carrier protein located in the stroma. Consequently, a wide variety of essential metabolic and signaling processes depend upon reduction by Fd. The currently available plant genomes of Arabidopsis and rice (Oryza sativa) contain several genes encoding putative Fds, although little is known about the proteins themselves. To establish whether this variety represents redundancy or specialized function, we have recombinantly expressed and purified the four conventional [2Fe-2S] Fd proteins encoded in the Arabidopsis genome and analyzed their physical and functional properties. Two proteins are leaf type Fds, having relatively low redox potentials and supporting a higher photosynthetic activity. One protein is a root type Fd, being more efficiently reduced under nonphotosynthetic conditions and supporting a higher activity of sulfite reduction. A further Fd has a remarkably positive redox potential and so, although redox active, is limited in redox partners to which it can donate electrons. Immunological analysis indicates that all four proteins are expressed in mature leaves. This holistic view demonstrates how varied and essential soluble electron transfer functions in higher plants are fulfilled through a diversity of Fd proteins.

Decreased content of leaf ferredoxin changes electron distribution and limits photosynthesis in transgenic potato plants

A complete ferredoxin (Fd) cDNA clone was isolated from potato (Solanum tuberosum L. cv Desiree) leaves. By molecular and immunoblot analysis, the gene was identified as the leaf-specific Fd isoform I. Transgenic potato plants were constructed by introducing the homologous potato fed 1 cDNA clone as an antisense construct under the control of the constitutive cauliflower mosaic virus 35S promoter. Stable antisense lines with Fd contents between 40% and 80% of the wild-type level were selected by northern- and western-blot analysis. In short-term experiments, the distribution of electrons toward their stromal acceptors was altered in the mutant plants. Cyclic electron transport, as determined by the quantum yields of photosystems I and II, was enhanced. The CO2 assimilation rate was decreased, but depending on the remaining Fd content, some lines showed photoinhibition. The leaf protein content remained largely constant, but the antisense plants had a lower total chlorophyll content per unit leaf area and an increased chlorophyll a/b ratio. In the antisense plants, the redox state of the quinone acceptor A in photosystem II (QA) was more reduced than that of the wild-type plants under all experimental conditions. Because the plants with lower Fd amounts reacted as if they were grown under a higher light intensity, the possibility that the altered chloroplast redox state affects light acclimation is discussed.

A ferredoxin Arg-Glu pair important for efficient electron transfer between ferredoxin and ferredoxin-NADP(+) reductase

In order to elucidate the importance of a ferredoxin (Fd) Arg-Glu pair involved in dynamic exchange from intra- to intermolecular salt bridges upon complex formation with ferredoxin-NADP(+) oxidoreductase (FNR), Equisetum arvense FdI and FdII were investigated as normal and the pair-lacking Fd, respectively. The FdI mutant lacking this pair was unstable and rapidly lost the [2Fe-2S] cluster. The catalytic constant (k(cat)) of the electron transfer for FdI is 5.5 times that for FdII and the introduction of this pair into FdII resulted in the increase of k(cat) to a level comparable to that for FdI, demonstrating directly that the Arg-Glu pair is important for efficient electron transfer between Fd and FNR.

Differential response of genes for ferredoxin and ferredoxin:NADP+ oxidoreductase to nitrate and light in maize leaves

In higher plants, ferredoxin (Fd) and Fd:NADP+ oxidoreductase (FNR, EC 1.18.1.2) are encoded by small multigene families, and the individuals transfer electrons to the dependent enzymes in the photosynthetic and the non-photosynthetic plastids. In maize, a C4 plant, expression of genes for the non-photosynthetic isoproteins, Fd VI and R-FNR, is responsive to nitrate in roots whereas the expression and the spatial distribution in the leaves have not been analysed. Here, we studied the expression pattern of a series of Fd and FNR genes in maize leaves in response to nitrate and light. Upon addition of nitrate, the transcripts for Fd VI and R-FNR rapidly accumulated in the leaves, whereas light did not induce accumulation. Expression of genes for the other isoproteins was not changed significantly by the nitrogen source. In the leaf, the transcripts for Fd VI and R-FNR were predominantly detected in mesophyll cells as were those for nitrate-assimilatory enzymes. Since R-FNR is an isoprotein transferring electrons from NADPH to non-photosynthetic type Fd, the redox equivalent is supplied in nitrate reduction, at least partially, via an oxidative pentose phosphate pathway, even in photosynthetic organs.

Isolation and characterization of cDNAs for differentially accumulated transcripts between mesophyll cells and bundle sheath strands of maize leaves

To characterize novel genes functioning specifically in mesophyll cells (MCs) or bundle sheath cells (BSCs) of C4 plants, differential screening of a maize cDNA library was conducted using 32P-labeled single-strand cDNAs prepared from MCs and bundle sheath strands (BSS) as probes. Ten genes encoding thylakoid membrane proteins in chloroplasts were identified as MC-abundant genes. These included genes for chlorophyll a/b binding proteins, plastocyanin, PsaD, PsbT, PsbR, PsbO, PsaK, PsaG, PsaN and ferredoxin. Seven genes identified as BSS-abundant genes encoded PEP carboxykinase, salt-inducible SalT homolog, heavy metal-inducible metallothionein-like protein, ABA- and drought-inducible glycine-rich protein, and three proteins of unknown function (one of which was named Bss1). In situ hybridization analyses for several selected genes revealed that mRNAs for the metallothionein-like protein and Bss1 were accumulated specifically in BSCs, and that mRNA for the SalT homolog was accumulated in vascular cells around phloem cells. Results suggest that the functional differentiation of MC chloroplasts accompany preferential expression of these small proteins in photosystem complexes and that BSCs are the major site of stress responses.

Differential electron flow around photosystem I by two C(4)-photosynthetic-cell-specific ferredoxins

In the C(4) plant maize (Zea mays L.), two ferredoxin isoproteins, Fd I and Fd II, are expressed specifically in mesophyll and bundle-sheath cells, respectively. cDNAs for these ferredoxins were introduced separately into the cyanobacterium Plectonema boryanum with a disrupted endogenous ferredoxin gene, yielding TM202 and KM2-9 strains expressing Fd I and Fd II. The growth of TM202 was retarded under high light (130 micromol/m(2)/s), whereas KM2-9 grew at a normal rate but exhibited a nitrogen-deficient phenotype. Measurement of photosynthetic O(2) evolution revealed that the reducing power was not efficiently partitioned into nitrogen assimilation in KM2-9. After starvation of the cells in darkness, the P700 oxidation level under far-red illumination increased significantly in TM202. However, it remained low in KM2-9, indicating an active cyclic electron flow. In accordance with this, the cellular ratio of ATP/ADP increased and that of NADPH/NADP(+) decreased in KM2-9 as compared with TM202. These results demonstrated that the two cell type-specific ferredoxins differentially modulate electron flow around photosystem I.