Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis.

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

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.

Mutation of UDP-glucose binding motif residues lead to increased affinity for ADP-glucose in sugarcane sucrose phosphate synthase.

Plant sucrose-phosphate synthase (SPS) contains a glycosyltransferase domain, which specifically catalyzes reactions with the nucleotide sugar uridine diphosphate glucose (UDP-G) as a donor substrate. Unlike plant SPS, bacterial SPS is predicted to bind other nucleotide sugars, such as adenosine diphosphate glucose (ADP-G). This study aimed to identify the UDP-G binding site of sugarcane (Saccharum officinarum) SPS (SoSPS1) and to improve its affinity for ADP-G by site-directed mutagenesis. To achieve targeted mutagenesis, amino acid distribution and comparative modeling studies were performed, followed by site-directed mutagenesis of SoSPS1 in the putative UDP-G binding motif. The N-terminal deletion of SoSPS1 (∆N-SoSPS1) was used for enzymatic analysis. The results showed that mutations in the R-X4-K, E-X7-E, and H-X5-V motifs significantly affect UDP-G and ADP-G binding. Mutations at R496 and K501 severely attenuate the affinity for UDP-G. Additionally, alanine substitutions at E591 and V570 decreased the UDP-G affinity but remarkably increased its ADP-G affinity. The R-X4-K motif plays a crucial role in the UDP-G binding site and catalytic activity of plant SPS; thus, its alteration to other amino acids was not viable. The E-X7-E and H-X5-V motifs may bind to the nucleotide glucose substrate, indicating that these motifs are involved in substrate specificity. These results agree with substrate docking simulations at the mutated residue positions, supporting the experimental results. These results demonstrate that mutation of E591 and V570 severely attenuated the UDP-G affinity, while retaining its activity against ADP-G, offering strategic insights into increasing sucrose synthesis and plant growth.

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.

A unique ferredoxin acts as a player in the low-iron response of photosynthetic organisms.

Iron chronically limits aquatic photosynthesis, especially in marine environments, and the correct perception and maintenance of iron homeostasis in photosynthetic bacteria, including cyanobacteria, is therefore of global significance. Multiple adaptive mechanisms, responsive promoters, and posttranscriptional regulators have been identified, which allow cyanobacteria to respond to changing iron concentrations. However, many factors remain unclear, in particular, how iron status is perceived within the cell. Here we describe a cyanobacterial ferredoxin (Fed2), with a unique C-terminal extension, that acts as a player in iron perception. Fed2 homologs are highly conserved in photosynthetic organisms from cyanobacteria to higher plants, and, although they belong to the plant type ferredoxin family of [2Fe-2S] photosynthetic electron carriers, they are not involved in photosynthetic electron transport. As deletion of fed2 appears lethal, we developed a C-terminal truncation system to attenuate protein function. Disturbed Fed2 function resulted in decreased chlorophyll accumulation, and this was exaggerated in iron-depleted medium, where different truncations led to either exaggerated or weaker responses to low iron. Despite this, iron concentrations remained the same, or were elevated in all truncation mutants. Further analysis established that, when Fed2 function was perturbed, the classical iron limitation marker IsiA failed to accumulate at transcript and protein levels. By contrast, abundance of IsiB, which shares an operon with isiA, was unaffected by loss of Fed2 function, pinpointing the site of Fed2 action in iron perception to the level of posttranscriptional regulation.

Inhibitory Activity and Docking Analysis of Antimalarial Agents from Stemona sp. toward Ferredoxin-NADP+ Reductase from Malaria Parasites.

Ferredoxin-NADP+ reductases (FNRs, EC 1.18.1.2) were found in the plastids of Plasmodium and have been considered as a target for the development of new antimalarial agents. Croomine, epi-croomine, tuberostemonine, javastemonine A, and isoprotostemonine are isolated alkaloids from the roots of Stemona sp. and their inhibitory effect on FNRs from Plasmodium falciparum (PfFNR) was investigated. Croomine showed the highest level of inhibition (33.9%) of electron transfer from PfFNR to PfFd, while tuberstemonine displayed the highest level of inhibition (55.4%) of diaphorase activity of PfFNR. Docking analysis represented that croomine is located at the middle position of PfFNR and PfFd. Croomine from S. tuberosa appeared to have potential as an antimalarial agent.

Full sequence of the coat protein gene is required for the induction of pathogen-derived resistance against sugarcane mosaic virus in transgenic sugarcane.

Sugarcane mosaic virus (SCMV) is a plant pathogenic virus of the family Potyviridae that causes chlorosis, stunting and significantly reduced sugar productivity in sugarcane. Pathogen-derived resistance is a method used to develop SCMV-resistant sugarcane by overexpression of viral DNA. In this study, the gene encoding the coat protein (CP) of SCMV was amplified by reverse transcriptase PCR from symptomatic sugarcane leaves and used to generate transgenic sugarcane. Nucleotide sequence analysis of amplified cDNA indicated that the 998-bp-long cDNA, termed ScMVCp cDNA, codes for the CP of SCMV from the PS881 isolate. The ScMVCp cDNA was inserted into the binary vector pRI101-ON with two constructs, a full nucleotide sequence (p927) and a sequence coding for N-terminally truncated protein (p702). The constructs were then introduced into sugarcane using Agrobacterium-mediated transformation. Southern blot analysis showed a single hybridized DNA copy inserted into the genome of transgenic sugarcane lines. The inserted genes were expressed at both the RNA transcript and protein levels in the transgenic sugarcane. The highest expression was found in transgenic lines 10, 11 and 13 from the p927 construct. Artificial infection by the virus showed that p927 generated a higher resistance to virus compared with p702. This resistance was passed on to the second generation of transgenic sugarcane with 100 and 20-40% levels of resistance in the p927 and p702 transgenic lines, respectively. This report shows that the full sequence of the CP gene is required to disrupt viral assembly and packaging, thereby generating resistance to SCMV infection.

Plasmodium-specific basic amino acid residues important for the interaction with ferredoxin on the surface of ferredoxin-NADP+ reductase.

The malaria parasite (Plasmodium falciparum) possesses a plastid-derived, essential organelle called the apicoplast, which contains a redox system comprising plant-type ferredoxin (Fd) and Fd-NADP+ reductase (FNR). This system supplies reducing power for the crucial metabolic pathways in this organelle. Electron transfer between P. falciparum Fd (PfFd) and FNR (PfFNR) is performed with higher affinity and specificity than that of plant Fd and FNR. To investigate the mechanism for such superior protein-protein interaction, we searched for the Fd interaction sites on the surface of PfFNR. Basic amino acid residues on the FAD binding side of PfFNR were comprehensively substituted to acidic amino acids by site-directed mutagenesis. Kinetic analysis of electron transfer to PfFd and plant Fds, physical binding to immobilized PfFd and thermodynamics of the PfFd binding using these PfFNR mutants revealed that several basic amino acid residues including those in Plasmodium-specific insertion region are important for the interaction with PfFd. Majority of these basic residues are Plasmodium-specific and not conserved among plant and cyanobacteria FNRs. These results suggest that the interaction mode of Fd and FNR is diverged during evolution so that PfFd: PfFNR interaction meets the physiological requirement in the cells of Plasmodium species.

Identification of UDP-glucose binding site in glycosyltransferase domain of sucrose phosphate synthase from sugarcane (Saccharum officinarum) by structure-based site-directed mutagenesis.

Sucrose phosphate synthase (SPS) is believed to be the key enzyme for controlling the biosynthesis of sucrose. SPSs consist of a functional glycosyltransferase domain that shares conserved residues with the glycosyltransferase domain of sucrose biosynthesis-related protein. The formation of sucrose-6-phosphate is catalyzed by SPS with the transfer of a glycosyl group of uridine diphosphate glucose (UDP-G) as an activated donor sugar to a fructose-6-phosphate as a sugar acceptor. However, understanding of the mechanism of catalytic and substrate binding in SPS is very limited. Based on amino acid sequence alignments with several enzymes that belong to the glycosyltransferase family, the UDP-G binding sites that might be critical for catalytic mechanism were identified. Here, we report that single point mutation of R496, D498, and V570 located in the proposed UDP-G binding site led to less active or complete loss of enzyme activity. Through structure-based site-directed mutagenesis and biochemical studies, the results indicated that these residues contribute to the catalytic activity of plant SPS. Moreover, understanding of the UDP-G binding site provides an insight into new strategies for enzyme engineering and redesigning a catalytic mechanism for UDP.

ASN1-encoded asparagine synthetase in floral organs contributes to nitrogen filling in Arabidopsis seeds.

Despite a general view that asparagine synthetase generates asparagine as an amino acid for long-distance transport of nitrogen to sink organs, its role in nitrogen metabolic pathways in floral organs during seed nitrogen filling has remained undefined. We demonstrate that the onset of pollination in Arabidopsis induces selected genes for asparagine metabolism, namely ASN1 (At3g47340), GLN2 (At5g35630), GLU1 (At5g04140), AapAT2 (At5g19950), ASPGA1 (At5g08100) and ASPGB1 (At3g16150), particularly at the ovule stage (stage 0), accompanied by enhanced asparagine synthetase protein, asparagine and total amino acids. Immunolocalization confined asparagine synthetase to the vascular cells of the silique cell wall and septum, but also to the outer and inner seed integuments, demonstrating the post-phloem transport of asparagine in these cells to developing embryos. In the asn1 mutant, aberrant embryo cell divisions in upper suspensor cell layers from globular to heart stages assign a role for nitrogen in differentiating embryos within the ovary. Induction of asparagine metabolic genes by light/dark and nitrate supports fine shifts of nitrogen metabolic pathways. In transgenic Arabidopsis expressing promoterCaMV35S ::ASN1 fusion, marked metabolomics changes at stage 0, including a several-fold increase in free asparagine, are correlated to enhanced seed nitrogen. However, specific promoterNapin2S ::ASN1 expression during seed formation and a six-fold increase in asparagine toward the desiccation stage result in wild-type seed nitrogen, underlining that delayed accumulation of asparagine impairs the timing of its use by releasing amide and amino nitrogen. Transcript and metabolite profiles in floral organs match the carbon and nitrogen partitioning to generate energy via the tricarboxylic acid cycle, GABA shunt and phosphorylated serine synthetic pathway.

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.

Characterization of two ferredoxin-dependent sulfite reductases having different substrate specificity in the red alga Cyanidioschyzon merolae.

Assimilatory sulfite reductase (SiR) and nitrite reductase (NiR), which are important determinants in biomass productivity, are homologous enzymes that catalyze the reduction of sulfite to sulfide and nitrite to ammonium, respectively. They have a siroheme and a [4Fe-4S] cluster as prosthetic groups in common. The red alga Cyanidioschyzon merolae encodes two SiR-like enzymes, CmSiRA and CmSiRB, which are likely products of recent gene duplication, but no homologues of NiR. The growth in a medium containing nitrate, however, must be supported by a nitrite reducing activity. CmSiRB was not detected in the ammonium medium, but, in the nitrate medium, it was present at a level 1/6 of that of constitutively expressed CmSiRA. Kinetic analysis of the two enzymes showed that CmSiRA has high kcat values with both sulfite and nitrite, but CmSiRB has virtually only the activity of nitrite reduction, although the Km value against nitrite was fairly high in both enzymes. The six amino acid residues that are specific to CmSiRB among various SiR-like enzymes in the active site were mutagenized to mimic partially CmSiRA. Among them, the mutation S217C in CmSiRB partially recovered sulfite reduction activity, suggesting that this residue is a major determinant of substrate specificity.

Energetic basis on interactions between ferredoxin and ferredoxin NADP+ reductase at varying physiological conditions.

In spite of a number of studies to characterize ferredoxin (Fd):ferredoxin NADP+ reductase (FNR) interactions at limited conditions, detailed energetic investigation on how these proteins interact under near physiological conditions and its linkage to FNR activity are still lacking. We herein performed systematic Fd:FNR binding thermodynamics using isothermal titration calorimetry (ITC) at distinct pH (6.0 and 8.0), NaCl concentrations (0-200 mM), and temperatures (19-28 °C) for mimicking physiological conditions in chloroplasts. Energetically unfavorable endothermic enthalpy changes were accompanied by Fd:FNR complexation at all conditions. This energetic cost was compensated by favorable entropy changes, balanced by conformational and hydrational entropy. Increases in the NaCl concentration and pH weakened interprotein affinity due to the less contribution of favorable entropy change regardless of energetic gains from enthalpy changes, suggesting that entropy drove complexation and modulated affinity. Effects of temperature on binding thermodynamics were much smaller than those of pH and NaCl. NaCl concentration and pH-dependent enthalpy and heat capacity changes provided clues for distinct binding modes. Moreover, decreases in the enthalpy level in the Hammond's postulate-based energy landscape implicated kinetic advantages for FNR activity. All these energetic interplays were comprehensively demonstrated by the driving force plot with the enthalpy-entropy compensation which may serve as an energetic buffer against outer stresses. We propose that high affinity at pH 6.0 may be beneficial for protection from proteolysis of Fd and FNR in rest states, and moderate affinity at pH 8.0 and proper NaCl concentrations with smaller endothermic enthalpy changes may contribute to increase FNR activity.

Biochemical and Biophysical Methods to Examine the Effects of Site-Directed Mutagenesis on Enzymatic Activities and Interprotein Interactions.

Mutations in proteins often affect interactions with partner molecules, sequentially changing their activities and functions. In order to examine mutagenic effects, we herein describe practical and detailed protocols for enzymatic activity assays using ferredoxin (Fd)-NADP+ reductase (FNR) and sulfite reductase (SiR), which are electron-transferring enzymes for the Calvin cycle and sulfur assimilation in various organisms, respectively. Methods for isothermal titration calorimetry and nuclear magnetic resonance spectroscopy, which are very useful thermodynamically and mechanically for investigating the effects of mutations on intermolecular interactions, are also described with practical examples of the Fd-FNR binding system.

Ferredoxin:NADP(H) Oxidoreductase Abundance and Location Influences Redox Poise and Stress Tolerance.

In linear photosynthetic electron transport, ferredoxin:NADP(H) oxidoreductase (FNR) transfers electrons from ferredoxin (Fd) to NADP+ Both NADPH and reduced Fd (Fdred) are required for reductive assimilation and light/dark activation/deactivation of enzymes. FNR is therefore a hub, connecting photosynthetic electron transport to chloroplast redox metabolism. A correlation between FNR content and tolerance to oxidative stress is well established, although the precise mechanism remains unclear. We investigated the impact of altered FNR content and localization on electron transport and superoxide radical evolution in isolated thylakoids, and probed resulting changes in redox homeostasis, expression of oxidative stress markers, and tolerance to high light in planta. Our data indicate that the ratio of Fdred to FNR is critical, with either too much or too little FNR potentially leading to increased superoxide production, and perception of oxidative stress at the level of gene transcription. In FNR overexpressing plants, which show more NADP(H) and glutathione pools, improved tolerance to high-light stress indicates that disturbance of chloroplast redox poise and increased free radical generation may help "prime" the plant and induce protective mechanisms. In fnr1 knock-outs, the NADP(H) and glutathione pools are more oxidized relative to the wild type, and the photoprotective effect is absent despite perception of oxidative stress at the level of gene transcription.

Arabidopsis Root-Type Ferredoxin:NADP(H) Oxidoreductase 2 is Involved in Detoxification of Nitrite in Roots.

Ferredoxin:NADP(H) oxidoreductase (FNR) plays a key role in redox metabolism in plastids. Whereas leaf FNR (LFNR) is required for photosynthesis, root FNR (RFNR) is believed to provide electrons to ferredoxin (Fd)-dependent enzymes, including nitrite reductase (NiR) and Fd-glutamine-oxoglutarate aminotransferase (Fd-GOGAT) in non-photosynthetic conditions. In some herbal species, however, most nitrate reductase activity is located in photosynthetic organs, and ammonium in roots is assimilated mainly by Fd-independent NADH-GOGAT. Therefore, RFNR might have a limited impact on N assimilation in roots grown with nitrate or ammonium nitrogen sources. AtRFNR genes are rapidly induced by application of toxic nitrite. Thus, we tested the hypothesis that RFNR could contribute to nitrite reduction in roots by comparing Arabidopsis thaliana seedlings of the wild type with loss-of-function mutants of RFNR2 When these seedlings were grown under nitrate, nitrite or ammonium, only nitrite nutrition caused impaired growth and nitrite accumulation in roots of rfnr2 Supplementation of nitrite with nitrate or ammonium as N sources did not restore the root growth in rfnr2 Also, a scavenger for nitric oxide (NO) could not effectively rescue the growth impairment. Thus, nitrite toxicity, rather than N depletion or nitrite-dependent NO production, probably causes the rfnr2 root growth defect. Our results strongly suggest that RFNR2 has a major role in reduction of toxic nitrite in roots. A specific set of genes related to nitrite reduction and the supply of reducing power responded to nitrite concomitantly, suggesting that the products of these genes act co-operatively with RFNR2 to reduce nitrite in roots.

Non-covalent forces tune the electron transfer complex between ferredoxin and sulfite reductase to optimize enzymatic activity.

Although electrostatic interactions between negatively charged ferredoxin (Fd) and positively charged sulfite reductase (SiR) have been predominantly highlighted to characterize complex formation, the detailed nature of intermolecular forces remains to be fully elucidated. We investigated interprotein forces for the formation of an electron transfer complex between Fd and SiR and their relationship to SiR activity using various approaches over NaCl concentrations between 0 and 400 mM. Fd-dependent SiR activity assays revealed a bell-shaped activity curve with a maximum ∼40-70 mM NaCl and a reverse bell-shaped dependence of interprotein affinity. Meanwhile, intrinsic SiR activity, as measured in a methyl viologen-dependent assay, exhibited saturation above 100 mM NaCl. Thus, two assays suggested that interprotein interaction is crucial in controlling Fd-dependent SiR activity. Calorimetric analyses showed the monotonic decrease in interprotein affinity on increasing NaCl concentrations, distinguished from a reverse bell-shaped interprotein affinity observed from Fd-dependent SiR activity assay. Furthermore, Fd:SiR complex formation and interprotein affinity were thermodynamically adjusted by both enthalpy and entropy through electrostatic and non-electrostatic interactions. A residue-based NMR investigation on the addition of SiR to 15N-labeled Fd at the various NaCl concentrations also demonstrated that a combination of electrostatic and non-electrostatic forces stabilized the complex with similar interfaces and modulated the binding affinity and mode. Our findings elucidate that non-electrostatic forces are also essential for the formation and modulation of the Fd:SiR complex. We suggest that a complex configuration optimized for maximum enzymatic activity near physiological salt conditions is achieved by structural rearrangement through controlled non-covalent interprotein interactions.

Structural and mutational studies of an electron transfer complex of maize sulfite reductase and ferredoxin.

The structure of the complex of maize sulfite reductase (SiR) and ferredoxin (Fd) has been determined by X-ray crystallography. Co-crystals of the two proteins prepared under different conditions were subjected to the diffraction analysis and three possible structures of the complex were solved. Although topological relationship of SiR and Fd varied in each of the structures, two characteristics common to all structures were found in the pattern of protein-protein interactions and positional arrangements of redox centres; (i) a few negative residues of Fd contact with a narrow area of SiR with positive electrostatic surface potential and (ii) [2Fe-2S] cluster of Fd and [4Fe-4S] cluster of SiR are in a close proximity with the shortest distance around 12 Å. Mutational analysis of a total of seven basic residues of SiR distributed widely at the interface of the complex showed their importance for supporting an efficient Fd-dependent activity and a strong physical binding to Fd. These combined results suggest that the productive electron transfer complex of SiR and Fd could be formed through multiple processes of the electrostatic intermolecular interaction and this implication is discussed in terms of the multi-functionality of Fd in various redox metabolisms.

Purification and characterization of recombinant sugarcane sucrose phosphate synthase expressed in E. coli and insect Sf9 cells: an importance of the N-terminal domain for an allosteric regulatory property.

Sucrose phosphate synthase (SPS) catalyses the transfer of glycosyl group of uridine diphosphate glucose to fructose-6-phosphate to form sucrose-6-phosphate. Plant SPS plays a key role in photosynthetic carbon metabolisms, which activity is modulated by an allosteric activator glucose-6-phosphate (G6P). We produced recombinant sugarcane SPS using Escherichia coli and Sf9 insect cells to investigate its structure-function relationship. When expressed in E. coli, two forms of SPS with different sizes appeared; the larger was comparable in size with the authentic plant enzyme and the shorter was trimmed the N-terminal 20 kDa region off. In the insect cells, only enzyme with the authentic size was produced. We purified the trimmed SPS and the full size enzyme from insect cells and found their enzymatic properties differed significantly; the full size enzyme was activated allosterically by G6P, while the trimmed one showed a high activity even without G6P. We further introduced a series of N-terminal truncations up to 171 residue and found G6P-independent activity was enhanced by the truncation. These combined results indicated that the N-terminal region of sugarcane SPS is crucial for the allosteric regulation by G6P and may function like a suppressor domain for the enzyme activity.