Characterizing the flavodoxin landscape in Clostridioides difficile

Clostridioides difficile infections have become a major challenge in medical facilities. The bacterium is capable of spore formation allowing the survival of antibiotic treatment. Therefore, research on the physiology of C. difficile is important for the development of alternative treatment strategies. In this study, we investigated eight putative flavodoxins of C. difficile 630. Flavodoxins are small electron transfer proteins of specifically low potential. The unusually high number of flavodoxins in C. difficile suggests that they are expressed under different conditions. We determined high transcription levels for several flavodoxins during the exponential growth phase, especially for floX. Since flavodoxins are capable of replacing ferredoxins under iron deficiency conditions in other bacteria, we also examined their expression in C. difficile under low iron and no iron levels. In particular, the amount of fldX increased with decreasing iron concentration and thus could possibly replace ferredoxins. Moreover, we demonstrated that fldX is increasingly expressed under different oxidative stress conditions and thus may play an important role in the oxidative stress response. While increased fldX expression was detectable at both RNA and protein level, CD2825 showed increased expression only at mRNA level under H2O2 stress with sufficient iron availability and may indicate hydroxyl radical-dependent transcription. Although the exact function of the individual flavodoxins in C. difficile needs to be further investigated, the present study shows that flavodoxins could play an important role in several physiological processes and under infection-relevant conditions.

IMPORTANCE

The gram-positive, anaerobic, and spore-forming bacterium Clostridioides difficile has become a vast problem in human health care facilities. The antibiotic-associated infection with this intestinal pathogen causes serious and recurrent inflammation of the intestinal epithelium, in many cases with a severe course. To come up with novel targeted therapies against C. difficile infections, a more detailed knowledge on the pathogen's physiology is mandatory. Eight putative flavodoxins, an extraordinarily high copy number of this type of small electron transfer proteins, are annotated for C. difficile. Flavodoxins are known to be essential electron carriers in other bacteria, for instance, during infection-relevant conditions such as iron limitation and oxidative stress. This work is a first and comprehensive overview on characteristics and expression profiles of the putative flavodoxins in the pathogen C. difficile.

Activation of Coq6p, a FAD Monooxygenase Involved in Coenzyme Q Biosynthesis, by Adrenodoxin Reductase/Ferredoxin

Adrenodoxin reductase (AdxR) plays a pivotal role in electron transfer, shuttling electrons between NADPH and iron/sulfur adrenodoxin proteins in mitochondria. This electron transport system is essential for P450 enzymes involved in various endogenous biomolecules biosynthesis. Here, we present an in-depth examination of the kinetics governing the reduction of human AdxR by NADH or NADPH. Our results highlight the efficiency of human AdxR when utilizing NADPH as a flavin reducing agent. Nevertheless, akin to related flavoenzymes such as cytochrome P450 reductase, we observe that low NADPH concentrations hinder flavin reduction due to intricate equilibrium reactions between the enzyme and its substrate/product. Remarkably, the presence of MgCl2 suppresses this complex kinetic behavior by decreasing NADPH binding to oxidized AdxR, effectively transforming AdxR into a classical Michaelis-Menten enzyme. We propose that the addition of MgCl2 may be adapted for studying the reductive half-reactions of other flavoenzymes with NADPH. Furthermore, in vitro experiments provide evidence that the reduction of the yeast flavin monooxygenase Coq6p relies on an electron transfer chain comprising NADPH-AdxR-Yah1p-Coq6p, where Yah1p shuttles electrons between AdxR and Coq6p. This discovery explains the previous in vivo observation that Yah1p and the AdxR homolog, Arh1p, are required for the biosynthesis of coenzyme Q in yeast.

[Correlation of serum ferredoxin 1 and lipoic acid levels with severity of coronary artery disease]

OBJECTIVE

To analyze the correlation of copper death inducer ferredoxin 1 (FDX1) and lipoic acid (LA) with the occurrence and severity of coronary atherosclerosis and explore their roles in coronary heart disease (CHD).

METHODS

We analyzed the data of 226 patients undergoing coronary artery angiography (CAG) in our hospital between October, 2021 and October, 2022, including 47 patients with normal CAG findings (control group) and 179 patients with mild, moderate or severe coronary artery stenosis (CHD group). Serum FDX1 and LA levels were determined with ELISA for all the patients. We also examined pathological changes in the aorta of normal and ApoE-/- mice using HE staining and observed collagen fiber deposition with Sirius red staining. Immunohistochemistry was used to detect the expression and distribution of FDX1 and LA in the aorta, and RT-PCR was performed to detect the expressions of FDX1, LIAS and ACO2 mRNAs in the myocardial tissues.

RESULTS

Compared with the control patients, CHD patients had significantly lower serum FDX1 and LA levels, which decreased progressively as coronary artery stenosis worsened (P < 0.01) and as the number of involved coronary artery branches increased (P < 0.05). Serum FDX1 and LA levels were positively correlated (r=0.451, P < 0.01) and they both negatively correlated with the Gensini score (r=-0.241 and -0.273, respectively; P < 0.01). Compared with normal mice, ApoE-/- mice showed significantly increased lipid levels (P < 0.01) and atherosclerosis index, obvious thickening, lipid aggregation, and collagen fiber hyperplasia in the aorta, and significantly reduced expressions of FDX1, LA, LIAS, and ACO2 (P < 0.05).

CONCLUSION

Serum FDX1 and LA levels decrease with worsening of coronary artery lesions, and theirs expressions are correlated with coronary artery lesions induced by hyperlipidemia.

CYP108N14: A Monoterpene Monooxygenase from Rhodococcus globerulus

Rhodococcus globerulus (R. globerulus) was isolated from the soil beneath a Eucalypt tree. Metabolic growth studies revealed that R. globerulus was capable of living on certain monoterpenes, including 1,8-cineole and p-cymene, as sole sources of carbon and energy. Multiple P450 genes were identified in the R. globerulus genome that shared homology to known bacterial, monoterpene hydroxylating P450s. To date, two of these P450s have been expressed and characterised as 1,8-cineole (CYP176A1) and p-cymene (CYP108N12) monooxygenases that are believed to initiate the biodegradation of these terpenes. In this work, another putative P450 gene (CYP108N14) was identified in R. globerulus genome. Given its amino acid sequence identity to other monoterpene hydroxylating P450s it was hypothesised to catalyse monoterpene hydroxylation. These include CYP108A1 from Pseudomonas sp. (47 % identity, 68 % similarity) which hydroxylates α-terpineol, and CYP108N12 also from R. globerulus (62 % identity, 77 % similarity). Also present in the operon containing CYP108N14 were putative ferredoxin and ferredoxin reductase genes, suggesting a typical Class I P450 system. CYP108N14 was successfully over-expressed heterologously and purified, resulting in a good yield of CYP108N14 holoprotein. However, neither the ferredoxin nor ferredoxin reductase could be produced heterologously. Binding studies with CYP108N14 revealed a preference for the monoterpenes p-cymene, (R)-limonene, (S)-limonene, (S)-α-terpineol and (S)-4-terpineol. An active catalytic system was reconstituted with the non-native redox partners cymredoxin (from the CYP108N12 system) and putidaredoxin reductase (from the CYP101A1 system). CYP108N14 when supported by these redox partners was able to catalyse the hydroxylation of the five aforementioned substrates selectively at the methyl benzylic/allylic positions.

Enlighting Electron Routes In Oxyfunctionalizing Synechocystis sp. PCC 6803

Phototrophic microorganisms, like cyanobacteria, are gaining attention as host organisms for biocatalytic processes with light as energy source and water as electron source. Redox enzymes, especially oxygenases, can profit from in-situ supply of co-substrates, i. e., reduction equivalents and O2 , by the photosynthetic light reaction. The electron transfer downstream of PS I to heterologous electron consuming enzymes in principle can involve NADPH, NADH, and/or ferredoxin, whereas most direct and efficient transfer is desirable. Here, we use the model organism Synechocystis sp. PCC 6803 to investigate, to what extent host and/or heterologous constituents are involved in electron transfer to a heterologous cytochrome P450 monooxygenase from Acidovorax sp. CHX100. Interestingly, in this highly active light-fueled cycloalkane hydroxylating biocatalyst, host-intrinsic enzymes were found capable of completely substituting the function of the Acidovorax ferredoxin reductase. To a certain extent (20 %), this also was true for the Acidovorax ferredoxin. These results indicate the presence of a versatile set of electron carriers in cyanobacteria, enabling efficient and direct coupling of electron consuming reactions to photosynthetic water oxidation. This will both simplify and promote the use of phototrophic microorganisms for sustainable production processes.

Systematic analysis based on the cuproptosis-related genes identifies ferredoxin 1 as an immune regulator and therapeutic target for glioblastoma

Glioblastoma multiforme (GBM) is recognized as the prevailing malignant and aggressive primary brain tumor, characterized by an exceedingly unfavorable prognosis. Cuproptosis, a recently identified form of programmed cell death, exhibits a strong association with cancer progression, therapeutic response, and prognostic outcomes. However, the specific impact of cuproptosis on GBM remains uncertain. To address this knowledge gap, we obtained transcriptional and clinical data pertaining to GBM tissues and their corresponding normal samples from various datasets, including TCGA, CGGA, GEO, and GTEx. R software was utilized for the analysis of various statistical techniques, including survival analysis, cluster analysis, Cox regression, Lasso regression, gene enrichment analysis, drug sensitivity analysis, and immune microenvironment analysis. Multiple assays were conducted to investigate the expression of genes related to cuproptosis and their impact on the proliferation, invasion, and migration of glioblastoma multiforme (GBM) cells. The datasets were obtained and prognostic risk score models were constructed and validated using differentially expressed genes (DEGs) associated with cuproptosis. To enhance the practicality of these models, a nomogram was developed.Patients with glioblastoma multiforme (GBM) who were classified as high risk exhibited a more unfavorable prognosis and shorter overall survival compared to those in the low risk group. Additionally, we specifically chose FDX1 from the differentially expressed genes (DEGs) within the high risk group to assess its expression, prognostic value, biological functionality, drug responsiveness, and immune cell infiltration. The findings demonstrated that FDX1 was significantly upregulated and associated with a poorer prognosis in GBM. Furthermore, its elevated expression appeared to be linked to various metabolic processes and the susceptibility to chemotherapy drugs. Moreover, FDX1 was found to be involved in immune cell infiltration and exhibited correlations with multiple immunosuppressive genes, including TGFBR1 and PDCD1LG2. The aforementioned studies offer substantial assistance in informing the chemotherapy and immunotherapy approaches for GBM. In summary, these findings contribute to a deeper comprehension of cuproptosis and offer novel perspectives on the involvement of cuproptosis-related genes in GBM, thereby presenting a promising therapeutic strategy for GBM patients.

Interactions of human mitochondrial Ferredoxin 1 (Adrenodoxin) by NMR; modulation by cytochrome P450 substrate and by truncation of the C-terminal tail

Human Ferredoxin 1, also referred to as Adrenodoxin (Adx), is the sole electron carrier supporting the function of all seven mitochondrial cytochrome P450 (CYP) enzymes. Adx utilizes conserved negatively charged residues along its α-helix3 to interact with either the proximal surface of CYP enzymes or the binding surface of Adrendodoxin Reductase (AdR). However, in the oxidized state, Adx assumes a monomer-homodimer equilibrium that requires the presence of its unstructured C-terminal tail. Crystallographic structures of full-length human Adx dimers indicate that part of the binding surface necessary for its interactions with CYPs or with AdR is partially occluded by the dimer interface. In this study, protein NMR spectroscopy was used to interrogate the interactions between full-length (2-124) or truncated monomeric (2-108) human Adx and human CYP24A1 (with and without its vitamin-D substrate) as well as interactions with AdR. Here, monomeric Adx induced a similar pattern of peak broadening as that induced by addition of CYP24A1 substrate, consistent with a 1:1 Adx:CYP interaction as the functional complex. Additionally, removal of the C-terminal tail appears to enhance the interaction with AdR, despite removal of some of the AdR contacts in the tail region. This finding was also supported by an NMR competition assay. These findings suggest that the Adx dimers do not undergo meaningful interactions with either CYP or AdR, but may instead be responsible for regulating access to monomeric Adx. These conclusions are discussed in the context of a revised model of the Adx electron shuttle mechanism.

Molecular architecture and electron transfer pathway of the Stn family transhydrogenase

The challenge of endergonic reduction of NADP+ using NADH is overcome by ferredoxin-dependent transhydrogenases that employ electron bifurcation for electron carrier adjustments in the ancient Wood-Ljungdahl pathway. Recently, an electron-bifurcating transhydrogenase with subunit compositions distinct from the well-characterized Nfn-type transhydrogenase was described: the Stn complex. Here, we present the single-particle cryo-EM structure of the Stn family transhydrogenase from the acetogenic bacterium Sporomusa ovata and functionally dissect its electron transfer pathway. Stn forms a tetramer consisting of functional heterotrimeric StnABC complexes. Our findings demonstrate that the StnAB subunits assume the structural and functional role of a bifurcating module, homologous to the HydBC core of the electron-bifurcating HydABC complex. Moreover, StnC contains a NuoG-like domain and a GltD-like NADPH binding domain that resembles the NfnB subunit of the NfnAB complex. However, in contrast to NfnB, StnC lost the ability to bifurcate electrons. Structural comparison allows us to describe how the same fold on one hand evolved bifurcation activity on its own while on the other hand combined with an associated bifurcating module, exemplifying modular evolution in anaerobic metabolism to produce activities critical for survival at the thermodynamic limit of life.

FDX1 enhances endometriosis cell cuproptosis via G6PD-mediated redox homeostasis

Cuproptosis is a new form of programmed cell death, which is associated with the mitochondrial TCA (tricarboxylic acid) cycle. But the functions of cuproptosis in endometriosis progression are still unknown. Here, we find that cuproptosis suppresses the growth of endometriosis cells and the growth of ectopic endometrial tissues in a mouse model. FDX1 as a key regulator in cuproptosis pathway could promote cuproptosis in endometriosis cells. Interestingly, FDX1 interacts with G6PD, and reduces its protein stability, which predominantly affects the cellular redox-regulating systems. Then, the reduced G6PD activity enhances cuproptosis via down-regulating NADPH and GSH levels. Collectively, our study demonstrates that FDX1 mediates cuproptosis in endometriosis via G6PD pathway, resulting in repression of endometriosis cell proliferation and metastasis.

Cymredoxin, a [2Fe-2S] ferredoxin, supports catalytic activity of the p-cymene oxidising P450 enzyme CYP108N12

Rhodococcus globerulus is a metabolically active organism that has been shown to utilise eucalypt oil as its sole source of carbon and energy. This oil includes 1,8-cineole, p-cymene and limonene. Two identified and characterised cytochromes P450 (P450s) from this organism initiate the biodegradation of the monoterpenes 1,8-cineole (CYP176A1) and p-cymene (CYP108N12). Extensive characterisation has been completed for CYP176A1 and it has been successfully reconstituted with its immediate redox partner, cindoxin, and E. coli flavodoxin reductase. Two putative redox partner genes are encoded in the same operon as CYP108N12 and here the isolation, expression, purification, and characterisation of its specific [2Fe-2S] ferredoxin redox partner, cymredoxin is presented. Reconstitution of CYP108N12 with cymredoxin in place of putidaredoxin, a [2Fe-2S] redox partner of another P450, improves both the rate of electron transfer (from 13 ± 2 to 70 ± 1 μM NADH/min/μM CYP108N12) and the efficiency of NADH utilisation (the so-called coupling efficiency increases from 13% to 90%). Cymredoxin improves the catalytic ability of CYP108N12 in vitro. Aldehyde oxidation products of the previously identified substrates p-cymene (4-isopropylbenzaldehyde) and limonene (perillaldehyde) were observed in addition to major hydroxylation products 4-isopropylbenzyl alcohol and perillyl alcohol respectively. These further oxidation products had not previously been seen with putidaredoxin supported oxidation. Furthermore, when supported by cymredoxin CYP108N12 is able to oxidise a wider range of substrates than previously reported. These include o-xylene, α-terpineol, (-)-carveol and thymol yielding o-tolylmethanol, 7-hydroxyterpineol, (4R)-7-hydroxycarveol and 5-hydroxymethyl-2-isopropylphenol, respectively. Cymredoxin is also capable of supporting CYP108A1 (P450_terp) and CYP176A1 activity, allowing them to catalyse the hydroxylation of their native substrates α-terpineol to 7-hydroxyterpineol and 1,8-cineole to 6β-hydroxycineole respectively. These results indicate that cymredoxin not only improves the catalytic capability of CYP108N12 but can also support the activity of other P450s and prove useful for their characterisation.

Functional spectrum and specificity of mitochondrial ferredoxins FDX1 and FDX2

Ferredoxins comprise a large family of iron-sulfur (Fe-S) proteins that shuttle electrons in diverse biological processes. Human mitochondria contain two isoforms of [2Fe-2S] ferredoxins, FDX1 (aka adrenodoxin) and FDX2, with known functions in cytochrome P450-dependent steroid transformations and Fe-S protein biogenesis. Here, we show that only FDX2, but not FDX1, is involved in Fe-S protein maturation. Vice versa, FDX1 is specific not only for steroidogenesis, but also for heme a and lipoyl cofactor biosyntheses. In the latter pathway, FDX1 provides electrons to kickstart the radical chain reaction catalyzed by lipoyl synthase. We also identified lipoylation as a target of the toxic antitumor copper ionophore elesclomol. Finally, the striking target specificity of each ferredoxin was assigned to small conserved sequence motifs. Swapping these motifs changed the target specificity of these electron donors. Together, our findings identify new biochemical tasks of mitochondrial ferredoxins and provide structural insights into their functional specificity.

Fast chlorophyll a fluorescence induction (OJIP) phenotyping of chlorophyll-deficient wheat suggests that an enlarged acceptor pool size of Photosystem I helps compensate for a deregulated photosynthetic electron flow

The wheat lines affected by a decrease in the leaf chlorophyll content typically experience a biomass loss. A known major problem of the chlorophyll-deficient wheat mutants is their limited prevention of Photosystem I (PSI) over-reduction brought about by an insufficient cyclic electron flow, potentially exposing them to a higher sensitivity to light fluctuations. However, the resistance of some mutant lines against fluctuating light suggests the occurrence of regulatory processes compensating for the defect in cyclic electron flow. In this study, a phenotyping approach based on fast chlorophyll a fluorescence induction (OJIP transient), corroborated by P700 redox kinetics, was applied to a collection of chlorophyll-deficient wheat lines, grown under continuous or fluctuating light. Quantitative parameters calculated from the OJIP transient are considered informative about Photosystem II (PSII) functional antenna size and photochemistry, as well as the functioning of the entire photosynthetic electron transport chain. The mutants tended to recover a wild-type-like chlorophyll content, and mature plants could hardly be distinguished based on their effective PSII antenna size. Nevertheless, specific OJIP-derived parameters were strongly correlated with the phenotype severity, in particular the amplitude of the I-P phase and the I-P/J-P amplitude ratio, which are indicative of a more capacitive pool of PSI final electron acceptors (ferredoxin and ferredoxin-NADP⁺ oxidoreductase, FNR). We propose that the enlargement of such pool of electron carriers is a compensatory response operating at the acceptor side of PSI to alleviate potentially harmful over-reduced states of PSI. Our results also suggest that, in chlorophyll-deficient mutants, higher F_V /F_M cannot prove a superior PSII photochemistry and wider I-P phase is not indicative of a higher relative content of PSI.

FDXR Gene Expression after in Vivo Radiation Exposure of Pediatric Patients Undergoing Interventional Cardiology Procedures

BACKGROUND

Ferredoxin reductase (FDXR) has already been reported as a promising biomarker for estimating radiation doses in radiotherapy. This study aimed to investigate the responsiveness of FDXR on pediatric population exposed to ionizing radiation (X-rays) during pediatric interventional cardiology (IC) procedures.

PATIENTS AND METHODS

Peripheral blood was collected by venipuncture from 24 pediatric donors before and 24 hours after the IC procedure. To estimate the effective dose, demographic data and Air Kerma-Area Product (PKA) were recorded for each patient. The relative quantification (RQ) of the FDXR gene in irradiated patient blood samples compared to the non-irradiated blood samples was determined using qPCR analysis. The relative values of FDXR were log- transformed.

RESULTS

The effective dose ranged from 0.002 mSv to 8.004 mSv. Over this radiation exposure range, the FDXR gene expression varied randomly with the effective dose. Up-regulation in FDXR expression was observed in 17 patients and down-regulation in 7 patients.

CONCLUSIONS

Further studies in a larger cohort of pediatric patients along with the record of clinical data are needed to determine whether FDXR gene expression is an effective biomarker for radiation exposure estimation in pediatric imaging.

Acetylacetone Interferes with Carbon and Nitrogen Metabolism of Microcystis aeruginosa by Cutting Off the Electron Flow to Ferredoxin

The regulation of photosynthetic machinery with a nonoxidative approach is a powerful but challenging strategy for the selective inhibition of bloom-forming cyanobacteria. Acetylacetone (AA) was recently found to be a target-selective cyanocide for Microcystis aeruginosa, but the cause and effect in the studied system are still unclear. By recording of the chemical fingerprints of the cells at two treatment intervals (12 and 72 h with 0.1 mM AA) with omics assays, the molecular mechanism of AA in inactivating Microcystis aeruginosa was elucidated. The results clearly reveal the effect of AA on ferredoxin and the consequent effects on the physiological and biochemical processes of Microcystis aeruginosa. In addition to its role as an electron acceptor of photosystem I, ferredoxin plays pivotal roles in the assimilation of nitrogen in cyanobacterial cells. The effect of AA on ferredoxin and on nonheme iron of photosystem II first cut off the photosynthetic electron transfer flow and then interrupted the synthesis of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), which ultimately might affect carbon fixation and nitrogen assimilation metabolisms. The results here provide missing pieces in the current knowledge on the selective inhibition of cyanobacteria, which should shed light on the better control of harmful blooms.

Effect of Ferredoxin Receptor FusA on the Virulence Mechanism of Pseudomonas plecoglossicida

Pseudomonas plecoglossicida is an aerobic Gram-negative bacterium, which is the pathogen of “Visceral white spot disease” in large yellow croaker. P. plecoglossicida is a temperature-dependent bacterial pathogen in fish, which not only reduces the yield of large yellow croaker but also causes continuous transmission of the disease, seriously endangering the healthy development of fisheries. In this study, a mutant strain of fusA was constructed using homologous recombination technology. The results showed that knockout of P. plecoglossicida fusA significantly affected the ability of growth, adhesion, and biofilm formation. Temperature, pH, H₂O₂, heavy metals, and the iron-chelating agent were used to treat the wild type of P. plecoglossicida; the results showed that the expression of fusA was significantly reduced at 4°C, 12°C, and 37°C. The expression of fusA was significantly increased at pH 4 and 5. Cu²⁺ has a significant inducing effect on the expression of fusA, but Pb²⁺ has no obvious effect; the expression of fusA was significantly upregulated under different concentrations of H₂O₂. The expression of the fusA gene was significantly upregulated in the 0.5~4-μmol/l iron-chelating agent. The expression level of the fusA gene was significantly upregulated after the logarithmic phase. It was suggested that fusA included in the TBDR family not only was involved in the transport of ferredoxin but also played important roles in the pathogenicity and environment adaptation of P. plecoglossicida.

Small Molecule Inhibitors of the Bacterioferritin (BfrB)-Ferredoxin (Bfd) Complex Kill Biofilm-Embedded Pseudomonas aeruginosa Cells

Bacteria depend on a well-regulated iron homeostasis to survive adverse environments. A key component of the iron homeostasis machinery is the compartmentalization of Fe3+ in bacterioferritin and its subsequent mobilization as Fe2+ to satisfy metabolic requirements. In Pseudomonas aeruginosa Fe3+ is compartmentalized in bacterioferritin (BfrB), and its mobilization to the cytosol requires binding of a ferredoxin (Bfd) to reduce the stored Fe3+ and release the soluble Fe2+. Blocking the BfrB-Bfd complex in P. aeruginosa by deletion of the bfd gene triggers an irreversible accumulation of Fe3+ in BfrB, concomitant cytosolic iron deficiency and significant impairment of biofilm development. Herein we report that small molecules developed to bind BfrB at the Bfd binding site block the BfrB-Bfd complex, inhibit the mobilization of iron from BfrB in P. aeruginosa cells, elicit a bacteriostatic effect on planktonic cells, and are bactericidal to cells embedded in mature biofilms.

Involvement of the chloroplast gene ferredoxin 1 in multiple responses of Nicotiana benthamiana to Potato virus X infection

The chloroplast protein ferredoxin 1 (FD1), with roles in the chloroplast electron transport chain, is known to interact with the coat proteins (CPs) of Tomato mosaic virus and Cucumber mosaic virus. However, our understanding of the roles of FD1 in virus infection remains limited. Here, we report that the Potato virus X (PVX) p25 protein interacts with FD1, whose mRNA and protein levels are reduced by PVX infection or by transient expression of p25. Silencing of FD1 by Tobacco rattle virus-based virus-induced gene silencing (VIGS) promoted the local and systemic infection of plants by PVX. Use of a drop-and-see (DANS) assay and callose staining revealed that the permeability of plasmodesmata (PDs) was increased in FD1-silenced plants together with a consistently reduced level of PD callose deposition. After FD1 silencing, quantitative reverse transcription-real-time PCR (qRT-PCR) analysis and LC-MS revealed these plants to have a low accumulation of the phytohormones abscisic acid (ABA) and salicylic acid (SA), which contributed to the decreased callose deposition at PDs. Overexpression of FD1 in transgenic plants manifested resistance to PVX infection, but the contents of ABA and SA, and the PD callose deposition were not increased in transgenic plants. Overexpression of FD1 interfered with the RNA silencing suppressor function of p25. These results demonstrate that interfering with FD1 function causes abnormal plant hormone-mediated antiviral processes and thus enhances PVX infection.

Ultrafast Charge-Transfer Dynamics in the Iron-Sulfur Complex of Rhodobacter capsulatus Ferredoxin VI

Iron-sulfur proteins play essential roles in various biological processes. Their electronic structure and vibrational dynamics are key to their rich chemistry but nontrivial to unravel. Here, the first ultrafast transient absorption and impulsive coherent vibrational spectroscopic (ICVS) studies on 2Fe-2S clusters in Rhodobacter capsulatus ferreodoxin VI are characterized. Photoexcitation initiated populations on multiple excited electronic states that evolve into each other in a long-lived charge-transfer state. This suggests a potential light-induced electron-transfer pathway as well as the possibility of using iron-sulfur proteins as photosensitizers for light-dependent enzymes. A tyrosine chain near the active site suggests potential hole-transfer pathways and affirms this electron-transfer pathway. The ICVS data revealed vibrational bands at 417 and 484 cm^-1, with the latter attributed to an excited-state mode. The temperature dependence of the ICVS modes suggests that the temperature effect on protein structure or conformational heterogeneities needs to be considered during cryogenic temperature studies.

Characterization of the [3Fe-4S](0/1+) cluster from the D14C variant of Pyrococcus furiosus ferredoxin via combined NRVS and DFT analyses

The D14C variant of Pyrococcus furiosus ferredoxin provides an extraordinary framework to investigate a [3Fe-4S] cluster at two oxidation levels and compare the results to its physiologic [4Fe-4S] counterpart in the very same protein. Our spectroscopic and computational study reveals vibrational property changes related to the electronic and structural aspects of both Fe-S clusters.

The chemical properties of out-of-equilibrium states of proteins and the role of these states in protein functioning

The out-of-equilibrium states of several iron-containing proteins (cytochromes c of different origin, haemoglobin, myoglobin, ferredoxin and other non-haem iron proteins, cytochrome c oxidase, horseradish peroxidase) were recorded after fast changes in the active centre (electron reduction of iron, ligand dissociation). Strained states result in which the active centre has already been changed and undergone vibrational relaxation but the main part of protein globule is in the 'old', now out-of-equilibrium, state. Protein structure and chemical properties in these states differ considerably from those in equilibrium states. As a rule, the rate constants of protein-specific chemical reactions increase in out-of-equilibrium states by 1--3 orders of magnitude in comparison with those in equilibrium states. Spectra and reactivity of these proteins change in the course of slow (up to 10(-1) s) conformational relaxation, continuously approaching the equilibrium values. It seems that this conformational relaxation is essentially the elementary act of many enzymic reactions for which the rate of substrate-product transformation is determined by the rate of this conformational change.

Functional expression of Spirulina-Delta6 desaturase gene in yeast, Saccharomyces cerevisiae

Spirulina-acyl-lipid desaturases are membrane-bound enzymes found in thylakoid and plasma membranes. These enzymes carry out the fatty acid desaturation process of Spirulina to yield gamma-linolenic acid (GLA) as the final desaturation product. In this study, Spirulina-Delta(6) desaturase encoded by the desD gene was heterologously expressed and characterized in Saccharomyces cerevisiae. We then conducted site-directed mutagenesis of the histidine residues in the three histidine boxes to determine the role of these amino acid residues in the enzyme function. Our results showed that while four mutants showed complete loss of Delta(6)-desaturase activity and two mutants showed only trace of the activity, the enzyme activity could be partially restored by chemical rescue using exogenously provided imidazole. This study reveals that the histidine residues (which have imidazole as their functional group) in the conserved clusters play a critical role in Delta(6)-desaturase activity, possibly by providing a di-iron catalytic center. In our previous study, this enzyme was expressed in Escherichia coli. The results reveal that the enzyme can function only in the presence of an exogenous cofactor, ferredoxin, provided in vitro. This evidence suggests that baker's yeast has a cofactor that can complement ferredoxin, thought to act as an electron donor for the Delta(6) desaturation in cyanobacteria, including Spirulina. The electron donor of the Spirulina-Delta(6) desaturation in yeast is more likely to be cytochrome b(5), which is absent in E. coli. This means that the enzyme expressed in S. cerevisiae can catalyze the biosynthesis of the product, GLA, in vivo.

Purifications and characterizations of a ferredoxin and its related 2-oxoacid:ferredoxin oxidoreductase from the hyperthermophilic archaeon, Sulfolobus solfataricus P1

The coenzyme A-acylating 2-oxoacid:ferredoxin oxidoreductase and ferredoxin (an effective electron acceptor) were purified from the hyperthermophilic archaeon, Sulfolobus solfataricus P1 (DSM1616). The purified ferredoxin is a monomeric protein with an apparent molecular mass of approximately 11 kDa by SDS-PAGE and of 11,180+/-50 Da by MALDI-TOF mass spectrometry. Ferredoxin was identified to be a dicluster, [3Fe-4S][4Fe-4S], type ferredoxin by spectrophotometric and EPR studies, and appeared to be zinc-containing based on the shared homology of its N-terminal sequence with those of known zinc-containing ferredoxins. On the other hand, the purified 2-oxoacid: ferredoxin oxidoreductase was found to be a heterodimeric enzyme consisting of 69 kDa alpha and 34 kDa beta subunits by SDS-PAGE and MALDI-TOF mass spectrometry. The purified enzyme showed a specific activity of 52.6 units/mg for the reduction of cytochrome c with 2-oxoglutarate as substrate at 55 degrees C, pH 7.0. Maximum activity was observed at 70 degrees C and the optimum pH for enzymatic activity was 7.0 -8.0. The enzyme displays broad substrate specificity toward 2-oxoacids, such as pyruvate, 2-oxobutyrate, and 2-oxoglutarate. Among the 2-oxoacids tested (pyruvate, 2-oxobutyrate, and 2-oxoglutarate), 2-oxoglutarate was found to be the best substrate with Km and kcat values of 163 microM and 452 min(-1), respectively. These results provide useful information for structural studies on these two proteins and for studies on the mechanism of electron transfer between the two.

Rubrerythrin and peroxiredoxin: two novel putative peroxidases in the hydrogenosomes of the microaerophilic protozoon Trichomonas vaginalis

The parasitic flagellate Trichomonas vaginalis contains hydrogenosomes, anaerobic organelles related to mitochondria, that generate ATP from the fermentative conversion of pyruvate to acetate, CO2 and molecular hydrogen. Although an essentially anaerobic organism, Trichomonas encounters low oxygen concentrations in its natural habitat and has to protect itself, and especially the oxygen-sensitve enzymes of hydrogenosomal metabolism, from oxidative damage. We have identified two novel proteins in the hydrogenosomal proteome with strong similarity to two putative prokaryotic peroxidases, rubrerythrin and periplasmic thiol peroxidase. Both proteins have previously been found in many prokaryotes but were not known from eukaryotes, suggesting a significant prokaryotic component in the oxygen-detoxification system of trichomonad hydrogenosomes.

Identification of O2-induced peptides in an obligatory anaerobe, Clostridium acetobutylicum

Clostridium acetobutylicum DSM792 (=ATCC824), a solvent producing obligate anaerobe, grew well after a shift in growth conditions from anoxic to microoxic at the mid exponential phase. In two-dimensional gel electrophoresis, a spot migrating at 45 kDa and three spots at 23 kDa accumulated after 30 min of flushing with 5% O(2)/95% N(2). Based on peptide mass fingerprints, the 45 kDa polypeptide was determined to be NP_347663 (A-type flavoprotein homologue) and the 23 kDa polypeptides were determined to be NP_350180 or NP_350181 (novel type rubrerythrin homologue). Northern blot analysis indicated that the expressions of these peptide transcripts were upregulated within 10 min after flushing with 5% O(2)/95% N(2).

Xenorhabdus nematophila requires an intact iscRSUA-hscBA-fdx operon to colonize Steinernema carpocapsae nematodes

An insertion between iscA and hscB of the Xenorhabdus nematophila iscRSUA-hscBA-fdx locus, predicted to encode Fe-S assembly machinery, prevented colonization of Steinernema carpocapsae nematodes. The insertion disrupted cotranscription of iscA and hscB, but did not reduce hscBA expression, suggesting that X. nematophila requires coordinated expression of the isc-hsc-fdx locus for colonization.

Assimilatory nitrate reductase from the haloarchaeon Haloferax mediterranei: purification and characterisation

Haloferax mediterranei can use nitrate as sole nitrogen source during aerobic growth. We report here the purification and biochemical characterisation of the assimilatory nitrate reductase (EC 1.6.6.2) from H. mediterranei. The enzyme, as isolated, was composed of two subunits (105+/-1.3 kDa and 50+/-1.3 kDa) and behaved as a dimer during gel filtration (132+/-6 kDa). A pH of 9 and elevated temperatures up to 80 degrees C (at 3.1 M NaCl) are necessary for optimum activity. The enzyme stability and activity of the enzyme depend upon the salt concentration. Reduced methyl viologen was as effective as the natural electron donor ferredoxin in the catalytic process. In contrast, NADPH and NADH, which are electron donors in nitrate reductases from different non-photosynthetic bacteria, were ineffective.

Purification and characterisation of a possible assimilatory nitrite reductase from the halophile archaeon Haloferax mediterranei

The nitrite reductase from the extreme halophilic archaeon, Haloferax mediterranei, has been purified and characterised. H. mediterranei is capable of growing in a minimal medium (inorganic salts and glucose as a carbon source) with nitrate as the only nitrogen source. The overall purification was 46-fold with about 4% recovery of activity. The enzyme is a monomeric protein of approximately 66 kDa. A pH of 7.5 and high temperatures up to 60 degrees C are necessary for optimum activity. Reduced methyl viologen has been found to be an electron donor as effective as ferredoxin. NADPH and NADH, which are electron donors in nitrite reductases from different non-photosynthetic bacteria, were not effective with nitrite reductase from H. mediterranei.

Phytobilin biosynthesis: cloning and expression of a gene encoding soluble ferredoxin-dependent heme oxygenase from Synechocystis sp. PCC 6803

The phytobilin chromophores of phycobiliproteins and phytochromes are biosynthesized from heme in a pathway that begins with the opening of the tetrapyrrole macrocycle of protoheme to form biliverdin IX alpha, in a reaction catalyzed by heme oxygenase. A gene containing an open reading frame with a predicted polypeptide that has a sequence similar to that of a conserved region of animal microsomal heme oxygenases was identified in the published genomic sequence of Synechocystis sp. PCC 6803. This gene, named ho1, was cloned and expressed in Escherichia coli under the control of the lacZ promoter. Cells expressing the gene became green colored due to the accumulation of biliverdin IX alpha. The size of the expressed protein was equal to the predicted size of the Synechocystis gene product, named HO1. Heme oxygenase activity was assayed in incubations containing extract of transformed E. coli cells. Incubations containing extract of induced cells, but not those containing extract of uninduced cells, had ferredoxin-dependent heme oxygenase activity. With mesoheme as the substrate, the reaction product was identified as mesobiliverdin IX alpha by spectrophotometry and reverse-phase HPLC. Heme oxygenase activity was not sedimented by centrifugation at 100, 000 g. Expression of HO1 increased several-fold during incubation of the cells for 72 h in iron-deficient medium.

ANAEROBIC FORMATE OXIDATION: A FERREDOXIN-DEPENDENT REACTION

The formate oxidizing system of Methanobacillus omelianskii is specific for nicotinamide-adenine dinucleotide and requires ferredoxin, a biological carrier with the lowest known potential. ferredoxin from other obligate anaerobes is active in the reaction studied.

FERREDOXIN OF CLOSTRIDIUM THERMOSACCHAROLYTICUM

Wilder, Martin (University of Kansas, Lawrence), R. C. Valentine, and J. M. Akagi. Ferredoxin of Clostridium thermosaccharolyticum. J. Bacteriol. 86:861-865. 1963.-An electron-transferring agent has been isolated from Clostridium thermosaccharolyticum. This factor was found to participate as an electron carrier in the phosphoroclastic reaction of pyruvate, with the subsequent formation of acetyl phosphate and molecular hydrogen. It can be employed interchangeably with the ferredoxin of C. pasteurianum in various reactions. Thermal-stability studies indicated that this factor from C. thermosaccharolyticum was comparatively more heat-resistant than the carrier obtained from C. pasteurianum. It was concluded that this carrier was ferredoxin or a ferredoxin-like substance.