Insight into the protein and solvent contributions to the reduction potentials of [4Fe-4S]2+/+ clusters: crystal structures of the Allochromatium vinosum ferredoxin variants C57A and V13G and the homologous Escherichia coli ferredoxin

Heterologous synthesis of a simplified nitrogenase analog in Escherichia coli

The heterologous synthesis of a nitrogen-fixing system in a non-diazotrophic organism is a long-sought-after goal because of the crucial importance of nitrogenase for agronomy, energy, and the environment. Here, we report the heterologous synthesis of a two-component nitrogenase analog from Azotobacter vinelandii, which consists of the reductase component (NifH) and the cofactor maturase (NifEN), in Escherichia coli. Metal, electron paramagnetic resonance, and activity analyses verify the cluster composition and functional competence of the heterologously expressed NifH and NifEN. Nuclear magnetic resonance, nanoscale secondary ion mass spectrometry, and growth experiments further illustrate the ability of the NifH/NifEN system to reduce N2 and incorporate the reduced N into the cellular mass. These results establish NifEN/NifH as a simplified nitrogenase analog that could be optimized and engineered to facilitate transgenic expression and biotechnological adaptations of this important metalloenzyme.

Probing the ferredoxin:hydrogenase electron transfer complex by infrared difference spectroscopy

Ferredoxins are small iron-sulfur proteins that engage in one-electron transfer with oxidoreductases across all domains of life. The catalyzed reactions often include multiple electrons, e.g., in the two-electron reduction of NADP+ during photosynthesis or the reduction of protons to H2 by the metalloenzyme hydrogenase. To date, the microscopic details of how ferredoxins facilitate multiple electron redox chemistry are unknown. Ferredoxins of the Allochromatium vinosum subfamily contain two [4Fe-4S] clusters, which allows for two one-electron transfer reactions. However, the iron-sulfur clusters of conventional 2[4Fe-4S]-type ferredoxins have very similar reduction potentials and conclusive evidence for the transfer of two electrons during a single protein-protein interaction (PPI) has not been reported. In this work, the electron transfer complexes between the clostridial 2[4Fe-4S] ferredoxin, CpFd, and [FeFe]-hydrogenases from both Clostridium pasteurianum (CpI) and Chlamydomonas reinhardtii (CrHydA), were investigated. Introducing a non-canonical amino acid near to one of the iron-sulfur clusters of CpFd permitted the quantification of electric field changes via the vibrational Stark effect by Fourier-transform infrared (FTIR) spectroscopy. Upon reduction, in situ FTIR difference spectroscopy reported on protein structural changes and microscale thermophoresis revealed that the affinity between ferredoxin and hydrogenase is modulated by redox-dependent PPIs. Prompted by these findings, we suggest a model how ferredoxin efficiently facilitates multiple electron redox chemistry based on individual one-electron transfer reactions.

Ferredoxins: Functions, Evolution, Potential Applications, and Challenges of Subtype Classification

Ferredoxins are proteins found in all biological kingdoms and are involved in essential biological processes including photosynthesis, lipid metabolism, and biogeochemical cycles. Ferredoxins are classified into different groups based on the iron-sulfur (Fe-S) clusters that they contain. A new subtype classification and nomenclature system, based on the spacing between amino acids in the Fe-S binding motif, has been proposed in order to better understand ferredoxins' biological diversity and evolutionary linkage across different organisms. This new classification system has revealed an unparalleled diversity between ferredoxins and has helped identify evolutionarily linked ferredoxins between species. The current review provides the latest insights into ferredoxin functions and evolution, and the new subtype classification, outlining their potential biotechnological applications and the future challenges in streamlining the process.

Characterization of ferredoxins involved in electron transfer pathways for nitrogen fixation implicates differences in electronic structure in tuning 2[4Fe4S] Fd activity

Ferredoxins (Fds) are small proteins which shuttle electrons to pathways like biological nitrogen fixation. Physical properties tune the reactivity of Fds with different pathways, but knowledge on how these properties can be manipulated to engineer new electron transfer pathways is lacking. Recently, we showed that an evolved strain of Rhodopseudomonas palustris uses a new electron transfer pathway for nitrogen fixation. This pathway involves a variant of the primary Fd of nitrogen fixation in R. palustris, Fer1, in which threonine at position 11 is substituted for isoleucine (Fer1T11I). To understand why this substitution in Fer1 enables more efficient electron transfer, we used in vivo and in vitro methods to characterize Fer1 and Fer1T11I. Electrochemical characterization revealed both Fer1 and Fer1T11I have similar redox transitions (-480 mV and - 550 mV), indicating the reduction potential was unaffected despite the proximity of T11 to an iron‑sulfur (FeS) cluster of Fer1. Additionally, disruption of hydrogen bonding around an FeS cluster in Fer1 by substituting threonine with alanine (T11A) or valine (T11V) did not increase nitrogenase activity, indicating that disruption of hydrogen bonding does not explain the difference in activity observed for Fer1T11I. Electron paramagnetic resonance spectroscopy studies revealed key differences in the electronic structure of Fer1 and Fer1T11I, which indicate changes to the high spin states and/or spin-spin coupling between the FeS clusters of Fer1. Our data implicates these electronic structure differences in facilitating electron flow and sets a foundation for further investigations to understand the connection between these properties and intermolecular electron transfer.

The energetics and evolution of oxidoreductases in deep time

The core metabolic reactions of life drive electrons through a class of redox protein enzymes, the oxidoreductases. The energetics of electron flow is determined by the redox potentials of organic and inorganic cofactors as tuned by the protein environment. Understanding how protein structure affects oxidation-reduction energetics is crucial for studying metabolism, creating bioelectronic systems, and tracing the history of biological energy utilization on Earth. We constructed ProtReDox (https://protein-redox-potential.web.app), a manually curated database of experimentally determined redox potentials. With over 500 measurements, we can begin to identify how proteins modulate oxidation-reduction energetics across the tree of life. By mapping redox potentials onto networks of oxidoreductase fold evolution, we can infer the evolution of electron transfer energetics over deep time. ProtReDox is designed to include user-contributed submissions with the intention of making it a valuable resource for researchers in this field.

Insights into Enzyme Reactions with Redox Cofactors in Biological Conversion of CO2

Carbon dioxide (CO2) is the most abundant component of greenhouse gases (GHGs) and directly creates environmental issues such as global warming and climate change. Carbon capture and storage have been proposed mainly to solve the problem of increasing CO2 concentration in the atmosphere; however, more emphasis has recently been placed on its use. Among the many methods of using CO2, one of the key environmentally friendly technologies involves biologically converting CO2 into other organic substances such as biofuels, chemicals, and biomass via various metabolic pathways. Although an efficient biocatalyst for industrial applications has not yet been developed, biological CO2 conversion is the needed direction. To this end, this review briefly summarizes seven known natural CO2 fixation pathways according to carbon number and describes recent studies in which natural CO2 assimilation systems have been applied to heterogeneous in vivo and in vitro systems. In addition, studies on the production of methanol through the reduction of CO2 are introduced. The importance of redox cofactors, which are often overlooked in the CO2 assimilation reaction by enzymes, is presented; methods for their recycling are proposed. Although more research is needed, biological CO2 conversion will play an important role in reducing GHG emissions and producing useful substances in terms of resource cycling.

Characterization of ferredoxins from the thermophilic, acetogenic bacterium Thermoanaerobacter kivui

A major electron carrier involved in energy and carbon metabolism in the acetogenic model organism Thermoanaerobacter kivui is ferredoxin, an iron-sulfur-containing, electron-transferring protein. Here, we show that the genome of T. kivui encodes four putative ferredoxin-like proteins (TKV_c09620, TKV_c16450, TKV_c10420 and TKV_c19530). All four genes were cloned, a His-tag encoding sequence was added and the proteins were produced from a plasmid in T. kivui. The purified proteins had an absorption peak at 430 nm typical for ferredoxins. The determined iron-sulfur content is consistent with the presence of two predicted [4Fe4S] clusters in TKV_c09620 and TKV_c19530 or one predicted [4Fe4S] cluster in TKV_c16450 and TKV_c10420 respectively. The reduction potential (Em ) for TKV_c09620, TKV_c16450, TKV_c10420 and TKV_c19530 was determined to be -386 ± 4 mV, -386 ± 2 mV, -559 ± 10 mV and -557 ± 3 mV, respectively. TKV_c09620 and TKV_c16450 served as electron carriers for different oxidoreductases from T. kivui. Deletion of the ferredoxin genes led to only a slight reduction of growth on pyruvate or autotrophically on H2  + CO2 . Transcriptional analysis revealed that TKV_c09620 was upregulated in a ΔTKV_c16450 mutant and vice versa TKV_c16450 in a ΔTKV_c09620 mutant, indicating that TKV_c09620 and TKV_c16450 can replace each other. In sum, our data are consistent with the hypothesis that TKV_c09620 and TKV_c16450 are ferredoxins involved in autotrophic and heterotrophic metabolism of T. kivui.

Evolving a New Electron Transfer Pathway for Nitrogen Fixation Uncovers an Electron Bifurcating-Like Enzyme Involved in Anaerobic Aromatic Compound Degradation

Nitrogenase is the key enzyme involved in nitrogen fixation and uses low potential electrons delivered by ferredoxin (Fd) or flavodoxin (Fld) to reduce dinitrogen gas (N₂) to produce ammonia, generating hydrogen gas (H₂) as an obligate product of this activity. Although the phototrophic alphaproteobacterium Rhodopseudomonas palustris encodes multiple proteins that can reduce Fd, the FixABCX complex is the only one shown to support nitrogen fixation, and R. palustris Fix^- mutants grow poorly under nitrogen-fixing conditions. To investigate how native electron transfer chains (ETCs) can be redirected toward nitrogen fixation, we leveraged the strong selective pressure of nitrogen limitation to isolate a suppressor of an R. palustris ΔfixC strain that grows under nitrogen-fixing conditions. We found two mutations were required to restore growth under nitrogen-fixing conditions in the absence of functional FixABCX. One mutation was in the gene encoding the primary Fd involved in nitrogen fixation, fer1, and the other mutation was in aadN, which encodes a homolog of NAD⁺-dependent Fd:NADPH oxidoreductase (Nfn). We present evidence that AadN plays a role in electron transfer to benzoyl coenzyme A reductase, the key enzyme involved in anaerobic aromatic compound degradation. Our data support a model where the ETC for anaerobic aromatic compound degradation was repurposed to support nitrogen fixation in the ΔfixC suppressor strain. IMPORTANCE There is increasing evidence that protein electron carriers like Fd evolved to form specific partnerships with select electron donors and acceptors to keep native electron transfer pathways insulated from one another. This makes it challenging to integrate a Fd-dependent pathway such as biological nitrogen fixation into non-nitrogen-fixing organisms and provide the high-energy reducing power needed to fix nitrogen. Here, we show that amino acid substitutions in an electron donor for anaerobic aromatic compound degradation and an Fd involved in nitrogen fixation enabled electron transfer to nitrogenase. This study provides a model system to understand electron transfer chain specificity and how new electron transfer pathways can be evolved for biotechnologically valuable pathways like nitrogen fixation.

Contrasting Health Effects of Bacteroidetes and Firmicutes Lies in Their Genomes: Analysis of P450s, Ferredoxins, and Secondary Metabolite Clusters

Species belonging to the bacterial phyla Bacteroidetes and Firmicutes represent over 90% of the gastrointestinal microbiota. Changes in the ratio of these two bacterial groups were found to have contrasting health effects, including obesity and inflammatory diseases. Despite the availability of many bacterial genomes, comparative genomic studies on the gene pools of these two bacterial groups concerning cytochrome P450 monooxygenases (P450s), ferredoxins, and secondary metabolite biosynthetic gene clusters (smBGCs) are not reported. This study is aimed to address this research gap. The study revealed the presence of diverse sets of P450s, ferredoxins, and smBGCs in their genomes. Bacteroidetes species have the highest number of P450 families, ferredoxin cluster-types, and smBGCs compared to Firmicutes species. Only four P450 families, three ferredoxin cluster types, and five smBGCs are commonly shared between these two bacterial groups. Considering the above facts, we propose that the contrasting effects of these two bacterial groups on the host are partly due to the distinct nature of secondary metabolites produced by these organisms. Thus, the cause of the contrasting health effects of these two bacterial groups lies in their gene pools.

Diversification of Ferredoxins across Living Organisms

Ferredoxins, iron-sulfur (Fe-S) cluster proteins, play a key role in oxidoreduction reactions. To date, evolutionary analysis of these proteins across the domains of life have been confined to observing the abundance of Fe-S cluster types (2Fe-2S, 3Fe-4S, 4Fe-4S, 7Fe-8S (3Fe-4s and 4Fe-4S) and 2[4Fe-4S]) and the diversity of ferredoxins within these cluster types was not studied. To address this research gap, here we propose a subtype classification and nomenclature for ferredoxins based on the characteristic spacing between the cysteine amino acids of the Fe-S binding motif as a subtype signature to assess the diversity of ferredoxins across the living organisms. To test this hypothesis, comparative analysis of ferredoxins between bacterial groups, Alphaproteobacteria and Firmicutes and ferredoxins collected from species of different domains of life that are reported in the literature has been carried out. Ferredoxins were found to be highly diverse within their types. Large numbers of alphaproteobacterial species ferredoxin subtypes were found in Firmicutes species and the same ferredoxin subtypes across the species of Bacteria, Archaea, and Eukarya, suggesting shared common ancestral origin of ferredoxins between Archaea and Bacteria and lateral gene transfer of ferredoxins from prokaryotes (Archaea/Bacteria) to eukaryotes. This study opened new vistas for further analysis of diversity of ferredoxins in living organisms.

Parsing redox potentials of five ferredoxins found within Thermotoga maritima

Most organisms contain multiple soluble protein-based redox carriers such as members of the ferredoxin (Fd) family, that contain one or more iron-sulfur clusters. The potential redundancy of Fd proteins is poorly understood, particularly in connection to the ability of Fd proteins to deliver reducing equivalents to members of the "radical SAM," or S-adenosylmethionine radical enzyme (ARE) superfamily, where the activity of all known AREs requires that an essential iron-sulfur cluster bound by the enzyme be reduced to the catalytically relevant [Fe₄ S₄ ]¹⁺ oxidation state. As it is still unclear whether a single Fd in a given organism is specific to individual redox partners, we have examined the five Fd proteins found within Thermotoga maritima via direct electrochemistry, to compare them in a side-by-side fashion for the first time. While a single [Fe₄ S₄ ]-cluster bearing Fd (TM0927) has a potential of -420 mV, the other four 2x[Fe₄ S₄ ]-bearing Fds (TM1175, TM1289, TM1533, and TM1815) have potentials that vary significantly, including cases where the two clusters of the same Fd are essentially coincident (e.g., TM1175) and those where the potentials are well separate (TM1815).

Identification and Characterization of Genes Required for 5-Hydroxyuridine Synthesis in Bacillus subtilis and Escherichia coli tRNA

In bacteria, tRNAs that decode 4-fold degenerate family codons and have uridine at position 34 of the anticodon are typically modified with either 5-methoxyuridine (mo⁵U) or 5-methoxycarbonylmethoxyuridine (mcmo⁵U). These modifications are critical for extended recognition of some codons at the wobble position. Whereas the alkylation steps of these modifications have been described, genes required for the hydroxylation of U34 to give 5-hydroxyuridine (ho⁵U) remain unknown. Here, a number of genes in Escherichia coli and Bacillus subtilis are identified that are required for wild-type (wt) levels of ho⁵U. The yrrMNO operon is identified in B. subtilis as important for the biosynthesis of ho⁵U. Both yrrN and yrrO are homologs to peptidase U32 family genes, which includes the rlhA gene required for ho⁵C synthesis in E. coli Deletion of either yrrN or yrrO, or both, gives a 50% reduction in mo⁵U tRNA levels. In E. coli, yegQ was found to be the only one of four peptidase U32 genes involved in ho⁵U synthesis. Interestingly, this mutant shows the same 50% reduction in (m)cmo⁵U as that observed for mo⁵U in the B. subtilis mutants. By analyzing the genomic context of yegQ homologs, the ferredoxin YfhL is shown to be required for ho⁵U synthesis in E. coli to the same extent as yegQ Additional genes required for Fe-S biosynthesis and biosynthesis of prephenate give the same 50% reduction in modification. Together, these data suggest that ho⁵U biosynthesis in bacteria is similar to that of ho⁵C, but additional genes and substrates are required for complete modification.IMPORTANCE Modified nucleotides in tRNA serve to optimize both its structure and function for accurate translation of the genetic code. The biosynthesis of these modifications has been fertile ground for uncovering unique biochemistry and metabolism in cells. In this work, genes that are required for a novel anaerobic hydroxylation of uridine at the wobble position of some tRNAs are identified in both Bacillus subtilis and Escherichia coli These genes code for Fe-S cluster proteins, and their deletion reduces the levels of the hydroxyuridine by 50% in both organisms. Additional genes required for Fe-S cluster and prephenate biosynthesis and a previously described ferredoxin gene all display a similar reduction in hydroxyuridine levels, suggesting that still other genes are required for the modification.

Structure and Function of a Bacterial Microcompartment Shell Protein Engineered to Bind a [4Fe-4S] Cluster

Bacterial microcompartments (BMCs) are self-assembling organelles composed of a selectively permeable protein shell and encapsulated enzymes. They are considered promising templates for the engineering of designed bionanoreactors for biotechnology. In particular, encapsulation of oxidoreductive reactions requiring electron transfer between the lumen of the BMC and the cytosol relies on the ability to conduct electrons across the shell. We determined the crystal structure of a component protein of a synthetic BMC shell, which informed the rational design of a [4Fe-4S] cluster-binding site in its pore. We also solved the structure of the [4Fe-4S] cluster-bound, engineered protein to 1.8 Å resolution, providing the first structure of a BMC shell protein containing a metal center. The [4Fe-4S] cluster was characterized by optical and EPR spectroscopies; it has a reduction potential of -370 mV vs the standard hydrogen electrode (SHE) and is stable through redox cycling. This remarkable stability may be attributable to the hydrogen-bonding network provided by the main chain of the protein scaffold. The properties of the [4Fe-4S] cluster resemble those in low-potential bacterial ferredoxins, while its ligation to three cysteine residues is reminiscent of enzymes such as aconitase and radical S-adenosymethionine (SAM) enzymes. This engineered shell protein provides the foundation for conferring electron-transfer functionality to BMC shells.

Electrochemical Resolution of the [4Fe-4S] Centers of the AdoMet Radical Enzyme BtrN: Evidence of Proton Coupling and an Unusual, Low-Potential Auxiliary Cluster

The S-adenosylmethionine (AdoMet) radical superfamily of enzymes includes over 113,500 unique members, each of which contains one indispensable iron-sulfur (FeS) cluster that is required to generate a 5'-deoxyadenosyl 5'-radical intermediate during catalysis. Enzymes within several subgroups of the superfamily, however, have been found to contain one or more additional FeS clusters. While these additional clusters are absolutely essential for enzyme activity, their exact roles in the function and/or mechanism of action of many of the enzymes are at best speculative, indicating a need to develop methods to characterize and study these clusters in more detail. Here, BtrN, an AdoMet radical dehydrogenase that catalyzes the two-electron oxidation of 2-deoxy-scyllo-inosamine to amino-dideoxy-scyllo-inosose, an intermediate in the biosynthesis of 2-deoxystreptamine antibiotics, is examined through direct electrochemistry, where the potential of both its AdoMet radical and auxiliary [4Fe-4S] clusters can be measured simultaneously. We find that the AdoMet radical cluster exhibits a midpoint potential of -510 mV, while the auxiliary cluster exhibits a midpoint potential of -765 mV, to our knowledge the lowest [4Fe-4S](2+/+) potential to be determined to date. The impact of AdoMet binding and the pH dependence of catalysis are also quantitatively observed. These data show that direct electrochemical methods can be used to further elucidate the chemistry of the burgeoning AdoMet radical superfamily in the future.

Heterometallic [AgFe(3)S (4)] ferredoxin variants: synthesis, characterization, and the first crystal structure of an engineered heterometallic iron-sulfur protein

Heterometallic [AgFe(3)S(4)] iron-sulfur clusters assembled in wild-type Pyrococcus furiosus ferredoxin and two variants, D14C and D14H, are characterized. The crystal structure of the [AgFe(3)S(4)] D14C variant shows that the silver(I) ion is indeed part of the cluster and is coordinated to the thiolate group of residue 14. Cyclic voltammetry shows one redox pair with a reduction potential of +220 mV versus the standard hydrogen electrode which is assigned to the [AgFe(3)S(4)](2+/+) couple. The oxidized form of the [AgFe(3)S(4)] D14C variant is stable in the presence of dioxygen, whereas the oxidized forms of the [AgFe(3)S(4)] wild type and D14H variants convert to the [Fe(3)S(4)] ferredoxin form. The monovalent d (10) silver(I) ion stabilizes the [Fe(3)S(4)](+/0) cluster fragment, as opposed to divalent d (10) metal ions, resulting in more than 0.4 V difference in reduction potentials between the silver(I) and, e.g., zinc(II) heterometallic [MFe(3)S(4)] ferredoxins. The trend in reduction potentials for the variants containing the [AgFe(3)S(4)] cluster is wild type ≤ D14C < D14H and shows the same trend as reported for the variants containing the [Fe(3)S(4)] cluster, but is different from the D14C < D14H < wild type trend reported for the [Fe(4)S(4)] ferredoxin. The similarity in the reduction potential trend for the variants containing the heterometallic [AgFe(3)S(4)] cluster and the [Fe(3)S(4)] cluster can be rationalized in terms of the electrostatic influence of the residue 14 side chains, rather than the dissociation constant of this residue, as is the case for [Fe(4)S(4)] ferredoxins. The trends in reduction potentials are in line with there being no electronic coupling between the silver(I) ion and the Fe(3)S(4) fragment.

Structural models of the [Fe4S4] clusters of homologous nitrogenase Fe proteins

The iron (Fe) proteins of molybdenum (Mo)-, vanadium (V)-, and iron (Fe)-only nitrogenases are encoded by nifH, vnfH, and anfH, respectively. While the nifH-encoded Fe protein has been extensively studied over recent years, information regarding the properties of the vnfH- and anfH-encoded Fe proteins has remained scarce. Here, we present a combined biochemical, electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) analysis of the [Fe(4)S(4)] clusters of NifH, VnfH, and AnfH of Azotobacter vinelandii . Our data show that all three Fe proteins contain [Fe(4)S(4)] clusters of very similar spectroscopic and geometric structural properties, although NifH differs more from VnfH and AnfH with regard to the electronic structure. These observations have an interesting impact on the theory of the plausible sequence of evolution of nitrogenase Fe proteins. More importantly, the results presented herein provide a platform for future investigations of the differential activities of the three Fe proteins in nitrogenase biosynthesis and catalysis.

Experimental and theoretical Mössbauer study of an extended family of [Fe8(μ4-O)4(μ-4-R-px)12X4] clusters

Six [Fe(8)(μ(4)-O)(4)(μ-4-R-pyrazolato)(12)X(4)] complexes containing an identical Fe(8)(μ(4)-O)(4) core have been structurally characterized and studied by Mössbauer spectroscopy. In each case, an inner μ(4)-O bridged Fe(III) cubane core is surrounded by four trigonal bipyramidal iron centers, the two distinct sites occurring in a 1:1 ratio. The Mössbauer spectrum of each of the clusters consists of two quadrupole doublets, which, with one exception (X = NCS, R = H), overlap to give three absorption lines. The systematic variation of X and R causes significant changes in the Mössbauer spectra. A comparison with values for the same clusters computed using density functional theory allows us to establish an unequivocal assignment of these peaks in terms of a nested model for the overlapping doublets. The changes in Mössbauer parameters (both experimental and computed) for the 1-electron reduced species [Fe(8)(μ(4)-O)(4)(μ-4-Cl-pyrazolato)(12)Cl(4)](-) are consistent with a redox event that is localized within the cubane core.

A bacteria-specific 2[4Fe-4S] ferredoxin is essential in Pseudomonas aeruginosa

Background

Ferredoxins are small iron-sulfur proteins belonging to all domains of life. A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials. These unusual properties are associated with two specific fragments of sequence. The functional importance of the very low potential ferredoxins is unknown.

Results

A bioinformatic screening of the sequence features defining very low potential 2[4Fe-4S] ferredoxins has revealed the almost exclusive presence of the corresponding fdx gene in the Proteobacteria phylum, without occurrence in Archaea and Eukaryota. The transcript was found to be monocistronic in Pseudomonas aeruginosa, and not part of an operon in most bacteria. Only fdx genes of bacteria which anaerobically degrade aromatic compounds belong to operons. As this pathway is not present in all bacteria having very low potential 2[4Fe-4S] ferredoxins, these proteins cannot exclusively be reductants of benzoyl CoA reductases. Expression of the ferredoxin gene did not change in response to varying growth conditions, including upon macrophage infection or aerobic growth with 4-hydroxy benzoate as carbon source. However, it increased along the growth curve in Pseudomonas aeruginosa and in Escherichia coli. The sequence immediately 5' upstream of the coding sequence contributed to the promotor activity. Deleting the fdx gene in Pseudomonas aeruginosa abolished growth, unless a plasmid copy of the gene was provided to the deleted strain.

Conclusions

The gene of the very low potential 2[4Fe-4S] ferredoxin displays characteristics of a housekeeping gene, and it belongs to the minority of genes that are essential in Pseudomonas aeruginosa. These data identify a new potential antimicrobial target in this and other pathogenic Proteobacteria.