Desulfoferrodoxin of Clostridium acetobutylicum functions as a superoxide reductase

Clostridioides difficile superoxide reductase mitigates oxygen sensitivity

Clostridioides difficile causes a serious diarrheal disease and is a common healthcare-associated bacterial pathogen. Although it has a major impact on human health, the mechanistic details of C. difficile intestinal colonization remain undefined. C. difficile is highly sensitive to oxygen and requires anaerobic conditions for in vitro growth. However, the mammalian gut is not devoid of oxygen, and C. difficile tolerates moderate oxidative stress in vivo. The C. difficile genome encodes several antioxidant proteins, including a predicted superoxide reductase (SOR) that is upregulated upon exposure to antimicrobial peptides. The goal of this study was to establish SOR enzymatic activity and assess its role in protecting C. difficile against oxygen exposure. Insertional inactivation of sor rendered C. difficile more sensitive to superoxide, indicating that SOR contributes to antioxidant defense. Heterologous C. difficile sor expression in Escherichia coli conferred protection against superoxide-dependent growth inhibition, and the corresponding cell lysates showed superoxide scavenging activity. Finally, a C. difficile SOR mutant exhibited global proteome changes under oxygen stress when compared to the parent strain. Collectively, our data establish the enzymatic activity of C. difficile SOR, confirm its role in protection against oxidative stress, and demonstrate SOR's broader impacts on the C. difficile vegetative cell proteome.IMPORTANCEClostridioides difficile is an important pathogen strongly associated with healthcare settings and capable of causing severe diarrheal disease. While considered a strict anaerobe in vitro, C. difficile has been shown to tolerate low levels of oxygen in the mammalian host. Among other well-characterized antioxidant proteins, the C. difficile genome encodes a predicted superoxide reductase (SOR), an understudied component of antioxidant defense in pathogens. The significance of the research reported herein is the characterization of SOR's enzymatic activity, including confirmation of its role in protecting C. difficile against oxidative stress. This furthers our understanding of C. difficile pathogenesis and presents a potential new avenue for targeted therapies.

Clostridium cellulovorans Proteomic Responses to Butanol Stress

Combination of butanol-hyperproducing and hypertolerant phenotypes is essential for developing microbial strains suitable for industrial production of bio-butanol, one of the most promising liquid biofuels. Clostridium cellulovorans is among the microbial strains with the highest potential for direct production of n-butanol from lignocellulosic wastes, a process that would significantly reduce the cost of bio-butanol. However, butanol exhibits higher toxicity compared to ethanol and C. cellulovorans tolerance to this solvent is low. In the present investigation, comparative gel-free proteomics was used to study the response of C. cellulovorans to butanol challenge and understand the tolerance mechanisms activated in this condition. Sequential Window Acquisition of all Theoretical fragment ion spectra Mass Spectrometry (SWATH-MS) analysis allowed identification and quantification of differentially expressed soluble proteins. The study data are available via ProteomeXchange with the identifier PXD024183. The most important response concerned modulation of protein biosynthesis, folding and degradation. Coherent with previous studies on other bacteria, several heat shock proteins (HSPs), involved in protein quality control, were up-regulated such as the chaperones GroES (Cpn10), Hsp90, and DnaJ. Globally, our data indicate that protein biosynthesis is reduced, likely not to overload HSPs. Several additional metabolic adaptations were triggered by butanol exposure such as the up-regulation of V- and F-type ATPases (involved in ATP synthesis/generation of proton motive force), enzymes involved in amino acid (e.g., arginine, lysine, methionine, and branched chain amino acids) biosynthesis and proteins involved in cell envelope re-arrangement (e.g., the products of Clocel_4136, Clocel_4137, Clocel_4144, Clocel_4162 and Clocel_4352, involved in the biosynthesis of saturated fatty acids) and a redistribution of carbon flux through fermentative pathways (acetate and formate yields were increased and decreased, respectively). Based on these experimental findings, several potential gene targets for metabolic engineering strategies aimed at improving butanol tolerance in C. cellulovorans are suggested. This includes overexpression of HSPs (e.g., GroES, Hsp90, DnaJ, ClpC), RNA chaperone Hfq, V- and F-type ATPases and a number of genes whose function in C. cellulovorans is currently unknown.

Role of Superoxide Reductase FA796 in Oxidative Stress Resistance in Filifactor alocis

Filifactor alocis, a Gram-positive anaerobic bacterium, is now a proposed diagnostic indicator of periodontal disease. Because the stress response of this bacterium to the oxidative environment of the periodontal pocket may impact its pathogenicity, an understanding of its oxidative stress resistance strategy is vital. Interrogation of the F. alocis genome identified the HMPREF0389_00796 gene that encodes for a putative superoxide reductase (SOR) enzyme. SORs are non-heme, iron-containing enzymes that can catalyze the reduction of superoxide radicals to hydrogen peroxide and are important in the protection against oxidative stress. In this study, we have functionally characterized the putative SOR (FA796) from F. alocis ATCC 35896. The recombinant FA796 protein, which is predicted to be a homotetramer of the 1Fe-SOR class, can reduce superoxide radicals. F. alocis FLL141 (∆FA796::ermF) was significantly more sensitive to oxygen/air exposure compared to the parent strain. Sensitivity correlated with the level of intracellular superoxide radicals. Additionally, the FA796-defective mutant had increased sensitivity to hydrogen peroxide-induced stress, was inhibited in its ability to form biofilm and had reduced survival in epithelial cells. Collectively, these results suggest that the F. alocis SOR protein is a key enzymatic scavenger of superoxide radicals and protects the bacterium from oxidative stress conditions.

How to define obligatory anaerobiosis? An evolutionary view on the antioxidant response system and the early stages of the evolution of life on Earth

One of the former definitions of "obligate anaerobiosis" was based on three main criteria: 1) it occurs in organisms, so-called obligate anaerobes, which live in environments without oxygen (O₂), 2) O₂-dependent (aerobic) respiration, and 3) antioxidant enzymes are absent in obligate anaerobes. In contrast, aerobes need O₂ in order to grow and develop properly. Obligate (or strict) anaerobes belong to prokaryotic microorganisms from two domains, Bacteria and Archaea. A closer look at anaerobiosis covers a wide range of microorganisms that permanently or in a time-dependent manner tolerate different concentrations of O₂ in their habitats. On this basis they can be classified as obligate/facultative anaerobes, microaerophiles and nanaerobes. Paradoxically, O₂ tolerance in strict anaerobes is usually, as in aerobes, associated with the activity of the antioxidant response system, which involves different antioxidant enzymes responsible for removing excess reactive oxygen species (ROS). In our opinion, the traditional definition of "obligate anaerobiosis" loses its original sense. Strict anaerobiosis should only be restricted to the occurrence of O₂-independent pathways involved in energy generation. For that reason, a term better than "obligate anaerobes" would be O₂/ROS tolerant anaerobes, where the role of the O₂/ROS detoxification system is separated from O₂-independent metabolic pathways that supply energy. Ubiquitous key antioxidant enzymes like superoxide dismutase (SOD) and superoxide reductase (SOR) in contemporary obligate anaerobes might suggest that their origin is ancient, maybe even the beginning of the evolution of life on Earth. It cannot be ruled out that c. 3.5 Gyr ago, local microquantities of O₂/ROS played a role in the evolution of the last universal common ancestor (LUCA) of all modern organisms. On the basis of data in the literature, the hypothesis that LUCA could be an O₂/ROS tolerant anaerobe is discussed together with the question of the abiotic sources of O₂/ROS and/or the early evolution of cyanobacteria that perform oxygenic photosynthesis.

Functional prediction, characterization, and categorization of operome from Acetoanaerobium sticklandii DSM 519

Acetoanaerobium sticklandii DSM 519 is a hyper-ammonia producing anaerobic bacterium that can be able utilizes amino acids as sole carbon and energy sources for its growth and energetic metabolism. A lack of knowledge on its molecular machinery and 30.5% conserved hypothetical proteins (HPs; operome) hinders the successful utility in biofuel applications. In this study, we have predicted, characterized and categorized its operome whose functions are still not determined accurately using a combined bioinformatics approach. The functions of 64 of the 359 predicted HPs are involved in diverse metabolic subsystems. A. sticklandii operome has consisted of 16% Rossmann fold and 46% miscellaneous folds. Subsystems-based technology has classified 51 HPs contributing to the small-molecular reactions, 26 in macromolecular reactions and 12 in the biosynthesis of cofactors, prosthetic groups and electron carriers. A generality of functions predicted from its operome contributed to the cell cycle, amino acid metabolism, membrane transport, and regulatory processes. Many of them have duplicated functions as paralogs in this genome. A. sticklandii has the ability to compete with invading microorganisms and tolerate abiotic stresses, which can be overwhelmed by the predicted functions of its operome. Results of this study revealed that it has specialized systems for amino acid catabolism-directed solventogenesis and acidogenesis but the level of gene expression may determine the metabolic function in amino acid fermenting niches in the rumina of cattle. As shown by our analysis, the predicted functions of its operome allow us for a better understanding of the growth and physiology at systems-scale.

Electron Bifurcation: A Long-Hidden Energy-Coupling Mechanism

A decade ago, a novel mechanism to drive thermodynamically unfavorable redox reactions was discovered that is used in prokaryotes to drive endergonic electron transfer reactions by a direct coupling to an exergonic redox reaction in one soluble enzyme complex. This process is referred to as flavin-based electron bifurcation, or FBEB. An important function of FBEB is that it allows the generation of reduced low-potential ferredoxin (Fd_red) from comparably high-potential electron donors such as NADH or molecular hydrogen (H₂). Fd_red is then the electron donor for anaerobic respiratory chains leading to the synthesis of ATP. In many metabolic scenarios, Fd is reduced by metabolic oxidoreductases and Fd_red then drives endergonic metabolic reactions such as H₂ production by the reverse, electron confurcation. FBEB is energetically more economical than ATP hydrolysis or reverse electron transport as a driving force for endergonic redox reactions; thus, it does "save" cellular ATP. It is essential for autotrophic growth at the origin of life and also allows for heterotrophic growth on certain low-energy substrates.

Quantitative proteome profiles help reveal efficient xylose utilization mechanisms in solventogenic Clostridium sp. strain BOH3

Development of sustainable biobutanol production platforms from lignocellulosic materials is impeded by inefficient five carbon sugar uptake by solventogenic bacteria. The recently isolated Clostridium sp. strain BOH3 is particularly advantaged in this regard as it serves as a model organism which can simultaneously utilize both glucose and xylose for high butanol (>15 g/L) production. Strain BOH3 was, therefore, investigated for its metabolic mechanisms for efficient five carbon sugar uptake using a quantitative proteomics based approach. The proteomics data show that proteins within the CAC1341-1349 operon play a pivotal role for efficient xylose uptake within the cells to produce butanol. Furthermore, up-regulation of key enzymes within the riboflavin synthesis pathway explained that xylose could induce higher riboflavin production capability of the bacteria (e.g., ∼80 mg/L from glucose vs. ∼120 mg/L from xylose). Overall results from the present experimental approach indicated that xylose-fed BOH3 cultures are subjected to high levels of redox stress which coupled with the solvent stress-trigger a sporulation response within the cells earlier than the glucose-fed cultures. The study lays the platform for metabolic engineering strategies in designing organisms for efficient butanol and other value-added chemicals such as riboflavin production. Biotechnol. Bioeng. 2017;114: 1959-1969. © 2017 Wiley Periodicals, Inc.

The Crystal Structure of Peroxiredoxin Asp f3 Provides Mechanistic Insight into Oxidative Stress Resistance and Virulence of Aspergillus fumigatus

Invasive aspergillosis and other fungal infections occur in immunocompromised individuals, including patients who received blood-building stem cell transplants, patients with chronic granulomatous disease (CGD), and others. Production of reactive oxygen species (ROS) by immune cells, which incidentally is defective in CGD patients, is considered to be a fundamental process in inflammation and antifungal immune response. Here we show that the peroxiredoxin Asp f3 of Aspergillus fumigatus inactivates ROS. We report the crystal structure and the catalytic mechanism of Asp f3, a two-cysteine type peroxiredoxin. The latter exhibits a thioredoxin fold and a homodimeric structure with two intermolecular disulfide bonds in its oxidized state. Replacement of the Asp f3 cysteines with serine residues retained its dimeric structure, but diminished Asp f3's peroxidase activity, and extended the alpha-helix with the former peroxidatic cysteine residue C61 by six residues. The asp f3 deletion mutant was sensitive to ROS, and this phenotype was rescued by ectopic expression of Asp f3. Furthermore, we showed that deletion of asp f3 rendered A. fumigatus avirulent in a mouse model of pulmonary aspergillosis. The conserved expression of Asp f3 homologs in medically relevant molds and yeasts prompts future evaluation of Asp f3 as a potential therapeutic target.

An anaerobic bacterium, Bacteroides thetaiotaomicron, uses a consortium of enzymes to scavenge hydrogen peroxide

Obligate anaerobes are periodically exposed to oxygen, and it has been conjectured that on such occasions their low-potential biochemistry will predispose them to rapid ROS formation. We sought to identify scavenging enzymes that might protect the anaerobe Bacteroides thetaiotaomicron from the H2 O2 that would be formed. Genetic analysis of eight candidate enzymes revealed that four of these scavenge H2 O2  in vivo: rubrerythrins 1 and 2, AhpCF, and catalase E. The rubrerythrins served as key peroxidases under anoxic conditions. However, they quickly lost activity upon aeration, and AhpCF and catalase were induced to compensate. The AhpCF is an NADH peroxidase that effectively degraded low micromolar levels of H2 O2 , while the catalytic cycle of catalase enabled it to quickly degrade higher concentrations that might arise from exogenous sources. Using a non-scavenging mutant we verified that endogenous H2 O2 formation was much higher in aerated B. thetaiotaomicron than in Escherichia coli. Indeed, the OxyR stress response to H2 O2 was induced when B. thetaiotaomicron was aerated, and in that circumstance this response was necessary to forestall cell death. Thus aeration is a serious threat for this obligate anaerobe, and to cope it employs a set of defences that includes a repertoire of complementary scavenging enzymes.

CAC2634-disrupted mutant of Clostridium acetobutylicum can be electrotransformed in air

AIMS

To simplify the electrotransformation process of Clostridium acetobutylicum, which currently needs to be performed in an anaerobic chamber, thus laborious and time-consuming.

METHODS AND RESULTS

The CAC2634 gene encoding PerR is a known peroxide regulon repressor in Cl. acetobutylicum. CAC2634 in a previously constructed Restriction-Modification system deficient Cl. acetobutylicum mutant SMB009 was disrupted using ClosTron method. The resulted mutant SMB012 can be electrotransformed in air with an efficiency of 1.2-3.1 × 10(3) transformants μg(-1) DNA.

CONCLUSIONS

We demonstrated that the disruption of CAC2634 in Cl. acetobutylicum enables its electrotransformation in air.

SIGNIFICANCE AND IMPACT OF THE STUDY

The electrotransformation process of Cl. acetobutylicum could be significantly simplified, especially when operating multiple electrotransformations.

SORGOdb: Superoxide Reductase Gene Ontology curated DataBase

BACKGROUND

Superoxide reductases (SOR) catalyse the reduction of superoxide anions to hydrogen peroxide and are involved in the oxidative stress defences of anaerobic and facultative anaerobic organisms. Genes encoding SOR were discovered recently and suffer from annotation problems. These genes, named sor, are short and the transfer of annotations from previously characterized neelaredoxin, desulfoferrodoxin, superoxide reductase and rubredoxin oxidase has been heterogeneous. Consequently, many sor remain anonymous or mis-annotated.

DESCRIPTION

SORGOdb is an exhaustive database of SOR that proposes a new classification based on domain architecture. SORGOdb supplies a simple user-friendly web-based database for retrieving and exploring relevant information about the proposed SOR families. The database can be queried using an organism name, a locus tag or phylogenetic criteria, and also offers sequence similarity searches using BlastP. Genes encoding SOR have been re-annotated in all available genome sequences (prokaryotic and eukaryotic (complete and in draft) genomes, updated in May 2010).

CONCLUSIONS

SORGOdb contains 325 non-redundant and curated SOR, from 274 organisms. It proposes a new classification of SOR into seven different classes and allows biologists to explore and analyze sor in order to establish correlations between the class of SOR and organism phenotypes. SORGOdb is freely available at http://sorgo.genouest.org/index.php.

Engineering the robustness of Clostridium acetobutylicum by introducing glutathione biosynthetic capability

To improve the aero- and solvent tolerance of the solvent-producing Clostridium acetobutylicum, glutathione biosynthetic capability was introduced into C. acetobutylicum DSM1731 by cloning and over-expressing the gshAB genes from Escherichia coli. Strain DSM1731(pITAB) produces glutathione, and shows a significantly improved survival upon aeration and butanol challenge, as compared with the control. In addition, strain DSM1731(pITAB) exhibited an improved butanol tolerance and an increased butanol production capability, as compared with the recombinant strains with only gshA or gshB gene. These results illustrated that introducing glutathione biosynthetic pathway, which is redundant for the metabolism of C. acetobutylicum, can increase the robustness of the host to achieve a better solvent production.

Methanoferrodoxin represents a new class of superoxide reductase containing an iron-sulfur cluster

Protein MM0632 from the methanogenic archaeon Methanosarcina mazei showed strong superoxide reductase activity and rapidly decomposed superoxide radicals to peroxides. The superoxide reductase activity of the heterologously produced enzyme was determined by a cytochrome c assay and in a test system with NADPH, ferredoxin:NADP(+) reductase, and rubredoxin. Furthermore, EPR spectroscopy showed that MM0632 is the first superoxide reductase that possesses an iron-sulfur cluster instead of a second mononuclear iron center. We propose the name methanoferrodoxin for this new class of superoxide reductase with an [Fe(NHis)(4)(SCys)] site as the catalytic center and a [4Fe-4S] cluster as second prosthetic group that is probably involved in electron transfer to the catalytic center. Methanosarcina mazei grows only under anaerobic conditions, but is one of the most aerotolerant methanogens. It is tempting to speculate that methanoferrodoxin contributes to the protection of cells from oxygen radicals formed by flavoproteins during periodic exposure to oxygen in natural environments.

Global regulation of gene expression in response to cysteine availability in Clostridium perfringens

Background

Cysteine has a crucial role in cellular physiology and its synthesis is tightly controlled due to its reactivity. However, little is known about the sulfur metabolism and its regulation in clostridia compared with other firmicutes. In Clostridium perfringens, the two-component system, VirR/VirS, controls the expression of the ubiG operon involved in methionine to cysteine conversion in addition to the expression of several toxin genes. The existence of links between the C. perfringens virulence regulon and sulfur metabolism prompted us to analyze this metabolism in more detail.

Results

We first performed a tentative reconstruction of sulfur metabolism in C. perfringens and correlated these data with the growth of strain 13 in the presence of various sulfur sources. Surprisingly, C. perfringens can convert cysteine to methionine by an atypical still uncharacterized pathway. We further compared the expression profiles of strain 13 after growth in the presence of cystine or homocysteine that corresponds to conditions of cysteine depletion. Among the 177 genes differentially expressed, we found genes involved in sulfur metabolism and controlled by premature termination of transcription via a cysteine specific T-box system (cysK-cysE, cysP1 and cysP2) or an S-box riboswitch (metK and metT). We also showed that the ubiG operon was submitted to a triple regulation by cysteine availability via a T-box system, by the VirR/VirS system via the VR-RNA and by the VirX regulatory RNA.

In addition, we found that expression of pfoA (theta-toxin), nagL (one of the five genes encoding hyaluronidases) and genes involved in the maintenance of cell redox status was differentially expressed in response to cysteine availability. Finally, we showed that the expression of genes involved in [Fe-S] clusters biogenesis and of the ldh gene encoding the lactate dehydrogenase was induced during cysteine limitation.

Conclusion

Several key functions for the cellular physiology of this anaerobic bacterium were controlled in response to cysteine availability. While most of the genes involved in sulfur metabolism are regulated by premature termination of transcription, other still uncharacterized mechanisms of regulation participated in the induction of gene expression during cysteine starvation.

Enhanced superoxide and hydrogen peroxide detection in biological assays

Superoxide reductase (SOR) is an enzyme that converts superoxide into hydrogen peroxide at a twofold higher yield than canonical superoxide dismutases (SOD). Superoxide radical detection was investigated using the Amplex red (AR)/peroxidase system to measure the difference in hydrogen peroxide production yield in the presence of SOR or SOD. We found that reduced SOR reacts with the AR oxidation intermediate, a one-electron reduced AR(*) radical, by reducing this intermediate back to the initial AR leuco compound. Ascorbate also quenched this radical in a concentration-dependent manner and could be used to compete efficiently with SOR; at concentrations of ascorbate higher than 5 microM, SOR no longer interfered with the detection of H(2)O(2). By using xanthine/xanthine oxidase as a superoxide-generating system, it was possible to successfully quantify superoxide and hydrogen peroxide in vitro using the AR/peroxidase/SOR system, either by visible absorption or by fluorescence emission, with a considerable low detection limit of 10nM/min. The use of enzymes with diffusion-limited reactivity toward superoxide substantially increases specificity and detection threshold for superoxide and turns this approach into a powerful system to detect ROS in biological systems.

The role of PerR in O2-affected gene expression of Clostridium acetobutylicum

In the strict anaerobe Clostridium acetobutylicum, a PerR-homologous protein has recently been identified as being a key repressor of a reductive machinery for the scavenging of reactive oxygen species and molecular O(2). In the absence of PerR, the full derepression of its regulon resulted in increased resistance to oxidative stress and nearly full tolerance of an aerobic environment. In the present study, the complementation of a Bacillus subtilis PerR mutant confirmed that the homologous protein from C. acetobutylicum acts as a functional peroxide sensor in vivo. Furthermore, we used a transcriptomic approach to analyze gene expression in the aerotolerant PerR mutant strain and compared it to the O(2) stimulon of wild-type C. acetobutylicum. The genes encoding the components of the alternative detoxification system were PerR regulated. Only few other targets of direct PerR regulation were identified, including two highly expressed genes encoding enzymes that are putatively involved in the central energy metabolism. All of them were highly induced when wild-type cells were exposed to sublethal levels of O(2). Under these conditions, C. acetobutylicum also activated the repair and biogenesis of DNA and Fe-S clusters as well as the transcription of a gene encoding an unknown CO dehydrogenase-like enzyme. Surprisingly few genes were downregulated when exposed to O(2), including those involved in butyrate formation. In summary, these results show that the defense of this strict anaerobe against oxidative stress is robust and by far not limited to the removal of O(2) and its reactive derivatives.

Pathway for H2O2 and O2 detoxification in Clostridium acetobutylicum

An unusual non-haem diiron protein, reverse rubrerythrin (revRbr), is known to be massively upregulated in response to oxidative stress in the strictly anaerobic bacterium Clostridium acetobutylicum. In the present study both in vivo and in vitro results demonstrate an H2O2 and O2 detoxification pathway in C. acetobutylicum involving revRbr, rubredoxin (Rd) and NADH : rubredoxin oxidoreductase (NROR). RevRbr exhibited both NADH peroxidase (NADH : H2O2 oxidoreductase) and NADH oxidase (NADH : O2 oxidoreductase) activities in in vitro assays using NROR as the electron-transfer intermediary from NADH to revRbr. Rd increased the NADH consumption rate by serving as an intermediary electron-transfer shuttle between NROR and revRbr. While H2O2 was found to be the preferred substrate for revRbr, its relative oxidase activity was found to be significantly higher than that reported for other Rbrs. A revRbr-overexpressing strain of C. acetobutylicum showed significantly increased tolerance to H2O2 and O2 exposure. RevRbr thus appears to protect C. acetobutylicum against oxidative stress by functioning as the terminal component of an NADH peroxidase and NADH oxidase.

O2 and reactive oxygen species detoxification complex, composed of O2-responsive NADH:rubredoxin oxidoreductase-flavoprotein A2-desulfoferrodoxin operon enzymes, rubperoxin, and rubredoxin, in Clostridium acetobutylicum

Clostridium acetobutylicum, an obligate anaerobe, grows normally under continuous-O(2)-flow culture conditions, where the cells consume O(2) proficiently. An O(2)-responsive NADH:rubredoxin oxidoreductase operon composed of three genes (nror, fprA2, and dsr), encoding NROR, functionally uncharacterized flavoprotein A2 (FprA2), and the predicted superoxide reductase desulfoferrodoxin (Dsr), has been proposed to participate in defense against O(2) stress. To functionally characterize these proteins, native NROR from C. acetobutylicum, recombinant NROR (rNROR), FprA2, Dsr, and rubredoxin (Rd) expressed in Escherichia coli were purified. Purified native NROR and rNROR both exhibited weak H(2)O(2)-forming NADH oxidase activity that was slightly activated by Rd. A mixture of NROR, Rd, and FprA2 functions as an efficient H(2)O-forming NADH oxidase with a high affinity for O(2) (the K(m) for O(2) is 2.9 +/- 0.4 microM). A mixture of NROR, Rd, and Dsr functions as an NADH-dependent O(2)(-) reductase. A mixture of NROR, Rd, and rubperoxin (Rpr, a rubrerythrin homologue) functions as an inefficient H(2)O-forming NADH oxidase but an efficient NADH peroxidase with a low affinity for O(2) and a high affinity for H(2)O(2) (the K(m)s for O(2) and H(2)O(2) are 303 +/- 39 microM and <or=1 microM, respectively). A gene encoding Rd is dicistronically transcribed with a gene encoding a glutaredoxin (Gd) homologue, and the expression levels of the genes encoding Gd and Rd were highly upregulated upon exposure to O(2). Therefore, nror operon enzymes, together with Rpr, efficiently function to scavenge O(2), O(2)(-), and H(2)O(2) by using an O(2)-responsive rubredoxin as a common electron carrier protein.

Reductive dioxygen scavenging by flavo-diiron proteins of Clostridium acetobutylicum

Two flavo-diiron proteins (FDPs), FprA1 and FprA2, are up-regulated when the strictly anaerobic solvent producer, Clostridium acetobutylicum, is exposed to dioxygen. These two FDPs were purified following heterologous overexpression in Escherichia coli as N-terminal Strep-tag fusion proteins. The recombinant FprA1 and FprA2 were found to be homodimeric and homotetrameric, respectively, and both FDPs functioned as terminal components of NADH oxidases (NADH:O(2) oxidoreductases) when using C. acetobutylicum NADH:rubredoxin oxidoreductase (NROR) and rubredoxin (Rd) as electron transport intermediaries. Both FDPs catalyzed the four-electron reduction of molecular oxygen to water with similar specific activities. The results are consistent with these FDPs functioning as efficient scavengers of intracellular dioxygen under aerobic or microoxic growth conditions.

How obligatory is anaerobiosis?

Historically many bacteria have been classified as obligate anaerobes. They have been construed as wholly intolerant of oxygen, a feature that was originally ascribed to their lack of superoxide dismutases and catalases. Clostridial species were regarded as classic examples. We now know that this view is quite wrong: enzymes that scavenge superoxide, hydrogen peroxide and even oxygen itself abound in anaerobes. In the current issue of Molecular Microbiology, Hillmann et al. demonstrate that full production of these proteins can allow Clostridium acetobutylicum to survive and even grow in oxygenated culture medium. Evidently, oxidative defences in anaerobes can be robust. In all likelihood, they are critical for the movement of bacteria through aerobic environments to new anaerobic habitats.