Sulfide quinone reductase (SQR) activity in Chlorobium

Photoautotrophic removal of hydrogen sulfide from biogas using purple and green sulfur bacteria

Biogas desulfurization based on anoxygenic photosynthetic processes represents an alternative to physicochemical technologies, decreasing the risk of O₂ and N₂ contamination. This work aimed at assessing the potential of Allochromatium vinosum and Chlorobium limicola for biogas desulfurization under different light intensities (10 and 25 klx) and H₂S concentrations (1 %, 1.5 % and 2 %) in batch photobioreactors. In addition, the influence of rising biogas flow rates (2.9, 5.8 and 11.5 L d^-1 in stage I, II and III, respectively) on the desulfurization performance in a 2.3 L photobioreactor utilizing C. limicola under continuous mode was assessed. The light intensity of 25 klx negatively influenced the growth of A. vinosum and C. limicola, resulting in decreased H₂S removal capacity. An increase in H₂S concentrations resulted in higher volumetric H₂S removal rates in C. limicola (2.9-5.3 mg L^-1 d^-1) tests compared to A. vinosum (2.4-4.6 mg L^-1 d^-1) tests. The continuous photobioreactor completely removed H₂S from biogas in stage I and II. The highest flow rate in stage III induced a deterioration in the desulfurization activity of C. limicola. Overall, the high H₂S tolerance of A. vinosum and C. limicola supports their use in H₂S desulfurization from biogas.

Unravelling microbial drivers of the sulfate-reduction process inside landfill using metagenomics

Hydrogen sulfide (H₂S) is one of the common landfill odor. This research demonstrates that the sulfate transformation behavior is significantly enhanced during the landfill process, accompanied by a shift in microbial structure. The relative abundance of dissimilatory sulfate reduction (DSR) and thiosulfate oxidation by SOX (sulfur-oxidation) complex gradually decreases through the landfill processes while the assimilatory sulfate reduction (ASR) demonstrates the opposite behavior. The major module for landfill sulfate reduction is ASR, accounting for 31.72% ± 2.84% of sulfate metabolism. Based on the functional genes for the sulfate pathway, the drivers for sulfate biotransformation in landfills were determined and further identified their contribution in the sulfate metabolism during landfill processes. Pseudomonas, Methylocaldum, Bacillus, Methylocystis and Hyphomicrobium were the top 5 contributors for ASR pathway, and only one genus Pseudomonas was found for DSR pathway. Among the 26 high-quality metagenome-assembled genomes of sulfate functional species, 24 were considered novel species for sulfuric metabolism. Overall, this study provides unique insight into the sulfate transformation process related to the H₂S odor control in landfill management.

Omics-Inferred Partitioning and Expression of Diverse Biogeochemical Functions in a Low-O2 Cyanobacterial Mat Community

Cyanobacterial mats profoundly influenced Earth's biological and geochemical evolution and still play important ecological roles in the modern world. However, the biogeochemical functioning of cyanobacterial mats under persistent low-O2 conditions, which dominated their evolutionary history, is not well understood. To investigate how different metabolic and biogeochemical functions are partitioned among community members, we conducted metagenomics and metatranscriptomics on cyanobacterial mats in the low-O2, sulfidic Middle Island sinkhole (MIS) in Lake Huron. Metagenomic assembly and binning yielded 144 draft metagenome assembled genomes, including 61 of medium quality or better, and the dominant cyanobacteria and numerous Proteobacteria involved in sulfur cycling. Strains of a Phormidium autumnale-like cyanobacterium dominated the metagenome and metatranscriptome. Transcripts for the photosynthetic reaction core genes psaA and psbA were abundant in both day and night. Multiple types of psbA genes were expressed from each cyanobacterium, and the dominant psbA transcripts were from an atypical microaerobic type of D1 protein from Phormidium. Further, cyanobacterial transcripts for photosystem I genes were more abundant than those for photosystem II, and two types of Phormidium sulfide quinone reductase were recovered, consistent with anoxygenic photosynthesis via photosystem I in the presence of sulfide. Transcripts indicate active sulfur oxidation and reduction within the cyanobacterial mat, predominately by Gammaproteobacteria and Deltaproteobacteria, respectively. Overall, these genomic and transcriptomic results link specific microbial groups to metabolic processes that underpin primary production and biogeochemical cycling in a low-O2 cyanobacterial mat and suggest mechanisms for tightly coupled cycling of oxygen and sulfur compounds in the mat ecosystem. IMPORTANCE Cyanobacterial mats are dense communities of microorganisms that contain photosynthetic cyanobacteria along with a host of other bacterial species that play important yet still poorly understood roles in this ecosystem. Although such cyanobacterial mats were critical agents of Earth's biological and chemical evolution through geological time, little is known about how they function under the low-oxygen conditions that characterized most of their natural history. Here, we performed sequencing of the DNA and RNA of modern cyanobacterial mat communities under low-oxygen and sulfur-rich conditions from the Middle Island sinkhole in Lake Huron. The results reveal the organisms and metabolic pathways that are responsible for both oxygen-producing and non-oxygen-producing photosynthesis as well as interconversions of sulfur that likely shape how much O2 is produced in such ecosystems. These findings indicate tight metabolic reactions between community members that help to explain the limited the amount of O2 produced in cyanobacterial mat ecosystems.

Molecular Physiology of Anaerobic Phototrophic Purple and Green Sulfur Bacteria

There are two main types of bacterial photosynthesis: oxygenic (cyanobacteria) and anoxygenic (sulfur and non-sulfur phototrophs). Molecular mechanisms of photosynthesis in the phototrophic microorganisms can differ and depend on their location and pigments in the cells. This paper describes bacteria capable of molecular oxidizing hydrogen sulfide, specifically the families Chromatiaceae and Chlorobiaceae, also known as purple and green sulfur bacteria in the process of anoxygenic photosynthesis. Further, it analyzes certain important physiological processes, especially those which are characteristic for these bacterial families. Primarily, the molecular metabolism of sulfur, which oxidizes hydrogen sulfide to elementary molecular sulfur, as well as photosynthetic processes taking place inside of cells are presented. Particular attention is paid to the description of the molecular structure of the photosynthetic apparatus in these two families of phototrophs. Moreover, some of their molecular biotechnological perspectives are discussed.

Anoxygenic Photosynthesis in Photolithotrophic Sulfur Bacteria and Their Role in Detoxication of Hydrogen Sulfide

Hydrogen sulfide is a toxic compound that can affect various groups of water microorganisms. Photolithotrophic sulfur bacteria including Chromatiaceae and Chlorobiaceae are able to convert inorganic substrate (hydrogen sulfide and carbon dioxide) into organic matter deriving energy from photosynthesis. This process takes place in the absence of molecular oxygen and is referred to as anoxygenic photosynthesis, in which exogenous electron donors are needed. These donors may be reduced sulfur compounds such as hydrogen sulfide. This paper deals with the description of this metabolic process, representatives of the above-mentioned families, and discusses the possibility using anoxygenic phototrophic microorganisms for the detoxification of toxic hydrogen sulfide. Moreover, their general characteristics, morphology, metabolism, and taxonomy are described as well as the conditions for isolation and cultivation of these microorganisms will be presented.

The Intersection of Geology, Geochemistry, and Microbiology in Continental Hydrothermal Systems

Decompressional boiling of ascending hydrothermal waters and separation into a vapor (gas) and a liquid phase drive extensive variation in the geochemical composition of hot spring waters. Yet little is known of how the process of phase separation influences the distribution of microbial metabolisms in springs. Here, we determined the variation in protein coding genes in 51 metagenomes from chemosynthetic hot spring communities that span geochemical gradients in Yellowstone National Park. The 51 metagenomes could be divided into 5 distinct groups that correspond to low and high temperatures and acidic and circumneutral/alkaline springs. A fifth group primarily comprised metagenomes from springs with moderate acidity and that are influenced by elevated volcanic gas input. Protein homologs putatively involved in the oxidation of sulfur compounds, a process that leads to acidification of spring waters, in addition to those involved in the reduction of sulfur compounds were enriched in metagenomes from acidic springs sourced by vapor phase gases. Metagenomes from springs with evidence for elevated volcanic gas input were enriched in protein homologs putatively involved in oxidation of those gases, including hydrogen and methane. Finally, metagenomes from circumneutral/alkaline springs sourced by liquid phase waters were enriched in protein homologs putatively involved in heterotrophy and respiration of oxidized nitrogen compounds and oxygen. These results indicate that the geological process of phase separation shapes the ecology of thermophilic communities through its influence on the availability of nutrients in the form of gases, solutes, and minerals. Microbial acidification of hot spring waters further influences the kinetic and thermodynamic stabilities of nutrients and their bioavailability. These data therefore provide an important framework to understand how geological processes have shaped the evolutionary history of chemosynthetic thermophiles and how these organisms, in turn, have shaped their geochemical environments.

Elucidating functional microorganisms and metabolic mechanisms in a novel engineered ecosystem integrating C, N, P and S biotransformation by metagenomics

Denitrifying sulfur conversion-associated enhanced biological phosphorous removal (DS-EBPR) system is not only a novel wastewater treatment process, but also an ideal model for microbial ecology in a community context. However, it exists the knowledge gap on the roles and interactions of functional microorganisms in the DS-EBPR system for carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) bioconversions. We use genome-resolved metagenomics to build up an ecological model of microbial communities in a lab-scale DS-EBPR system with stable operation for more than 400 days. Our results yield 11 near-complete draft genomes that represent a substantial portion of the microbial community (39.4%). Sulfate-reducing bacteria (SRB) and sulfide-oxidizing bacteria (SOB) promote complex metabolic processes and interactions for C, N, P and S conversions. Bins 1-4 and 10 are considered as new potential polyphosphate-accumulating organisms (PAOs), in which Bins 1-4 can be considered as S-related PAOs (S-PAOs) with no previously cultivated or reported members. Our findings give an insight into a new ecological system with C, N, P and S simultaneous bioconversions and improve the understanding of interactions among SRB, SOB, denitrifiers and PAOs within a community context.

A novel enzyme of type VI sulfide:quinone oxidoreductases in purple sulfur photosynthetic bacteria

Sulfide detoxification can be catalyzed by ancient membrane-bound flavoproteins, sulfide:quinone oxidoreductases (Sqr), which have important roles in sulfide homeostasis and sulfide-dependent energy conservation processes by transferring electrons from sulfide to respiratory or photosynthetic membrane electron flow. Sqr enzymes have been categorized into six groups. Several members of the groups I, II, III, and V are well-known, but type IV and VI Sqrs are, as yet, uncharacterized or hardly characterized at all. Here, we report detailed characterization of a type VI sulfide:quinone oxidoreductase (TrSqrF) from a purple sulfur bacterium, Thiocapsa roseopersicina. Phylogenetic analysis classified this enzyme in a special group composed of SqrFs of endosymbionts, while a weaker relationship could be observed with SqrF of Chlorobaculum tepidum which is the only type VI enzyme characterized so far. Directed mutagenesis experiments showed that TrSqrF contributed substantially to the sulfide:quinone oxidoreductase activity of the membranes. Expression of the sqrF gene could be induced by sulfide. Homologous recombinant TrSqrF protein was expressed and purified from the membranes of a SqrF-deleted T. roseopersicina strain. The purified protein contains redox-active covalently bound FAD cofactor. The recombinant TrSqrF enzyme catalyzes sulfur-dependent quinone reduction and prefers ubiquinone-type quinone compounds. Kinetic parameters of TrSqrF show that the affinity of the enzyme is similar to duroquinone and decylubiquinone, but the reaction has substantially lower activation energy with decylubiquinone, indicating that the quinone structure has an effect on the catalytic process. TrSqrF enzyme affinity for sulfide is low, therefore, in agreement with the gene expressional analyis, SqrF could play a role in energy-conserving sulfide oxidation at high sulfide concentrations. TrSqrF is a good model enzyme for the subgroup of type VI Sqrs of endosymbionts and its characterization might provide deeper insight into the molecular details of the ancient, anoxic, energy-gaining processes using sulfide as an electron donor.

Thousands of microbial genomes shed light on interconnected biogeochemical processes in an aquifer system

The subterranean world hosts up to one-fifth of all biomass, including microbial communities that drive transformations central to Earth's biogeochemical cycles. However, little is known about how complex microbial communities in such environments are structured, and how inter-organism interactions shape ecosystem function. Here we apply terabase-scale cultivation-independent metagenomics to aquifer sediments and groundwater, and reconstruct 2,540 draft-quality, near-complete and complete strain-resolved genomes that represent the majority of known bacterial phyla as well as 47 newly discovered phylum-level lineages. Metabolic analyses spanning this vast phylogenetic diversity and representing up to 36% of organisms detected in the system are used to document the distribution of pathways in coexisting organisms. Consistent with prior findings indicating metabolic handoffs in simple consortia, we find that few organisms within the community can conduct multiple sequential redox transformations. As environmental conditions change, different assemblages of organisms are selected for, altering linkages among the major biogeochemical cycles.

Sulfide Consumption in Sulfurimonas denitrificans and Heterologous Expression of Its Three Sulfide-Quinone Reductase Homologs

Sulfurimonas denitrificans is a sulfur-oxidizing epsilonproteobacterium. It has been reported to grow with sulfide and to harbor genes that encode sulfide-quinone reductases (SQRs) (catalyze sulfide oxidation). However, the actual sulfide concentrations at which S. denitrificans grows and whether its SQRs are functional remain enigmatic. Here, we illustrate the sulfide concentrations at which S. denitrificans exhibits good growth, namely, 0.18 mM to roughly 1.7 mM. Around 2.23 mM, sulfide appears to inhibit growth. S. denitrificans harbors three SQR homolog genes on its genome (Suden_2082 for type II SQR, Suden_1879 for type III SQR, and Suden_619 for type IV SQR). They are all transcribed in S. denitrificans. According to our experiments, they appear to be loosely bound to the membrane. Each individual S. denitrificans SQR was heterologously expressed in the Rhodobacter capsulatus SB1003 sqr deletion mutant, and all exhibited SQR activities individually. This suggests that all of these three genes encode functional SQRs. This study also provides the first experimental evidence of a functional bacterial type III SQR.

IMPORTANCE

Although the epsilonproteobacterium Sulfurimonas denitrificans has been described as using many reduced sulfur compounds as electron donors, there is little knowledge about its growth with sulfide. In many bacteria, the sulfide-quinone reductase (SQR) is responsible for catalyzing sulfide oxidation. S. denitrificans has an array of different types of sqr genes on its genome and so do several other sulfur-oxidizing Epsilonproteobacteria. However, whether these SQRs are functional has remained unknown. Here, we shed light on sulfide metabolism in S. denitrificans. Our study provides the first experimental evidence of active epsilonproteobacterial SQRs and also gives the first report of a functional bacterial type III SQR.

Coregulated genes link sulfide:quinone oxidoreductase and arsenic metabolism in Synechocystis sp. strain PCC6803

Although the biogeochemistry of the two environmentally hazardous compounds arsenic and sulfide has been extensively investigated, the biological interference of these two toxic but potentially energy-rich compounds has only been hypothesized and indirectly proven. Here we provide direct evidence for the first time that in the photosynthetic model organism Synechocystis sp. strain PCC6803 the two metabolic pathways are linked by coregulated genes that are involved in arsenic transport, sulfide oxidation, and probably in sulfide-based alternative photosynthesis. Although Synechocystis sp. strain PCC6803 is an obligate photoautotrophic cyanobacterium that grows via oxygenic photosynthesis, we discovered that specific genes are activated in the presence of sulfide or arsenite to exploit the energy potentials of these chemicals. These genes form an operon that we termed suoRSCT, located on a transposable element of type IS4 on the plasmid pSYSM of the cyanobacterium. suoS (sll5036) encodes a light-dependent, type I sulfide:quinone oxidoreductase. The suoR (sll5035) gene downstream of suoS encodes a regulatory protein that belongs to the ArsR-type repressors that are normally involved in arsenic resistance. We found that this repressor has dual specificity, resulting in 200-fold induction of the operon upon either arsenite or sulfide exposure. The suoT gene encodes a transmembrane protein similar to chromate transporters but in fact functioning as an arsenite importer at permissive concentrations. We propose that the proteins encoded by the suoRSCT operon might have played an important role under anaerobic, reducing conditions on primordial Earth and that the operon was acquired by the cyanobacterium via horizontal gene transfer.

Modulation of h(2)s metabolism by statins: a new aspect of cardiovascular pharmacology

SIGNIFICANCE

Statins (3-hydroxy-3-methylglutarylcoenzyme A reductase inhibitors) are commonly used in the treatment of cardiovascular diseases. Statins reduce plasma low-density lipoproteins, inhibit inflammatory reaction, improve endothelial function, ameliorate oxidative stress, and reduce platelet activity. Consequently, statins markedly decrease the risk of acute cardiovascular events. H(2)S is synthesized in all layers of the vascular wall, including the endothelium, smooth muscle cells, and perivascular adipose tissue (PVAT).

RECENT ADVANCES

Recent studies demonstrate that PVAT-derived H(2)S decreases vascular tone by activating K(ATP) and/or KCNQ potassium channels in smooth muscle cells. Lipophilic atorvastatin, but not hydrophilic pravastatin, increases net H(2)S production in PVAT by inhibiting its mitochondrial oxidation, and augments the anticontractile effect of PVAT. Inhibition of H(2)S metabolism results from atorvastatin-induced decrease in coenzyme Q, which is a cofactor of H(2)S oxidation by sulfide:quinone oxidoreductase. In contrast to H(2)S, statins do not impair mitochondrial oxidation of organic substrates.

CRITICAL ISSUES

Taking into account antiatherosclerotic and anti-inflammatory effect of H(2)S, the gas may mediate some of the beneficial effects of statins on the cardiovascular system. In addition, specific statins differ in their ability to enhance H(2)S signaling.

FUTURE DIRECTIONS

Since both statins and H(2)S reduce ischemia-reperfusion injury, the possible effect of statins on H(2)S oxidation in other tissues such as the heart and the kidney needs to be examined. Inhibition of H(2)S metabolism may be a new therapeutic strategy to improve H(2)S signaling, especially in the mitochondrial compartment.

Beyond the genome: functional studies of phototrophic sulfur oxidation

The increasing availability of complete genomic sequences for cultured phototrophic bacteria and assembled metagenomes from environments dominated by phototrophs has reinforced the need for a "post-genomic" analytical effort to test models of cellular structure and function proposed from genomic data. Comparative genomics has produced a testable model for pathways of sulfur compound oxidation in the phototrophic bacteria. In the case of sulfide, two enzymes are predicted to oxidize sulfide: sulfide:quinone oxidoreductase and flavocytochrome c sulfide dehydrogenase. However, these models do not predict which enzyme is important under what conditions. In Chlorobaculum tepidum, a model green sulfur bacterium, a combination of genetics and physiological analysis of mutant strains has led to the realization that this organism contains at least two active sulfide:quinone oxidoreductases and that there is significant interaction between sulfide oxidation and light harvesting. In the case of elemental sulfur, an organothiol intermediate of unknown structure has been proposed to activate elemental sulfur for transport into the cytoplasm where it can be oxidized or assimilated, and recent approaches using classical metabolite analysis have begun to shed light on this issue both in C. tepidum and the purple sulfur bacterium Allochromatium vinosum.

Inorganic sulfur oxidizing system in green sulfur bacteria

Green sulfur bacteria use various reduced sulfur compounds such as sulfide, elemental sulfur, and thiosulfate as electron donors for photoautotrophic growth. This article briefly summarizes what is known about the inorganic sulfur oxidizing systems of these bacteria with emphasis on the biochemical aspects. Enzymes that oxidize sulfide in green sulfur bacteria are membrane-bound sulfide-quinone oxidoreductase, periplasmic (sometimes membrane-bound) flavocytochrome c sulfide dehydrogenase, and monomeric flavocytochrome c (SoxF). Some green sulfur bacteria oxidize thiosulfate by the multienzyme system called either the TOMES (thiosulfate oxidizing multi-enzyme system) or Sox (sulfur oxidizing system) composed of the three periplasmic proteins: SoxB, SoxYZ, and SoxAXK with a soluble small molecule cytochrome c as the electron acceptor. The oxidation of sulfide and thiosulfate by these enzymes in vitro is assumed to yield two electrons and result in the transfer of a sulfur atom to persulfides, which are subsequently transformed to elemental sulfur. The elemental sulfur is temporarily stored in the form of globules attached to the extracellular surface of the outer membranes. The oxidation pathway of elemental sulfur to sulfate is currently unclear, although the participation of several proteins including those of the dissimilatory sulfite reductase system etc. is suggested from comparative genomic analyses.

Biochemistry and molecular biology of lithotrophic sulfur oxidation by taxonomically and ecologically diverse bacteria and archaea

Lithotrophic sulfur oxidation is an ancient metabolic process. Ecologically and taxonomically diverged prokaryotes have differential abilities to utilize different reduced sulfur compounds as lithotrophic substrates. Different phototrophic or chemotrophic species use different enzymes, pathways and mechanisms of electron transport and energy conservation for the oxidation of any given substrate. While the mechanisms of sulfur oxidation in obligately chemolithotrophic bacteria, predominantly belonging to Beta- (e.g. Thiobacillus) and Gammaproteobacteria (e.g. Thiomicrospira), are not well established, the Sox system is the central pathway in the facultative bacteria from Alphaproteobacteria (e.g. Paracoccus). Interestingly, photolithotrophs such as Rhodovulum belonging to Alphaproteobacteria also use the Sox system, whereas those from Chromatiaceae and Chlorobi use a truncated Sox complex alongside reverse-acting sulfate-reducing systems. Certain chemotrophic magnetotactic Alphaproteobacteria allegedly utilize such a combined mechanism. Sulfur-chemolithotrophic metabolism in Archaea, largely restricted to Sulfolobales, is distinct from those in Bacteria. Phylogenetic and biomolecular fossil data suggest that the ubiquity of sox genes could be due to horizontal transfer, and coupled sulfate reduction/sulfide oxidation pathways, originating in planktonic ancestors of Chromatiaceae or Chlorobi, could be ancestral to all sulfur-lithotrophic processes. However, the possibility that chemolithotrophy, originating in deep sea, is the actual ancestral form of sulfur oxidation cannot be ruled out.

Sulfur metabolism in phototrophic sulfur bacteria

Phototrophic sulfur bacteria are characterized by oxidizing various inorganic sulfur compounds for use as electron donors in carbon dioxide fixation during anoxygenic photosynthetic growth. These bacteria are divided into the purple sulfur bacteria (PSB) and the green sulfur bacteria (GSB). They utilize various combinations of sulfide, elemental sulfur, and thiosulfate and sometimes also ferrous iron and hydrogen as electron donors. This review focuses on the dissimilatory and assimilatory metabolism of inorganic sulfur compounds in these bacteria and also briefly discusses these metabolisms in other types of anoxygenic phototrophic bacteria. The biochemistry and genetics of sulfur compound oxidation in PSB and GSB are described in detail. A variety of enzymes catalyzing sulfur oxidation reactions have been isolated from GSB and PSB (especially Allochromatium vinosum, a representative of the Chromatiaceae), and many are well characterized also on a molecular genetic level. Complete genome sequence data are currently available for 10 strains of GSB and for one strain of PSB. We present here a genome-based survey of the distribution and phylogenies of genes involved in oxidation of sulfur compounds in these strains. It is evident from biochemical and genetic analyses that the dissimilatory sulfur metabolism of these organisms is very complex and incompletely understood. This metabolism is modular in the sense that individual steps in the metabolism may be performed by different enzymes in different organisms. Despite the distant evolutionary relationship between GSB and PSB, their photosynthetic nature and their dependency on oxidation of sulfur compounds resulted in similar ecological roles in the sulfur cycle as important anaerobic oxidizers of sulfur compounds.

Functional analysis of three sulfide:quinone oxidoreductase homologs in Chlorobaculum tepidum

Sulfide:quinone oxidoreductase (SQR) catalyzes sulfide oxidation during sulfide-dependent chemo- and phototrophic growth in bacteria. The green sulfur bacterium Chlorobaculum tepidum (formerly Chlorobium tepidum) can grow on sulfide as the sole electron donor and sulfur source. C. tepidum contains genes encoding three SQR homologs: CT0117, CT0876, and CT1087. This study examined which, if any, of the SQR homologs possess sulfide-dependent ubiquinone reduction activity and are required for growth on sulfide. In contrast to CT0117 and CT0876, transcripts of CT1087 were detected only when cells actively oxidized sulfide. Mutation of CT0117 or CT1087 in C. tepidum decreased SQR activity in membrane fractions, and the CT1087 mutant could not grow with >or=6 mM sulfide. Mutation of both CT0117 and CT1087 in C. tepidum completely abolished SQR activity, and the double mutant failed to grow with >or=4 mM sulfide. A C-terminal His(6)-tagged CT1087 protein was membrane localized, as was SQR activity. Epitope-tagged CT1087 was detected only when sulfide was actively consumed by cells. Recombinantly produced CT1087 and CT0117 proteins had SQR activity, while CT0876 did not. In summary, we conclude that, under the conditions tested, both CT0117 and CT1087 function as SQR proteins in C. tepidum. CT0876 may support the growth of C. tepidum at low sulfide concentrations, but no evidence was found for SQR activity associated with this protein.

Molecular analysis of the diversity of the sulfide : quinone reductase (sqr) gene in sediment environments

Our newly designed primers were evaluated for the molecular analysis of specific groups of the sqr gene encoding sulfide : quinone reductase (SQR) in sediment environments. Based on the phylogenetic analysis, we classified the sqr sequences into six groups. PCR primers specific for each group were developed. We successfully amplified sqr-like gene sequences related to groups 1, 2 and 4 from diverse sediments including a marine sediment (SW), a tidal flat (TS), a river sediment (RS) and a lake sediment (FW). We recovered a total of 82 unique phylotypes (based on a 95 % amino acid sequence similarity cutoff) from 243 individual sqr-like gene sequences. Phylotype richness varied widely among the groups of sqr-like gene sequences (group 1>group 2>group 4) and sediments (SW>TS>RS>FW). Most of the sqr-like gene sequences were affiliated with the Proteobacteria clade and were distantly related to the reference sqr gene sequences from cultivated strains (less than approximately 80 % amino acid sequence similarity). Unique sqr-like gene sequences were associated with individual sediment samples in groups 1 and 2. This molecular tool has also enabled us to detect sqr-like genes in a sulfur-oxidizing enrichment from marine sediments. Collectively, our results support the presence of previously unrecognized sqr gene-containing micro-organisms that play important roles in the global biogeochemical cycle of sulfur.

SoxAX binding protein, a novel component of the thiosulfate-oxidizing multienzyme system in the green sulfur bacterium Chlorobium tepidum

From the photosynthetic green sulfur bacterium Chlorobium tepidum (pro synon. Chlorobaculum tepidum), we have purified three factors indispensable for the thiosulfate-dependent reduction of the small, monoheme cytochrome c(554). These are homologues of sulfur-oxidizing (Sox) system factors found in various thiosulfate-oxidizing bacteria. The first factor is SoxYZ that serves as the acceptor for the reaction intermediates. The second factor is monomeric SoxB that is proposed to catalyze the hydrolytic cleavage of sulfate from the SoxYZ-bound oxidized product of thiosulfate. The third factor is the trimeric cytochrome c(551), composed of the monoheme cytochrome SoxA, the monoheme cytochrome SoxX, and the product of the hypothetical open reading frame CT1020. The last three components were expressed separately in Escherichia coli cells and purified to homogeneity. In the presence of the other two Sox factors, the recombinant SoxA and SoxX showed a low but discernible thiosulfate-dependent cytochrome c(554) reduction activity. The further addition of the recombinant CT1020 protein greatly increased the activity, and the total activity was as high as that of the native SoxAX-CT1020 protein complex. The recombinant CT1020 protein participated in the formation of a tight complex with SoxA and SoxX and will be referred to as SAXB (SoxAX binding protein). Homologues of the SAXB gene are found in many strains, comprising roughly about one-third of the thiosulfate-oxidizing bacteria whose sox gene cluster sequences have been deposited so far and ranging over the Chlorobiaciae, Chromatiaceae, Hydrogenophilaceae, Oceanospirillaceae, etc. Each of the deduced SoxA and SoxX proteins of these bacteria constitute groups that are distinct from those found in bacteria that apparently lack SAXB gene homologues.

Parallel electron donation pathways to cytochrome c(z) in the type I homodimeric photosynthetic reaction center complex of Chlorobium tepidum

We studied the regulation mechanism of electron donations from menaquinol:cytochrome c oxidoreductase and cytochrome c-554 to the type I homodimeric photosynthetic reaction center complex of the green sulfur bacterium Chlorobium tepidum. We measured flash-induced absorption changes of multiple cytochromes in the membranes prepared from a mutant devoid of cytochrome c-554 or in the reconstituted membranes by exogenously adding cytochrome c-555 purified from Chlorobium limicola. The results indicated that the photo-oxidized cytochrome c(z) bound to the reaction center was rereduced rapidly by cytochrome c-555 as well as by the menaquinol:cytochrome c oxidoreductase and that cytochrome c-555 did not function as a shuttle-like electron carrier between the menaquinol:cytochrome c oxidoreductase and cytochrome c(z). It was also shown that the rereduction rate of cytochrome c(z) by cytochrome c-555 was as high as that by the menaquinol:cytochrome c oxidoreductase. The two electron-transfer pathways linked to sulfur metabolisms seem to function independently to donate electrons to the reaction center.

Menaquinone reduction by an HMT2-like sulfide dehydrogenase from Bacillus stearothermophilus

The gene-encoding HMT2-like sulfide dehydrogenase from Bacillus stearothermophilus JCM2501 was amplified and expressed in Escherichia coli and the enzymatic features were examined. The enzyme was detected mainly in the membrane fraction. It catalyzed the sulfide-dependent menaquinone (MK) reduction showing special enzymatic features distinct from other sulfide–quinone oxidoreductases (SQRs) from autotrophic bacteria. The purified protein from E. coli brought about the sulfide-dependent 2,3-dimethyl-1,4-naphthoquinone (DMN) reduction in vitro. The reduction was accelerated in the presence of either cyanide or 2-mercaptoethanol and phospholipids. The high reduction was followed by a change in Kmvalues for sulfide and DMN. The purified enzyme utilized MK as an electron acceptor in the membrane fraction from E. coli. Under anaerobic conditions, sulfide was oxidized with reduction of fumarate or nitrate via the MK pool. The dehydrogenase was different from SQR in autotrophic bacteria in terms of the low affinity for sulfide and the activity enhancement in the presence of cyanide or 2-mercaptoethanol. The sulfide oxidation via MK in the cellular membrane of Gram-positive bacteria was certified.

Soluble cytochrome c-554, CycA, is not essential for photosynthetic electron transfer in Chlorobium tepidum

We constructed a mutant lacking soluble cytochrome c-554 (CycA) by disruption of the cycA gene in the green sulfur bacterium Chlorobium tepidum. The mutant grew phototrophically with a growth rate slower than that of the wild type, suggesting that CycA is not essential for photosynthetic electron transfer even though CycA is known to work as an electron donor to the reaction center. The re-reduction of photo-oxidized cytochrome c(z) by quinol oxidoreductase was inhibited almost completely by the addition of stigmatellin in the mutant cells. This result indicates that, in the mutant cells, the linear electron transfer can occur from the quinol oxidoreductase to cytochrome c(z), and to reaction center P840 with no participation of CycA.

Efficient removal of sulfide following integration of multiple copies of the sulfide-quinone oxidoreductase gene (sqr) into the Escherichia coli chromosome

For the oxidation and removal of hydrogen sulfide, which causes an offensive odor from the contents of animal intestines, recombinant strains of Escherichia coli were constructed. The sulfide-quinone oxidoreductase gene (sqr) from Rhodobacter capsulatus was integrated in low copy numbers into the chromosome of Escherichia coli W3110. Multiple copies of sqr on plasmids were also delivered into the cytoplasm of the same strain. The sqr genes were homologously transducted onto the chromosomal lacZ region and their existence there was verified by Southern blot analysis. Sulfide oxidation in a chemical medium effectively increased for the recombinant strains which carried 2 approximately 3 copies of sqr under the control of the lac or tac promoter in the chromosome, and also for strains which carried 10 copies of sqr under the control of the lac or tac promoter on plasmids. In both types of recombinant, the tac promoter was more effective for SQR expression than the lac promoter. Construction of a recombinant with 3 copies of sqr under the control of the tac promoter in the chromosome was unsuccessful. In recombinants with SQR activity lower than 700 nmol/mg cell protein/min, oxygen consumption increased proportionally to SQR activity. An elevation in SQR activity in this range resulted in an increase in oxygen consumption and a decrease in sulfide concentration. When the recombinant cells were cultured until the 160th generation, WL2, WL3 and WT2, which carried 2, 3 and 2 copies of sqr in the chromosome, respectively, retained SQR activity similar to that of the first generation. For WL300 and WT20 which carried multi-copies of sqr in plasmids SQR activity was undetectable. The recombinant with 2 copies of sqr in the chromosome regulated by the tac promoter was most suitable for sulfide oxidation and growth of the cells.

The reaction center of green sulfur bacteria(1)

The composition of the P840-reaction center complex (RC), energy and electron transfer within the RC, as well as its topographical organization and interaction with other components in the membrane of green sulfur bacteria are presented, and compared to the FeS-type reaction centers of Photosystem I and of Heliobacteria. The core of the RC is homodimeric, since pscA is the only gene found in the genome of Chlorobium tepidum which resembles the genes psaA and -B for the heterodimeric core of Photosystem I. Functionally intact RC can be isolated from several species of green sulfur bacteria. It is generally composed of five subunits, PscA-D plus the BChl a-protein FMO. Functional cores, with PscA and PscB only, can be isolated from Prostecochloris aestuarii. The PscA-dimer binds P840, a special pair of BChl a-molecules, the primary electron acceptor A(0), which is a Chl a-derivative and FeS-center F(X). An equivalent to the electron acceptor A(1) in Photosystem I, which is tightly bound phylloquinone acting between A(0) and F(X), is not required for forward electron transfer in the RC of green sulfur bacteria. This difference is reflected by different rates of electron transfer between A(0) and F(X) in the two systems. The subunit PscB contains the two FeS-centers F(A) and F(B). STEM particle analysis suggests that the core of the RC with PscA and PscB resembles the PsaAB/PsaC-core of the P700-reaction center in Photosystem I. PscB may form a protrusion into the cytoplasmic space where reduction of ferredoxin occurs, with FMO trimers bound on both sides of this protrusion. Thus the subunit composition of the RC in vivo should be 2(FMO)(3)(PscA)(2)PscB(PscC)(2)PscD. Only 16 BChl a-, four Chl a-molecules and two carotenoids are bound to the RC-core, which is substantially less than its counterpart of Photosystem I, with 85 Chl a-molecules and 22 carotenoids. A total of 58 BChl a/RC are present in the membranes of green sulfur bacteria outside the chlorosomes, corresponding to two trimers of FMO (42 Bchl a) per RC (16 BChl a). The question whether the homodimeric RC is totally symmetric is still open. Furthermore, it is still unclear which cytochrome c is the physiological electron donor to P840(+). Also the way of NAD(+)-reduction is unknown, since a gene equivalent to ferredoxin-NADP(+) reductase is not present in the genome.

Sulfide oxidation in gram-negative bacteria by expression of the sulfide–quinone reductase gene ofRhodobacter capsulatusand by electron transport to ubiquinone

The oxidation of sulfide was studied in recombinant bacteria expressing the sulfide–quinone reductase gene (sqr) from Rhodobacter capsulatus. Sulfide was oxidized by the Escherichia coli strain W3110 harboring the sqr construct (pKKSQ) under anaerobic conditions and nitrate was utilized as a terminal electron acceptor. Following the oxidation, elemental sulfur and nitrite were produced as the final reaction products. This activity was retained in the membrane preparation and was sensitive towards antimycin A, stigmatellin, and azide. As a consequence of the ubiquinone deficiency, this activity was markedly decreased. In additon, by recovery of ubiquinone, the oxidation was also restored to rates similar to those of the wild-type strain. These results indicate that sulfide oxidation in this strain occurs via the quinone pool in vivo, and that this sulfide–quinone reductase (SQR) in particular utilizes ubiquinone as a more appropriate electron acceptor than menaquinone or demetylmenaquinone. To our knowledge, this is the first study to show a direct interaction between SQR and ubiquinone in cells. When expressed in Pseudomonas putida and Rhizobium meliloti, the SQR conferred on these organisms the ability to oxidize sulfide as well as E. coli in vivo.Key words: SQR, quinone, anaerobic sulfide oxidation, heterogeneous expression.

Novel genes of the sox gene cluster, mutagenesis of the flavoprotein SoxF, and evidence for a general sulfur-oxidizing system in Paracoccus pantotrophus GB17

The novel genes soxFGH were identified, completing the sox gene cluster of Paracoccus pantotrophus coding for enzymes involved in lithotrophic sulfur oxidation. The periplasmic SoxF, SoxG, and SoxH proteins were induced by thiosulfate and purified to homogeneity from the soluble fraction. soxF coded for a protein of 420 amino acids with a signal peptide containing a twin-arginine motif. SoxF was 37% identical to the flavoprotein FccB of flavocytochrome c sulfide dehydrogenase of Allochromatium vinosum. The mature SoxF (42,832 Da) contained 0.74 mol of flavin adenine dinucleotide per mol. soxG coded for a novel protein of 303 amino acids with a signal peptide containing a twin-arginine motif. The mature SoxG (29,657 Da) contained two zinc binding motifs and 0.90 atom of zinc per subunit of the homodimer. soxH coded for a periplasmic protein of 317 amino acids with a double-arginine signal peptide. The mature SoxH (32,317 Da) contained two metal binding motifs and 0.29 atom of zinc and 0.20 atom of copper per subunit of the homodimer. SoxXA, SoxYZ, SoxB, and SoxCD (C. G. Friedrich, A. Quentmeier, F. Bardischewsky, D. Rother, R. Kraft, S. Kostka, and H. Prinz, J. Bacteriol. 182:4476-4487, 2000) reconstitute a system able to perform thiosulfate-, sulfite-, sulfur-, and hydrogen sulfide-dependent cytochrome c reduction, and this system is the first described for oxidizing different inorganic sulfur compounds. SoxF slightly inhibited the rate of hydrogen sulfide oxidation but not the rate of sulfite or thiosulfate oxidation. From use of a homogenote mutant with an in-frame deletion in soxF and complementation analysis, it was evident that the soxFGH gene products were not required for lithotrophic growth with thiosulfate.

A membrane-bound flavocytochrome c-sulfide dehydrogenase from the purple phototrophic sulfur bacterium Ectothiorhodospira vacuolata

The amino acid sequence of Ectothiorhodospira vacuolata cytochrome c-552, isolated from membranes with n-butanol, shows that it is a protein of 77 amino acid residues with a molecular mass of 9,041 Da. It is closely related to the cytochrome subunit of Chlorobium limicola f. sp. thiosulfatophilum flavocytochrome c-sulfide dehydrogenase (FCSD), having 49% identity. These data allowed isolation of a 5.5-kb subgenomic clone which contains the cytochrome gene and an adjacent flavoprotein gene as in other species which have an FCSD. The cytochrome subunit has a signal peptide with a normal cleavage site, but the flavoprotein subunit has a signal sequence which suggests that the mature protein has an N-terminal cysteine, characteristic of a diacyl glycerol-modified lipoprotein. The membrane localization of FCSD was confirmed by Western blotting with antibodies raised against Chromatium vinosum FCSD. When aligned according to the three-dimensional structure of Chromatium FCSD, all but one of the side chains near the flavin are conserved. These include the Cys 42 flavin adenine dinucleotide binding site; the Cys 161-Cys 337 disulfide; Glu 167, which modulates the reactivity with sulfite; and aromatic residues which may function as charge transfer acceptors from the flavin-sulfite adduct (C. vinosum numbering). The genetic context of FCSD is different from that in other species in that flanking genes are not conserved. The transcript is only large enough to encode the two FCSD subunits. Furthermore, Northern hybridization showed that the production of E. vacuolata FCSD mRNA is regulated by sulfide. All cultures that contained sulfide in the medium had elevated levels of FCSD RNA compared with cells grown on organics (acetate, malate, or succinate) or thiosulfate alone, consistent with the role of FCSD in sulfide oxidation.

Cyanobacterial sulfide-quinone reductase: cloning and heterologous expression

The gene encoding sulfide-quinone reductase (SQR; E.C.1.8.5.'), the enzyme catalyzing the first step of anoxygenic photosynthesis in the filamentous cyanobacterium Oscillatoria limnetica, was cloned by use of amino acid sequences of tryptic peptides as well as sequences conserved in the Rhodobacter capsulatus SQR and in an open reading frame found in the genome of Aquifex aeolicus. SQR activity was also detected in the unicellular cyanobacterium Aphanothece halophytica following sulfide induction, with a V(max) of 180 micromol of plastoquinone-1 (PQ-1) reduced/mg of chlorophyll/h and apparent K(m) values of 20 and 40 microM for sulfide and quinone, respectively. Based on the conserved sequences, the gene encoding A. halophytica SQR was also cloned. The SQR polypeptides deduced from the two cyanobacterial genes consist of 436 amino acids for O. limnetica SQR and 437 amino acids for A. halophytica SQR and show 58% identity and 74% similarity. The calculated molecular mass is about 48 kDa for both proteins; the theoretical isoelectric points are 7.7 and 5.6 and the net charges at a neutral pH are 0 and -14 for O. limnetica SQR and A. halophytica SQR, respectively. A search of databases showed SQR homologs in the genomes of the cyanobacterium Anabaena PCC7120 as well as the chemolithotrophic bacteria Shewanella putrefaciens and Thiobacillus ferrooxidans. All SQR enzymes contain characteristic flavin adenine dinucleotide binding fingerprints. The cyanobacterial proteins were expressed in Escherichia coli under the control of the T7 promoter. Membranes isolated from E. coli cells expressing A. halophytica SQR performed sulfide-dependent PQ-1 reduction that was sensitive to the quinone analog inhibitor 2n-nonyl-4-hydroxyquinoline-N-oxide. The wide distribution of SQR genes emphasizes the important role of SQR in the sulfur cycle in nature.

Oxidative phosphorylation supported by an alternative respiratory pathway in mitochondria from Euglena

The effect of antimycin, myxothiazol, 2-heptyl-4-hydroxyquinoline-N-oxide, stigmatellin and cyanide on respiration, ATP synthesis, cytochrome c reductase, and membrane potential in mitochondria isolated from dark-grown Euglena cells was determined. With L-lactate as substrate, ATP synthesis was partially inhibited by antimycin, but the other four inhibitors completely abolished the process. Cyanide also inhibited the antimycin-resistant ATP synthesis. Membrane potential was collapsed (<60 mV) by cyanide and stigmatellin. However, in the presence of antimycin, a H(+)60 mV) that sufficed to drive ATP synthesis remained. Cytochrome c reductase, with L-lactate as donor, was diminished by antimycin and myxothiazol. Cytochrome bc(1) complex activity was fully inhibited by antimycin, but it was resistant to myxothiazol. Stigmatellin inhibited both L-lactate-dependent cytochrome c reductase and cytochrome bc(1) complex activities. Respiration was partially inhibited by the five inhibitors. The cyanide-resistant respiration was strongly inhibited by diphenylamine, n-propyl-gallate, salicylhydroxamic acid and disulfiram. Based on these results, a model of the respiratory chain of Euglena mitochondria is proposed, in which a quinol-cytochrome c oxidoreductase resistant to antimycin, and a quinol oxidase resistant to antimycin and cyanide are included.

A fission yeast gene for mitochondrial sulfide oxidation

A cadmium-hypersensitive mutant of the fission yeast Schizosaccharomyces pombe was found to accumulate abnormally high levels of sulfide. The gene required for normal regulation of sulfide levels, hmt2(+), was cloned by complementation of the cadmium-hypersensitive phenotype of the mutant. Cell fractionation and immunocytochemistry indicated that HMT2 protein is localized to mitochondria. Sequence analysis revealed homology between HMT2 and sulfide dehydrogenases from photosynthetic bacteria. HMT2 protein, produced in and purified from Escherichia coli, was soluble, bound FAD, and catalyzed the reduction of quinone (coenzyme Q2) by sulfide. HMT2 activity was also detected in isolated fission yeast mitochondria. We propose that HMT2 functions as a sulfide:quinone oxidoreductase. Homologous enzymes may be widespread in higher organisms, as sulfide-oxidizing activities have been described previously in animal mitochondria, and genes of unknown function, but with similarity to hmt2(+), are present in the genomes of flies, worms, rats, mice, and humans.

Sulfide oxidation by gene expressions of sulfide-quinone oxidoreductase and ubiquinone-8 biosynthase in Escherichia coli

Sulfides (S2),SH-) such as hydrogen sulfide belong to a class of sulfur compounds with unpleasant odors. In order to confer sulfide-oxidizing ability on the intestine-inhabiting bacteria, the sulfide-quinone oxidoreductase gene (sqr) in Rhodobacter capsulatus DSM-155 and genes for quinone biosynthesis (ubiC, ubiA and ispB) in Escherichia coli XL1 Blue-MRF' were transduced into E. coli BL21(DE3). Plasmids pT7-7 and pSTV were used as vectors of sqr, and ubiCA and ispB, respectively. The recombinants sqr-BL21(DE3) and ubiCA,ispB-sqr-BL21(DE3) were successfully constructed. The maximal sulfide-removing activities of the whole cells and membrane fractions of sqr-BL21(DE3) attained at pH 8.0 and 7.8, were 267 nmol/mg cells (dry weight)/min and 1250 nmol/mg membrane fraction (protein)/min, respectively. The molecular ratio of sulfide (S2-) oxidized and oxygen (O2) consumed was 2:1. SQR activity in the recombinant cells was positively restricted under anaerobic conditions and also by the addition of electron transfer inhibitors. Ubiquinone-8 (UQ-8) biosynthesis in the cells of ubiCA,ispB-sqr-BL21(DE3) increased as much as 2.2-fold compared with that of (pSTV)-sqr-BL21(DE3) during the 12-16 h incubation period. The maximal sulfide removal in the quinone-raised E. coli was attained slightly earlier, however, SQR activities thereafter were lower than those in (pSTV)-sqr-BL21(DE3).

Membrane-bound cytochrome cz couples quinol oxidoreductase to the P840 reaction center complex in isolated membranes of the green sulfur bacterium Chlorobium tepidum

The reaction of quinol oxidoreductase and membrane-bound c-type cytochromes was studied in chlorosome-depleted membranes isolated from Chlorobium tepidum. Rapid oxidations of c-type cytochromes were detected after flash excitation. Their re-reductions occurred in parallel with the reduction of cytochrome b, especially in the presence of antimycin A, whereas reductions of both cytochromes c and b were suppressed by added stigmatellin. These results indicate the tight coupling between the photosynthetic reaction center and quinol oxidoreductase. Turnovers of two types of cytochromes c were detected. One was assigned to the monoheme-type cytochrome c (designated cytochrome cz), which is known to be tightly bound to the reaction center complex. The other was a new c-type cytochrome, cytochrome c-556, which functions the same as cytochrome c1. The steps of electron-transfer scheme, menaquinol --> Rieske FeS center --> cytochrom c-556 --> cytochrome cz --> P840, are estimated to have reaction times of 20 ms and 560, 150, and 40 microseconds, respectively. We conclude that quinol oxidoreductase and the reaction center complex in Chlorobium tepidum are linked by two distinct membrane-bound cytochromes, cz and c-556, with no involvement of water-soluble cytochromes.

Physiology and genetics of sulfur-oxidizing bacteria

Reduced inorganic sulfur compounds are oxidized by members of the domains Archaea and Bacteria. These compounds are used as electron donors for anaerobic phototrophic and aerobic chemotrophic growth, and are mostly oxidized to sulfate. Different enzymes mediate the conversion of various reduced sulfur compounds. Their physiological function in sulfur oxidation is considered (i) mostly from the biochemical characterization of the enzymatic reaction, (ii) rarely from the regulation of their formation, and (iii) only in a few cases from the mutational gene inactivation and characterization of the resulting mutant phenotype. In this review the sulfur-metabolizing reactions of selected phototrophic and of chemotrophic prokaryotes are discussed. These comprise an archaeon, a cyanobacterium, green sulfur bacteria, and selected phototrophic and chemotrophic proteobacteria. The genetic systems are summarized which are presently available for these organisms, and which can be used to study the molecular basis of their dissimilatory sulfur metabolism. Two groups of thiobacteria can be distinguished: those able to grow with tetrathionate and other reduced sulfur compounds, and those unable to do so. This distinction can be made irrespective of their phototrophic or chemotrophic metabolism, neutrophilic or acidophilic nature, and may indicate a mechanism different from that of thiosulfate oxidation. However, the core enzyme for tetrathionate oxidation has not been identified so far. Several phototrophic bacteria utilize hydrogen sulfide, which is considered to be oxidized by flavocytochrome c owing to its in vitro activity. However, the function of flavocytochrome c in vivo may be different, because it is missing in other hydrogen sulfide-oxidizing bacteria, but is present in most thiosulfate-oxidizing bacteria. A possible function of flavocytochrome c is discussed based on biophysical studies, and the identification of a flavocytochrome in the operon encoding enzymes involved in thiosulfate oxidation of Paracoccus denitrificans. Adenosine-5'-phosphosulfate reductase thought to function in the 'reverse' direction in different phototrophic and chemotrophic sulfur-oxidizing bacteria was analysed in Chromatium vinosum. Inactivation of the corresponding gene does not affect the sulfite-oxidizing ability of the mutant. This result questions the concept of its 'reverse' function, generally accepted for over three decades.

Sulfide-quinone reductase from Rhodobacter capsulatus. Purification, cloning, and expression

A sulfide-quinone oxidoreductase (SQR, EC 1.8.5.'.) has been purified to homogeneity from chromatophores of the non-sulfur purple bacterium Rhodobacter capsulatus DSM 155. It is composed of a single polypeptide with an apparent molecular mass of about 55 kDa, exhibiting absorption and fluorescence spectra typical for a flavoprotein and similar to the SQR from the cyanobacterium Oscillatoria limnetica. From N-terminal and tryptic peptide sequences of the pure protein a genomic DNA clone was obtained by polymerase chain reaction amplification. Its sequence contains an open reading frame of 1275 base pairs (EMBL nucleotide sequence data base, accession no. X97478X97478) encoding the SQR of R. capsulatus. The deduced polypeptide consists of 425 amino acid residues with a molecular mass of 47 kDa and a net charge of +9. The high similarity (72%)/identity (48%) between the N termini of the cyanobacterial and the bacterial enzyme was confirmed and extended. Both enzymes exhibit the FAD/NAD(P) binding betaalphabeta-fold (Wierenga, R. K., Terpstra, P., and Hol, W. G. S. (1986) J. Mol. Biol. 187, 101-107). The complete sequence of the SQR from R. capsulatus shows further similarity to flavoproteins, in particular glutathione reductase and lipoamide dehydrogenase. The cloned sqr was expressed in Escherichia coli in a functional form.

Mitochondrial sulfide oxidation in Arenicola marina. Evidence for alternative electron pathways

Sulfide is oxidized in the mitochondria of the lugworm Arenicola marina. Mitochondrial sulfide oxidation is coupled with oxygen consumption and with an equimolar production of thiosulfate [Völkel, S. & Grieshaber, M. K. (1994) Mar. Biol. 118, 137-147]. Mitochondrial respiration in the presence of malate (or succinate) and ADP but without sulfide could be completely inhibited by rotenone, antimycin, cyanide, and sulfide. Only 40% inhibition was achieved by salicylhydroxamic acid. Sulfide oxidation (with sulfide as the only substrate) was fully inhibited by antimycin and by salicylhydroxamic acid but not by rotenone or sulfide. Moreover, sulfide oxidation was 3-4-fold less sensitive to cyanide as compared to normal respiration. The data indicate that sulfide oxidation in A. marina is linked to the respiratory electron transport chain. We suggest that electrons from sulfide enter the respiratory chain via ubiquinone or at the ubiquinol-cytochrome-c oxidoreductase. At sulfide concentrations higher than 10 microM, the cytochrome-c oxidase is blocked and electrons from sulfide are transferred to oxygen via an alternative terminal oxidase.

Reduction of cytochromes with menaquinol and sulfide in membranes from green sulfur bacteria

Reduction of cytochromes in chlorosome-free membranes of Chlorobia was studied anaerobically, with an LED array spectrophotometer. For Chlorobium tepidum these membranes contained 0.2 moles cytochrome per mole of bacteriochlorophyll a. The observed change upon complete reduction of oxidized membranes with dithionite could be satisfactorily fitted with three cytochrome components having absorption peaks at 553 (cyt c), 558 and 563 nm (cyt b), in relative amounts of 5:1:2. About 20% of total cytochrome 553 were reducible by ascorbate. Menaquinol reduced all of the 553-component, and this reduction was sensitive to stigmatellin, NQNO and antimycin A. The reduction was insensitive to KCN. However, it was transient at low concentrations of menaquinol in the absence of KCN, but permanent in its presence, demonstrating that electron transport into an oxidation pool was blocked. The 563-component was only slightly reduced by menaquinol unless NQNO or antimycin were present. The stimulation of cytochrome 563-reduction by these inhibitors was more pronounced in the presence of ferricyanide. This phenomenon reflects 'oxidant-induced reduction' of cytochrome b and demonstrates that a Q-cycle is operative in Chlorobia. Also, sulfide fully reduced cytochrome 553, but more slowly than menaquinol. KCN inhibited in this case, as did stigmatellin, NQNO and antimycin A. NQNO was a better inhibitor than antimycin A. Cytochrome 563 again was hardly reduced unless antimycin A was added. The effect was more difficult to observe with NQNO. This supports the conclusion that sulfide oxidation proceeds via the quinone pool and the cytochrome bc-complex in green sulfur bacteria.

Photo-oxidation of membrane-bound and soluble cytochromec in the green sulfur bacteriumChlorobium tepidum

We studied the photosynthetic electron transfer system of membrane-bound and soluble cytochromec inChlorobium tepidum, a thermophilic green sulfur bacterium, using whole cells and membrane preparations. Sulfide and thiosulfate, physiological electron donors, enhanced flash-induced photo-oxidation ofc-type cytochromes in whole cells. In membranes,c-553 cytochromes with two (or three) heme groups served as immediate electron donors for photo-oxidized bacteriochlorophyll (P840) in the reaction center, and appeared to be closely associated with the reaction center complex. The membrane-bound cytochromec-553 had anE m-value of 180 mV. When isolated soluble cytochromec-553, which has an apparent molecular weight of 10 kDa and seems to correspond to the cytochromec-555 inChlorobium limicola andChlorobium vibrioforme, was added to a membrane suspension, rapid photo-oxidation of both soluble and membrane-bound cytochromesc-553 was observed. The oxidation of soluble cytochromec-553 was inhibited by high salt concentrations. In whole cells, photo-oxidation was observed in the absence of exogenous electron donors and re-reduction was inhibited by stigmatellin, an inhibitor of the cytochromebc complex. These results suggest that the role of membrane-bound and soluble cytochromec inC. tepidum is similar to the role of cytochromec in the photosynthetic electron transfer system of purple bacteria.

Sulfide-quinone and sulfide-cytochrome reduction in Rhodobacter capsulatus

The reduction by sulfide of exogenous ubiquinone is compared to the reduction of cytochromes in chromatophores of Rhodobacter capsulatus. From titrations with sulfide values for Vmax of 300 and 10 μmoles reduced/mg bacteriochlorophyll a·h, and for Km of 5 and 3 μM were estimated, for decyl-ubiquinone-and cytochrome c-reduction, respectively. Both reactions are sensitive to KCN, as has been found for sulfide-quinone reductase (SQR) in Oscillatoria limnetica, which is a flavoprotein. Effects of inhibitors interfering with quinone binding sites suggest that at least part of the electron transport from sulfide in R. capsulatus employs the cytochrome bc 1-complex via the ubiquinone pool.