Dimethyl sulfoxide as an electron acceptor for anaerobic growth

Emerging Chemical Diversity and Potential Applications of Enzymes in the DMSO Reductase Superfamily

Molybdenum- and tungsten-dependent proteins catalyze essential processes in living organisms and biogeochemical cycles. Among these enzymes, members of the dimethyl sulfoxide (DMSO) reductase superfamily are considered the most diverse, facilitating a wide range of chemical transformations that can be categorized as oxygen atom installation, removal, and transfer. Importantly, DMSO reductase enzymes provide high efficiency and excellent selectivity while operating under mild conditions without conventional oxidants such as oxygen or peroxides. Despite the potential utility of these enzymes as biocatalysts, such applications have not been fully explored. In addition, the vast majority of DMSO reductase enzymes still remain uncharacterized. In this review, we describe the reactivities, proposed mechanisms, and potential synthetic applications of selected enzymes in the DMSO reductase superfamily. We also highlight emerging opportunities to discover new chemical activity and current challenges in studying and engineering proteins in the DMSO reductase superfamily. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Halo(natrono)archaea from hypersaline lakes can utilize sulfoxides other than DMSO as electron acceptors for anaerobic respiration

Dimethylsulfoxide (DMSO) has long been known to support anaerobic respiration in a few species of basically aerobic extremely halophilic euryarchaea living in hypersaline lakes. Recently, it has also been shown to be utilized as an additional electron acceptor in basically anaerobic sulfur-reducing haloarchaea. Here we investigated whether haloarchaea would be capable of anaerobic respiration with other two sulfoxides, methionine sulfoxide (MSO) and tetramethylene sulfoxide (TMSO). For this, anaerobic enrichment cultures were inoculated with sediments from hypersaline salt and soda lakes in southwestern Siberia and southern Russia. Positive enrichments were obtained for both MSO and TMSO with yeast extract but not with formate or acetate as the electron donor. Two pure cultures obtained from salt lakes, either with MSO or TMSO, were obligate anaerobes closely related to sulfur-reducing Halanaeroarchaeum sulfurireducens, although the type strain of this genus was unable to utilize any sulfoxides. Two pure cultures isolated from soda lakes were facultatively anaerobic alkaliphilic haloarchaea using O₂, sulfur and sulfoxides as the electron acceptors. One isolate was identical to the previously described sulfur-reducing Natrarchaeobaculum sulfurireducens, while another one, enriched at lower alkalinity, is forming a new species in the genus Halobiforma. Since all isolates enriched with either MSO or TMSO were able to respire all three sulfoxides including DMSO and the corresponding activities were cross-induced, it suggest that a single enzyme of the DMSO-reductase family with a broad substrate specificity is responsible for various sulfoxide-dependent respiration in haloarchaea.

Design of mutualistic microbial consortia for stable conversion of carbon monoxide to value-added chemicals

Carbon monoxide (CO) is a promising carbon source for producing value-added biochemicals via microbial fermentation. However, its microbial conversion has been challenging because of difficulties in genetic engineering of CO-utilizing microorganisms and, more importantly, maintaining CO consumption which is negatively affected by the toxicity of CO and accumulated byproducts. To overcome these issues, we devised mutualistic microbial consortia, co-culturing Eubacterium limosum and genetically engineered Escherichia coli for the production of 3-hydroxypropionic acid (3-HP) and itaconic acid (ITA). During the co-culture, E. limosum assimilated CO and produced acetate, a toxic by-product, while E. coli utilized acetate as a sole carbon source. We found that this mutualistic interaction dramatically stabilized and improved CO consumption of E. limosum compared to monoculture. Consequently, the improved CO consumption allowed successful production of 3-HP and ITA from CO. This study is the first demonstration of value-added biochemical production from CO using a microbial consortium. Moreover, it suggests that synthetic mutualistic microbial consortium can serve as a powerful platform for the valorization of CO.

Pterin function in bacteria

Pterins are widely conserved biomolecules that play essential roles in diverse organisms. First described as enzymatic cofactors in eukaryotic systems, bacterial pterins were discovered in cyanobacteria soon after. Several pterin structures unique to bacteria have been described, with conjugation to glycosides and nucleotides commonly observed. Despite this significant structural diversity, relatively few biological functions have been elucidated. Molybdopterin, the best studied bacterial pterin, plays an essential role in the function of the Moco cofactor. Moco is an essential component of molybdoenzymes such as sulfite oxidase, nitrate reductase, and dimethyl sulfoxide reductase, all of which play important roles in bacterial metabolism and global nutrient cycles. Outside of the molybdoenzymes, pterin cofactors play important roles in bacterial cyanide utilization and aromatic amino acid metabolism. Less is known about the roles of pterins in nonenzymatic processes. Cyanobacterial pterins have been implicated in phenotypes related to UV protection and phototaxis. Research describing the pterin-mediated control of cyclic nucleotide metabolism, and their influence on virulence and attachment, points to a possible role for pterins in regulation of bacterial behavior. In this review, we describe the variety of pterin functions in bacteria, compare and contrast structural and mechanistic differences, and illuminate promising avenues of future research.

S- and N-Oxide Reductases

Escherichia coli is a versatile facultative anaerobe that can respire on a number of terminal electron acceptors, including oxygen, fumarate, nitrate, and S- and N-oxides. Anaerobic respiration using S- and N-oxides is accomplished by enzymatic reduction of these substrates by dimethyl sulfoxide reductase (DmsABC) and trimethylamine N-oxide reductase (TorCA). Both DmsABC and TorCA are membrane-associated redox enzymes that couple the oxidation of menaquinol to the reduction of S- and N-oxides in the periplasm. DmsABC is membrane bound and is composed of a membrane-extrinsic dimer with a 90.4-kDa catalytic subunit (DmsA) and a 23.1-kDa electron transfer subunit (DmsB). These subunits face the periplasm and are held to the membrane by a 30.8-kDa membrane anchor subunit (DmsC). The enzyme provides the scaffold for an electron transfer relay composed of a quinol binding site, five [4Fe-4S] clusters, and a molybdo-bis(molybdopterin guanine dinucleotide) (present nomenclature: Mo-bis-pyranopterin) (Mo-bisMGD) cofactor. TorCA is composed of a soluble periplasmic subunit (TorA, 92.5 kDa) containing a Mo-bis-MGD. TorA is coupled to the quinone pool via a pentaheme c subunit (TorC, 40.4 kDa) in the membrane. Both DmsABC and TorCA require system-specific chaperones (DmsD or TorD) for assembly, cofactor insertion, and/or targeting to the Tat translocon. In this chapter, we discuss the complex regulation of the dmsABC and torCAD operons, the poorly understood paralogues, and what is known about the assembly and translocation to the periplasmic space by the Tat translocon.

Characterization of odor emission from alternating aerobic and anoxic activated sludge systems using real-time total reduced sulfur analyzer

Anaerobic biodegradation of sulfur-containing compounds always generates volatile sulfur compounds (VSCs) including H2S, methyl mercaptan, and dimethyl sulfide (DMS). VSC emissions from wastewater treatment plants (WWTPs) result in odor complaints from people living nearby. To control odor-causing compounds in WWTPs, it is important to know the odor emission quantity particularly with continuous monitoring. Since modified activated sludge processes always include anaerobic, anoxic and aerobic conditions for nutrient removal, odor emission from these different environmental settings is expected. In this study, continuous monitoring of VSCs from the headspace of an alternating aerobic and anoxic (AAA) activated sludge process via total reduced sulfur (TRS) analyzer was performed. There is clear pattern of the initial TRS peak immediately after the initiation of the aeration in the AAA system and TRS concentration begins to drop through the remaining air-on cycle. On the other hand, during the air-off period, TRS concentrations increase with time. In particular, a clear inflection point in the TRS profile could be observed after complete removal of nitrate during air-off, meaning more VSCs formation. Since the highest odor emission occurs after the initiation of aeration, the future control of exhausted air should only deal with air collected during the initial aeration period (e.g., 30min), a similar concept for the treatment of first flush in combined sewer overflow. In addition, application of a control scheme to initiate aeration immediately after denitrification is completed during air-off should be beneficial in reducing odor emission.

Linking TFT-LCD wastewater treatment performance to microbial population abundance of Hyphomicrobium and Thiobacillus spp

This study investigated the linkage between performance of two full-scale membrane bioreactor (MBR) systems treating thin-film transistor liquid crystal display (TFT-LCD) wastewater and the population dynamics of dimethylsulfoxide (DMSO)/dimethylsulfide (DMS) degrading bacteria. High DMSO degradation efficiencies were achieved in both MBRs, while the levels of nitrification inhibition due to DMS production from DMSO degradation were different in the two MBRs. The results of real-time PCR targeting on DMSO/DMS degrading populations, including Hyphomicrobium and Thiobacillus spp., indicated that a higher DMSO oxidation efficiency occurred at a higher Hyphomicrobium spp. abundance in the systems, suggesting that Hyphomicrobium spp. may be more important for complete DMSO oxidation to sulfate compared with Thiobacillus spp. Furthermore, Thiobacillus spp. was more abundant during poor nitrification, while Hyphomicrobium spp. was more abundant during good nitrification. It is suggested that microbial population of DMSO/DMS degrading bacteria is closely linking to both DMSO/DMS degradation efficiency and nitrification performance.

Gas chromatographic-mass spectrometry method for the detection of busulphan and its metabolites in plasma and urine

Busulphan is an alkylating agent used as conditioning regimen prior to stem cell transplantation. Busulphan is metabolized in the liver and four major metabolites have been identified. The first metabolite is tetrahydrothiophene which is oxidized to tetrahydrothiophene 1-oxide, then sulfolane and finally 3-hydroxy sulfolane. Despite the low molecular weight and wide polarity range of busulphan and its four metabolites, the use of a fused silica non-polar column significantly enhanced the automated gas chromatography-mass spectrometry of their detection in one simple method. The limit of quantification was 0.5μM for busulphan and all its metabolites except 3-OH sulfolane, which was 1.25μM. This method was validated for all the compounds in both human plasma and urine. Lower limits of quantifications (LLOQs) were run in pentaplicate per compound and all results were within 20% of the nominal values. The recovery was determined by comparing the peak area of two quality control (QC) samples, before and after extraction in plasma and urine, in triplicate. Acceptable precision and accuracy have been obtained; at least 3 standard curves have been run for each compound using three different QCs covering the calibration curve in triplicate. The QC values were within 15% (SD) of the nominal values. Selectivity and sensitivity of all compounds have been measured. Compounds were stable up to 50 days after extraction in -20°C and 48h at RT. Moreover, the compounds were stable for three cycles of freezing and thawing. The method was applied in a clinical case where the patient received high dose busulphan; all the compounds have been detected, identified and quantified both in plasma and urine.

Global transcriptome analysis of the tetrachloroethene-dechlorinating bacterium Desulfitobacterium hafniense Y51 in the presence of various electron donors and terminal electron acceptors

Desulfitobacterium hafniense Y51 is a dechlorinating bacterium that encodes an unusually large set of O-demethylase paralogs and specialized respiratory systems including specialized electron donors and acceptors. To use this organism in bioremediation of tetrachloroethene (PCE) or trichloroethene (TCE) pollution, expression patterns of its 5,060 genes were determined under different conditions using 60-mer probes in DNA microarrays. PCE, TCE, fumarate, nitrate, and dimethyl sulfoxide (DMSO) respiration all sustain the growth of strain Y51. Global transcriptome analyses were thus performed using various electron donor and acceptor couples (respectively, pyruvate and either fumarate, TCE, nitrate, or DMSO, and vanillate/fumarate). When TCE is used as terminal electron acceptor, resulting in its detoxification, a series of electron carriers comprising a cytochrome bd-type quinol oxidase (DSY4055-4056), a ferredoxin (DSY1451), and four Fe-S proteins (DSY1626, DSY1629, DSY0733, DSY3309) are upregulated, suggesting that the products of these genes are involved in PCE oxidoreduction. Interestingly, the PCE dehalogenase cluster (pceABCT) is constitutively expressed in the media tested, with pceT being upregulated and pceC downregulated in pyruvate/TCE-containing medium. In addition, another dehalogenation enzyme (DSY1155 coding for a putative chlorophenol reductive dehalogenase), is induced 225-fold in that medium, despite not being involved in PCE respiration. Remarkably since the reducing equivalents formed during pyruvate conversion to acetyl-CoA are channeled to electron acceptors including halogenated compounds, pyruvate induces expression of a pyruvate:ferredoxin oxidoreductase. This study paves the way to understanding the physiology of D. hafniense, optimizing this microbe as a bioremediation agent, and designing bioarray sensors to monitor the presence of dechlorinating organisms in the environment.

Aerobic degradation of mercaptosuccinate by the gram-negative bacterium Variovorax paradoxus strain B4

The Gram-negative bacterium Variovorax paradoxus strain B4 was isolated from soil under mesophilic and aerobic conditions to elucidate the so far unknown catabolism of mercaptosuccinate (MS). During growth with MS this strain released significant amounts of sulfate into the medium. Tn5::mob-induced mutagenesis was successfully employed and yielded nine independent mutants incapable of using MS as a carbon source. In six of these mutants, Tn5::mob insertions were mapped in a putative gene encoding a molybdenum (Mo) cofactor biosynthesis protein (moeA). In two further mutants the Tn5::mob insertion was mapped in the gene coding for a putative molybdopterin (MPT) oxidoreductase. In contrast to the wild type, these eight mutants also showed no growth on taurine. In another mutant a gene putatively encoding a 3-hydroxyacyl-coenzyme A dehydrogenase (paaH2) was disrupted by transposon insertion. Upon subcellular fractionation of wild-type cells cultivated with MS as sole carbon and sulfur source, MPT oxidoreductase activity was detected in only the cytoplasmic fraction. Cells grown with succinate, taurine, or gluconate as a sole carbon source exhibited no activity or much lower activity. MPT oxidoreductase activity in the cytoplasmic fraction of the Tn5::mob-induced mutant Icr6 was 3-fold lower in comparison to the wild type. Therefore, a new pathway for MS catabolism in V. paradoxus strain B4 is proposed: (i) MPT oxidoreductase catalyzes the conversion of MS first into sulfinosuccinate (a putative organo-sulfur compound composed of succinate and a sulfino group) and then into sulfosuccinate by successive transfer of oxygen atoms, (ii) sulfosuccinate is cleaved into oxaloacetate and sulfite, and (iii) sulfite is oxidized to sulfate.

Complete oxidation of propionate, valerate, succinate, and other organic compounds by newly isolated types of marine, anaerobic, mesophilic, gram-negative, sulfur-reducing eubacteria

Anaerobic enrichment cultures with either propionate, succinate, lactate, or valerate and elemental sulfur and inocula from shallow marine or deep-sea sediments were dominated by rod-shaped motile bacteria after three transfers. By application of deep-agar dilutions, five eubacterial strains were obtained in pure culture and designated Kyprop, Gyprop, Kysw2, Gylac, and Kyval. All strains were gram negative and grew by complete oxidation of the electron donors and concomitant stoichiometric reduction of elemental sulfur to hydrogen sulfide. The isolates used acetate, propionate, succinate, lactate, pyruvate, oxaloacetate, maleate, glutamate, alanine, aspartate, and yeast extract. All isolates, except strain Gylac, used citrate as an electron donor but valerate was oxidized only by strain Kyval. Fumarate and malate were degraded by all strains without an additional electron donor or acceptor. Kyprop, Gyprop, and Gylac utilized elemental sulfur as the sole inorganic electron acceptor, while Kysw2 and Kyval also utilized nitrate, dimethyl sulfoxide, or Fe(III)-citrate as an electron acceptor.

Inhibition of Salmonella host cell invasion by dimethyl sulfide

We show that dimethyl sulfoxide (DMSO) inhibits Salmonella hilA expression and that this inhibition is stronger under anaerobiosis. Because DMSO can be reduced to dimethyl sulfide (DMS) during anaerobic growth, we hypothesized that DMS was responsible for hilA inhibition. Indeed, DMS strongly inhibited the expression of hilA and multiple Salmonella pathogenicity island 1 (SPI-1)-associated genes as well as the invasion of cultured epithelial cells. Because DMSO and DMS are widespread in nature, we hypothesize that this phenomenon may contribute to environmental sensing by Salmonella.

Microbial degradation of dimethylsulphide and related C1-sulphur compounds: organisms and pathways controlling fluxes of sulphur in the biosphere

Dimethylsulphide (DMS) plays a major role in the global sulphur cycle. It has important implications for atmospheric chemistry, climate regulation, and sulphur transport from the marine to the atmospheric and terrestrial environments. In addition, DMS acts as an info-chemical for a wide range of organisms ranging from micro-organisms to mammals. Micro-organisms that cycle DMS are widely distributed in a range of environments, for instance, oxic and anoxic marine, freshwater and terrestrial habitats. Despite the importance of DMS that has been unearthed by many studies since the early 1970s, the understanding of the biochemistry, genetics, and ecology of DMS-degrading micro-organisms is still limited. This review examines current knowledge on the microbial cycling of DMS and points out areas for future research that should shed more light on the role of organisms degrading DMS and related compounds in the biosphere.

Probing the role of copper in the biosynthesis of the molybdenum cofactor in Escherichia coli and Rhodobacter sphaeroides

The crystal structure of Cnx1G, an enzyme involved in the biosynthesis of the molybdenum cofactor (Moco) in Arabidopsis thaliana, revealed the remarkable feature of a copper ion bound to the dithiolene unit of a molybdopterin intermediate (Kuper et al. Nature 430:803-806, 2004). To characterize further the role of copper in Moco biosynthesis, we examined the in vivo and/or in vitro activity of two Moco-dependent enzymes, dimethyl sulfoxide reductase (DMSOR) and nitrate reductase (NR), from cells grown under a variety of copper conditions. We found the activities of DMSOR and NR were not affected when copper was depleted from the media of either Escherichia coli or Rhodobacter sphaeroides. These data suggest that while copper may be utilized during Moco biosynthesis when it is available, copper does not appear to be strictly required for Moco biosynthesis in these two organisms.

Anaerobic biodegradation of vegetable oil and its metabolic intermediates in oil-enriched freshwater sediments

Anaerobic biodegradation of vegetable oil in freshwater sediments is strongly inhibited by high concentrations of oil, but the presence of ferric hydroxide relieves the inhibition. The effect of ferric hydroxide is not due to physical or chemical interactions with long-chain fatty acids (LCFAs) that are produced as intermediates during metabolism of vegetable-oil triglycerides. The anaerobic biodegradation of canola oil and mixtures of acetic and oleic acids, two important intermediates of vegetable-oil metabolism, were investigated using sediments enriched on canola oil under methanogenic and iron-reducing conditions to determine whether the effect of ferric hydroxide has a biological basis. Sediments enriched under both conditions rapidly and completely converted canola oil to methane when the initial oil concentration was relatively low (1.9 g oil/kg sediments), but the biotransformation was strongly inhibited in sediments enriched under methanogenic conditions when the initial concentration was 19 g/kg (< 30% of the oil-derived electron equivalents were transferred to methane in a 420-day incubation period). Sediments enriched under iron-reducing conditions, however, completely transformed canola oil to methane in about 250 days at this initial oil concentration. The anaerobic biotransformation of mixtures of acetate and oleic acid followed a similar pattern: the rate and extent of conversion of these electron-donor substrates to methane was always higher in sediments enriched under iron-reducing than under methanogenic conditions. These results suggest that enrichment on canola oil in the presence of ferric hydroxide selects a microbial community that is less sensitive to inhibition by LCFAs than the community that develops during enrichment under methanogenic conditions.

Dimethyl sulfoxide exposure facilitates phospholipid biosynthesis and cellular membrane proliferation in yeast cells

Me2SO is a polar solvent that is widely used in biochemistry, pharmacology, and industry. Although there are several reports in the literature concerning the biological effects of Me2SO, the total cellular response remains unclear. In this paper, DNA microarray technology combined with the hierarchical clustering bioinformatics tool was used to assess the effects of Me2SO on yeast cells. We found that yeast exposed to Me2SO increased phospholipid biosynthesis through up-regulated gene expression. It was confirmed by Northern blotting that the level of INO1 and OPI3 gene transcripts, encoding key enzymes in phospholipid biosynthesis, were significantly elevated following treatment with Me2SO. Furthermore, the phospholipid content of the cells increased during exposure to Me2SO as shown by conspicuous incorporation of a lipophilic fluorescent dye (3,3'-dihexyloxacarbocyanine iodide) into the cell membranes. From these results we propose that Me2SO treatment induces membrane proliferation in yeast cells to alleviate the adverse affects of this chemical on membrane integrity.

Microbial activity in aquatic environments measured by dimethyl sulfoxide reduction and intercomparison with commonly used methods

A new method to determine microbial (bacterial and fungal) activity in various freshwater habitats is described. Based on microbial reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS), our DMSO reduction method allows measurement of the respiratory activity in interstitial water, as well as in the water column. DMSO is added to water samples at a concentration (0.75% [vol/vol] or 106 mM) high enough to compete with other naturally occurring electron acceptors, as determined with oxygen and nitrate, without stimulating or inhibiting microbial activity. Addition of NaN(3), KCN, and formaldehyde, as well as autoclaving, inhibited the production of DMS, which proves that the reduction of DMSO is a biotic process. DMSO reduction is readily detectable via the formation of DMS even at low microbial activities. All water samples showed significant DMSO reduction over several hours. Microbially reduced DMSO is recovered in the form of DMS from water samples by a purge and trap system and is quantified by gas chromatography and detection with a flame photometric detector. The DMSO reduction method was compared with other methods commonly used for assessment of microbial activity. DMSO reduction activity correlated well with bacterial production in predator-free batch cultures. Cell-production-specific DMSO reduction rates did not differ significantly in batch cultures with different nutrient regimes but were different in different growth phases. Overall, a cell-production-specific DMSO reduction rate of 1.26 x 10(-17) +/- 0. 12 x 10(-17) mol of DMS per produced cell (mean +/- standard error; R(2) = 0.78) was calculated. We suggest that the relationship of DMSO reduction rates to thymidine and leucine incorporation is linear (the R(2) values ranged from 0.783 to 0.944), whereas there is an exponential relationship between DMSO reduction rates and glucose uptake, as well as incorporation (the R(2) values ranged from 0.821 to 0.931). Based on our results, we conclude that the DMSO reduction method is a nonradioactive alternative to other methods commonly used to assess microbial activity.

Microbial formation of dimethyl sulfide in anoxic sphagnum peat

Peat bogs dominated by Sphagnum spp. have relatively high areal rates of dimethyl sulfide (DMS) emission to the atmosphere. DMS was produced in anoxic slurries of Sphagnum peat with a linear time course and with an average rate of 40.4 (range, 22.0 to 68.6) nmol per liter of slurry (middot) day(sup-1) observed in nine batches of slurry. Methanethiol (MeSH) was produced at roughly similar rates over the typical 4- to 8-day incubations. DMS and MeSH production in these acidic (pH 4.2 to 4.6) peats were biological, as they were stopped completely by autoclaving and inhibited strongly by addition of antibiotics and 500 (mu)M chloroform. Endogenous DMS production may be due to the degradation of S-methyl-methionine, dimethyl sulfoxide, or methoxyaromatic compounds (e.g., syringic acid), each of which stimulated DMS formation when added at 5 to 10 (mu)M concentrations. However, on the basis of the high rates of thiol (MeSH and ethanethiol) methylation activity that we observed and the availability of endogenous MeSH, we suggest that methylation of MeSH is the major pathway leading to DMS formation in anaerobic peat. Solid-phase adsorption of MeSH plays a key role in its availability for biomethylation reactions. Additions of acetate (1.5 mM) or compounds which could cause acetate to accumulate (e.g., glucose, alanine, and 2-bromoethanesulfonate) suppressed DMS formation. It is likely that acetogenic bacteria are involved in DMS formation, but our data are insufficient to allow firm conclusions about the metabolic pathways or organisms involved. Our observations are the first which point to the methylation of MeSH as the major mechanism for endogenous DMS production in any environment. The rates of net DMS production observed are sufficient to explain the relatively high fluxes of DMS emitted to the atmosphere from Sphagnum sp.-dominated wetlands.

Sulphoxide reduction by rat intestinal flora and by Escherichia coli in vitro

The caecal microflora from female rats show a greater ability to reduce the sulphoxide group of sulindac than either the liver or kidneys. Studies on sulphoxide reduction by Escherichia coli showed that NADH, NADPH and dithiothreitol (DTT), but not acetaldehyde could act as cofactors. The cytosolic fraction was responsible for about 90%, 80% and 60% of the total reducing activity with sulindac, diphenyl sulphoxide and sulphinpyrazone, respectively. The main NADPH linked activity in the E. coli cytosol was dependent on thioredoxin, since the activity was essentially abolished by passing through a G50 column or by the addition of anti-thioredoxin anti-serum. Partial purification and separation of sulphoxide reducing activity by DEAE-cellulose chromatography separated two main protein bands, each of which possessed sulindac reducing activity. The importance of thioredoxin for much of the NADPH dependent activity was confirmed but the eluate fractions also showed the presence of other activities with NADH, NADPH and DTT that were independent of thioredoxin. Incubation of the DEAE-cellulose eluate fractions with flosequinan and sulphinpyrazone showed that the reducing activity in the two main protein peaks showed different substrate specificities and that there were multiple sulphoxide reductase systems present in E. coli cytosol.

DMSO respiration by the anaerobic rumen bacterium Wolinella succinogenes

The anaerobic rumen bacterium Wolinella succinogenes was able to grow by respiration with dimethylsulphoxide (DMSO) as electron acceptor and formate or H2 as electron donors. The growth yield amounted to 6.7 g and 6.4 g dry cells/mol DMSO with formate or H2 as the donors, respectively. This suggested an ATP yield of about 0.7 mol ATP/mol DMSO. Cell homogenates and the membrane fraction contained DMSO reductase activity with a high Km (43 mM) for DMSO. The electron transport from H2 to DMSO in the membranes was inhibited by 2-(heptyl)-4-hydroxyquinoline N-oxide, indicating the participation of menaquinone. Formation of DMSO reductase activity occurred only during growth on DMSO, presence of other electron acceptors (fumarate, nitrate, nitrite, N2O, and sulphur) repressed the DMSO reductase activity. DMSO can therefore be used by W. succinogenes as an acceptor for phosphorylative electron transport, but other electron acceptors are used preferentially.

Thiosulfate, polythionates and elemental sulfur assimilation and reduction in the bacterial world

Among sulfur compounds, thiosulfate and polythionates are present at least transiently in many environments. These compounds have a similar chemical structure and their metabolism appears closely related. They are commonly used as energy sources for photoautotrophic or chemolithotrophic microorganisms, but their assimilation has been seldom studied and their importance in bacterial physiology is not well understood. Almost all bacterial strains are able to cleave these compounds since they possess thiosulfate sulfur transferase, thiosulfate reductase or S-sulfocysteine synthase activities. However, the role of these enzymes in the assimilation of thiosulfate or polythionates has not always been clearly established. Elemental sulfur is, on the contrary, very common in the environment. It is an energy source for sulfur-reducing eubacteria and archaebacteria and many sulfur-oxidizing archaebacteria. A phenomenon still not well understood is the 'excessive assimilatory sulfur metabolism' as observed in methanogens which perform a sulfur reduction which exceeds their anabolic needs without any apparent benefit. In heterotrophs, assimilation of elemental sulfur is seldom described and it is uncertain whether this process actually has a physiological significance. Thus, reduction of thiosulfate and elemental sulfur is a common but incompletely understood feature among bacteria. These activities could give bacteria a selective advantage, but further investigations are needed to clarify this possibility. Presence of thiosulfate, polythionates and sulfur reductase activities does not imply obligatorily that these activities play a role in thiosulfate, polythionates or sulfur assimilation as these compounds could be merely intermediates in bacterial metabolism. The possibility also exists that the assimilation of these sulfur compounds is just a side effect of an enzymatic activity with a completely different function. As long as these questions remain unanswered, our understanding of sulfur and thiosulfate metabolism will remain incomplete.

Pyrogallol-to-phloroglucinol conversion and other hydroxyl-transfer reactions catalyzed by cell extracts of Pelobacter acidigallici

Permeabilized cells and cell extracts of Pelobacter acidigallici catalyzed the conversion of pyrogallol (1,2,3-trihydroxybenzene) to phloroglucinol (1,3,5-trihydroxybenzene) in the presence of 1,2,3,5-tetrahydroxybenzene. Pyrogallol consumption by resting cells stopped after lysis by French press or mild detergent (cetyltrimethylammonium bromide [CTAB]) treatment. Addition of 1,2,3,5-tetrahydroxybenzene to the assay mixture restored pyrogallol consumption and led to stoichiometric phloroglucinol accumulation. The stoichiometry of pyrogallol conversion to phloroglucinol was independent of the amount of tetrahydroxybenzene added. The tetrahydroxybenzene concentration limited the velocity of the transhydroxylation reaction, which reached a maximum at 1.5 mM tetrahydroxybenzene (1 U/mg of protein). Transhydroxylation was shown to be reversible. The equilibrium constant of the reaction was determined, and the free-energy change (delta G degree') of phloroglucinol formation from pyrogallol was calculated to be -15.5 kJ/mol. Permeabilized cells and cell extracts also catalyzed the transfer of hydroxyl moieties between other hydroxylated benzenes. Tetrahydroxybenzene and hydroxyhydroquinone participated as hydroxyl donors and as hydroxyl acceptors in the reaction, whereas pyrogallol, resorcinol, and phloroglucinol were hydroxylated by both donors. A novel mechanism deduced from these data involves intermolecular transfer of the hydroxyl moiety from the cosubstrate (1,2,3,5-tetrahydroxybenzene) to the substrate (pyrogallol), thus forming the product (phloroglucinol) and regenerating the cosubstrate.

The inducible trimethylamine N-oxide reductase of Escherichia coli K12: its localization and inducers

We used an anti-trimethylamine-N-oxide reductase (EC 1.6.6.9) serum and different immunological techniques (Ouchterlony, rocket immunoelectrophoresis, immunoblotting) to show that dimethylsulphoxide (DMSO), tetrahydrothiophene 1-oxide (THTO) and pyridine N-oxide (PNO) were effective inducers of the inducible form of trimethylamine N-oxide reductase. We confirmed this genetically and biochemically using a strain in which phage MudII 1734 carrying lacZ was inserted into torA, the structural gene for inducible trimethylamine-N-oxide reductase. By subcellular fractionation and quantitation with rocket immunoelectrophoresis, we showed that the enzyme was principally localized in the periplasmic fraction. Constitutive trimethylamine-N-oxide reductase was localized in the membrane fraction and, like the inducible enzyme showed a broad specificity with respect to various compounds such as DMSO, THTO and PNO. Apart from their immunological properties, the two enzymes could be clearly differentiated by their temperature stability.

Microbial transformations of methylated sulfur compounds in anoxic salt marsh sediments

Anoxic salt marsh sediments were amended with several methylated sulfur compounds. Sediment microbes transformed the added compounds into other volatile methylated sulfur compounds and eventually mineralized the compounds to CH4 and presumably to CO2 and H2S. The principal methyl-sulfur product of dimethylsulfoniopropionate (DMSP) was found to be dimethylsulfide (DMS), with only small amounts of methane thiol (MSH) produced. By contrast, methionine and S-methyl cysteine were degraded mostly to MSH and to lesser amounts of DMS. Dimethylsulfoxide (DMSO) was biologically converted to DMS. Dimethyldisulfide (DMDS) was rapidly reduced to MSH by the sediment microflora, and some DMS was also produced. DMS, whether added directly or when derived from other precursors, was metabolized with the production of MSH. Methane thiol was also metabolized, and evidence suggests that MSH may be biologically methylated to form DMS. Experiments with selective microbial inhibitors were used to ascertain which microbial groups were responsible for the observed transformations. Based on these experiments, it appears that both sulfate-reducing and methane-producing bacteria may be involved in transforming and mineralizing methylated sulfur compounds. A simple scheme of how methylated sulfur compounds may be transformed in the environment is presented.

Molecular cloning and expression of the Escherichia coli dimethyl sulfoxide reductase operon

The dimethyl sulfoxide (DMSO) reductase operon coding for a membrane-bound iron-sulfur, molybdoenzyme, which functions as a terminal reductase in Escherichia coli, has been isolated and cloned from an E. coli gene bank. Two clones, MV12(pLC19-36) and MV12(pLC43-43), overexpressed both DMSO and trimethylamine N-oxide (TMAO) reductase activities 13- to 15-fold compared with wild-type cells. Amplification was highest in cells grown anaerobically on fumarate, while cells grown on DMSO or TMAO displayed reduced levels of enzyme amplification. Growth on nitrate or aerobic growth repressed expression of the enzyme. A 6.5-kilobase-pair DNA restriction endonuclease fragment was subcloned from pLC19-36 into the vector pBR322, yielding a recombinant DMSO reductase plasmid, pDMS159. Two polypeptides were amplified and identified on sodium dodecyl sulfate-polyacrylamide gels of proteins from E. coli HB101 harboring pDMS159: a membrane-bound protein with molecular weight 82,600 and a soluble polypeptide with molecular weight 23,600. Three plasmid-encoded polypeptides with molecular weights of 87,500, 23,300, and 22,600 were detected by in vivo transcription/translation studies. The smallest subunit was poorly defined and not detectable by Coomassie blue staining. The DMSO reductase operon was localized to the 20.0-min position on the E. coli linkage map.

Tetrahydrothiophene 1-oxide as an electron acceptor for Escherichia coli

Escherichia coli used tetrahydrothiophene 1-oxide (THTO) as an electron acceptor for anaerobic growth with glycerol as a carbon source; the THTO was reduced to tetrahydrothiophene. Cell extracts also reduced THTO to tetrahydrothiophene in the presence of a variety of electron donors. Chlorate-resistant (chl) mutants (chlA, chlB, chlD, and chlE) were unable to grow with THTO as the electron acceptor. However, growth and THTO reduction by the chlD mutant were restored by high concentrations of molybdate. Similarly, mutants of E. coli that are blocked in the menaquinone (vitamin K2) biosynthetic pathway, i.e., menB, menC, and menD mutants, did not grow with THTO as an electron acceptor. Growth and THTO reduction were restored in these mutants by the presence of appropriate intermediates of the vitamin K biosynthetic pathway.

Proton translocation coupled to dimethyl sulfoxide reduction in anaerobically grown Escherichia coli HB101

Proton translocation coupled to dimethyl sulfoxide (DMSO) reduction was examined in Escherichia coli HB101 grown anaerobically on glycerol and DMSO. Rapid acidification of the medium was observed when an anaerobic suspension of cells, preincubated with glycerol, was pulsed with DMSO, methionine sulfoxide, nitrate, or trimethylamine N-oxide. The DMSO-induced acidification was sensitive to the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (60 microM) and was inhibited by the quinone analog 2-n-heptyl-4-hydroxy-quinoline-N-oxide (5.6 microM). Neither sodium azide nor potassium cyanide inhibited the DMSO response. An apparent----H+/2e- ratio of 2.9 was obtained for DMSO reduction with glycerol as the reductant. Formate and H2(g), but not lactate, could serve as alternate electron donors for DMSO reduction. Cells grown anaerobically on glycerol and fumarate displayed a similar response to pulses of DMSO, methionine sulfoxide, nitrate, and trimethylamine N-oxide with either glycerol or H2(g) as the electron donor. However, fumarate pulses did not result in acidification of the suspension medium. Proton translocation coupled to DMSO reduction was also demonstrated in membrane vesicles by fluorescence quenching. The addition of DMSO to hydrogen-saturated everted membrane vesicles resulted in a carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone-sensitive fluorescence quenching of quinacrine dihydrochloride. The data indicate that reduction of DMSO by E. coli is catalyzed by an anaerobic electron transport chain, resulting in the formation of a proton motive force.

Dimethyl sulfoxide reductase activity by anaerobically grown Escherichia coli HB101

Escherichia coli grew anaerobically on a minimal medium with glycerol as the carbon and energy source and dimethyl sulfoxide (DMSO) as the terminal electron acceptor. DMSO reductase activity, measured with an artificial electron donor (reduced benzyl viologen), was preferentially associated with the membrane fraction (77 +/- 10% total cellular activity). A Km for DMSO reduction of 170 +/- 60 microM was determined for the membrane-bound activity. Methyl viologen, reduced flavin mononucleotide, and reduced flavin adenine dinucleotide also served as electron donors for DMSO reduction. Methionine sulfoxide, a DMSO analog, could substitute for DMSO in both the growth medium and in the benzyl viologen assay. DMSO reductase activity was present in cells grown anaerobically on DMSO but was repressed by the presence of nitrate or by aerobic growth. Anaerobic growth on DMSO coinduced nitrate, fumarate, and and trimethylamine-N-oxide reductase activities. The requirement of a molybdenum cofactor for DMSO reduction was suggested by the inhibition of growth and a 60% reduction in DMSO reductase activity in the presence of 10 mM sodium tungstate. Furthermore, chlorate-resistant mutants chlA, chlB, chlE, and chlG were unable to grow anaerobically on DMSO. DMSO reduction appears to be under the control of the fnr gene.