Inductive properties of polypyridyl ruthenium complexes significantly regulate various protein distributions in Escherichia coli

Ruthenium complexes with similar octahedral structures but different intrinsic inductive properties significantly influence the total cellular protein distributions, which may affect different metabolic pathways. A systematic study of the relationship between ruthenium complexes and Escherichia coli was undertaken, using two-dimensional gel electrophoresis analysis and the identification of various proteins by mass data mining. Based on the low similarities (<40%) between the total protein distributions, the inductive properties of the ruthenium complexes are relevant to the formation of the protein-Ru interaction in addition to the Ru-DNA interaction. Two major protein functions in E. coli BL21 that were reduced by compound 1 were oxidoreductases and transporters, corresponding to 29% and 25% of the 24 down-regulated proteins. The main biological processes of the proteins down-regulated by compound 1 were related to carbohydrate reactions, including in transport, tricarboxylic acid (TCA) cycle, glycolysis, and gluconeogenesis. All four ruthenium complexes shared similar up-regulated proteins, including clpB and kpyk1, and down-regulated similar proteins, including ompA and ybbN. This result supports that the presence of Ru-protein interactions is a major factor affecting bacteria growth, and particularly transport and carbohydrate-related reactions.

The dual-targeted plant sulfiredoxin retroreduces the sulfinic form of atypical mitochondrial peroxiredoxin

Sulfiredoxin (Srx) couples the energy of ATP hydrolysis to the energetically unfavorable process of reducing the inactive sulfinic form of 2-cysteine peroxiredoxins (Prxs) to regenerate its active form. In plants, Srx as well as typical 2-cysteine Prx have been considered as enzymes with exclusive chloroplast localization. This work explores the subcellular localization of Srx in pea (Pisum sativum) and Arabidopsis (Arabidopsis thaliana). Immunocytochemistry, analysis of protein extracts from isolated intact organelles, and cell-free posttranslational import assays demonstrated that plant Srx also localizes to the mitochondrion in addition to plastids. The dual localization was in line with the prediction of a signal peptide for dual targeting. Activity tests and microcalorimetric data proved the interaction between Srx and its mitochondrial targets Prx IIF and thioredoxin. Srx catalyzed the retroreduction of the inactive sulfinic form of atypical Prx IIF using thioredoxin as reducing agent. Arabidopsis Srx also reduced overoxidized human Prx V. These results suggest that plant Srx could play a crucial role in the regulation of Prx IIF activity by controlling the regeneration of its overoxidized form in mitochondria, which are sites of efficient reactive oxygen species production in plants.

Evaluation of biofilm performance as a protective barrier against biocorrosion using an enzyme electrode

Sulfide is known to be an important factor in microbiologically influenced corrosion (MIC) of metals and concrete deterioration in wastewater treatment structures and sewer pipelines. A sulfide biosensor was used to determine the effectiveness of Escherichia coli DH5 alpha biofilm as a protective barrier against MIC. The biofilm was shown to be effective in protecting surfaces from sulfide and helping to reduce MIC using amperometric measurements. The results also indicated that the growth conditions of E. coli DH5 alpha may have an impact on the performance of the biofilm as a sulfide barrier. The simple method provided in this work enables the comparison of several microbial biofilms and selection of the ones with potential to prevent MIC in a relatively short time.

Control of sulfur partitioning between primary and secondary metabolism

Sulfur is an essential nutrient for all organisms. Plants take up most sulfur as inorganic sulfate, reduce it and incorporate it into cysteine during primary sulfate assimilation. However, some of the sulfate is partitioned into the secondary metabolism to synthesize a variety of sulfated compounds. The two pathways of sulfate utilization branch after activation of sulfate to adenosine 5'-phosphosulfate (APS). Recently we showed that the enzyme APS kinase limits the availability of activated sulfate for the synthesis of sulfated secondary compounds in Arabidopsis. To further dissect the control of sulfur partitioning between the primary and secondary metabolism, we analysed plants in which activities of enzymes that use APS as a substrate were increased or reduced. Reduction in APS kinase activity led to reduced levels of glucosinolates as a major class of sulfated secondary metabolites and an increased concentration of thiols, products of primary reduction. However, over-expression of this gene does not affect the levels of glucosinolates. Over-expression of APS reductase had no effect on glucosinolate levels but did increase thiol levels, but neither glucosinolate nor thiol levels were affected in mutants lacking the APR2 isoform of this enzyme. Measuring the flux through sulfate assimilation using [(35) S]sulfate confirmed the larger flow of sulfur to primary assimilation when APS kinase activity was reduced. Thus, at least in Arabidopsis, the interplay between APS reductase and APS kinase is important for sulfur partitioning between the primary and secondary metabolism.

Molecular characterization and expression analysis of interferon-gamma-inducible lysosomal thiol reductase (GILT) gene from pearl oyster Pinctada fucata

Interferon-gamma-inducible lysosomal thiol reductase (GILT) is an important thiol reductase, involved in class, MHC-restricted antigen processing by catalyzing disulfide bond reduction in mammals. Herein, we describe the identification and characterization of pearl oyster Pinctada fucata GILT (designated as poGILT). The poGILT cDNA was 1273bp long and consisted of a 5'-untranslated region (UTR) of 24bp, a 3'-UTR of 484bp with two cytokine RNA instability motifs (ATTTA), and an open reading frame (ORF) of 765bp encoding a polypeptide of 254 amino acids with an estimated molecular mass of 28.9kDa and a theoretical isoelectric point of 7.4. The N-terminus of the poGILT was found to have a putative signal peptide with a cleavage site amino acid position at 19-20. SMART analysis showed that the poGILT contained a GILT active-site C(69)PDC(72) motif and a GILT signature motif C(115)QHGKEECIGNLIETC(130). Homology analysis of the deduced amino acid sequence of the poGILT with other known GILT sequences by MatGAT software revealed that the poGILT shared 42.9-67.3% similarity and 22.9-49.8% identity to the other known GILT sequences. The expression level of poGILT mRNA was higher in digestive gland, moderate in adductor muscle, gills, gonad, intestine and mantle, and lower in hemocytes. The poGILT mRNA expression was significantly up-regulated in gill and digestive gland after LPS or V. alginolyticus stimulation, respectively. These results suggested that the poGILT was a constitutively expressed acute-phase protein, the expression of which can be enhanced after LPS or V. algrinolyticus stimulation, perhaps involved in the innate immune response of pearl oyster.

Regulation of sulfate assimilation in Physcomitrella patens: mosses are different!

Sulfur is an essential nutrient, taken up as sulfate from soil, reduced and incorporated into bioorganic compounds in plant cells. The pathway of sulfate assimilation is highly regulated in a demand-driven manner in seed plants. To test the evolutionary conservation of the regulatory mechanisms, we analyzed regulation of the pathway in the model for basal plants, the moss Physcomitrella patens. While in Arabidopsis the key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase (APR), is feedback repressed by thiols and induced by reduced levels of glutathione, in P. patens such regulation does not occur. The control of the pathway was not moved to other components as these conditions affected neither mRNA accumulation of other genes of sulfate assimilation nor sulfate uptake. Other treatments known to regulate APR, O-acetylserine, cadmium and sulfur deficiency affected APR transcript levels, but not enzyme activity. It appears that the sulfate assimilation pathway in P. patens is much more robust than in seed plants. Thus, the regulatory networks controlling the pathway have probably evolved only later in the evolution of the seed plants after separation of the bryophytes.

Identification of rice Al-responsive genes by semi-quantitative polymerase chain reaction using sulfite reductase as a novel endogenous control

Based on the evidence that Al resistance is an inducible process and rice is an Al-resistant crop, identification of Al-responsive genes from rice may help to further clone Al-resistant genes in plants. Semi-quantitative and real-time polymerase chain reaction (PCR) is widely applied in gene transcriptional analyses, particularly for those genes with low transcript abundance. Normalization with proper endogenous control (EC) genes is critical for these two approaches in terms of reliability and precision. We first noticed that the expression of several commonly-used EC genes was depressed under Al stress, while sulfite reductase gene (SR) was stable throughout the Al treatment. The reliability of SR as an EC gene was further tested by analyzing the expression of a number of genes in response to Al challenge. Except for the consistent results obtained for the four previously-identified genes, nine additional genes were newly defined as Al-responsive in this study. Collectively, our results suggest that SR can be used as a novel EC gene for semi-quantitative and real-time PCR analysis of Al responsive genes, and that activated transport of silicon and stimulated metabolism of carotenoid and terpenoid could be involved in Al resistance in rice plants.

Nitrate assimilatory genes and their transcriptional regulation in a unicellular red alga Cyanidioschyzon merolae: genetic evidence for nitrite reduction by a sulfite reductase-like enzyme

Cyanidioschyzon merolae is a unicellular red alga living in acid hot springs, which is able to grow on ammonium, as well as nitrate as sole nitrogen source. Based on the complete genome sequence, proteins for nitrate utilization, nitrate transporter (NRT) and nitrate reductase (NR), were predicted to be encoded by the neighboring nuclear genes CMG018C and CMG019C, respectively, but no typical nitrite reductase (NiR) gene was found by similarity searches. On the other hand, two candidate genes for sulfite reductase (SiR) were found, one of which (CMG021C) is located next to the above-noted nitrate-related genes. Given that transcripts of CMG018C, CMG019C and CMG021C accumulate in nitrate-containing media, but are repressed by ammonium, and that SiR and NiR are structurally related enzymes, we hypothesized that the CMG021C gene product functions as an NiR in C. merolae. To test this hypothesis, we developed a method for targeted gene disruption in C. merolae. In support of our hypothesis, we found that a CMG021G null mutant in comparison with the parental strain showed decreased cell growth in nitrate-containing but not in ammonium-containing media. Furthermore, expression of CMG021C in the nirA mutant of a cyanobacterium, Leptolyngbya boryana (formerly Plectonema boryanum), could genetically complement the NiR defect. Immunofluorescent analysis indicated the localization of CMG021C in chloroplasts, and hence we propose an overall scheme for nitrate assimilation in C. merolae.

Copper exposure interferes with the regulation of the uptake, distribution and metabolism of sulfate in Chinese cabbage

Exposure of Chinese cabbage (Brassica pekinensis) to enhanced Cu(2+) concentrations (1-10 microM) resulted in leaf chlorosis, a loss of photosynthetic capacity and lower biomass production at > or = 5 microM. The decrease in pigment content was likely not the consequence of degradation, but due to hindered chloroplast development upon Cu exposure. The Cu content of the root increased with the Cu(2+) concentration (up to 40-fold), though only a minor proportion (4%) was transferred to the shoot. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+). The nitrogen content of the root was affected little at lower Cu(2+) levels, whereas that in the shoot was decreased at > or = 5 microM Cu(2+). Cu affected the uptake, distribution and metabolism of sulfate in Chinese cabbage. The total sulfur content of the shoot was increased at > or = 2 microM Cu(2+), which could be attributed mainly to an increase in sulfate content. Moreover, there was a strong increase in water-soluble non-protein thiol content in the root and, to a lesser extent, in the shoot at > or = 1 microM, which could only partially be ascribed to a Cu-induced enhancement of the phytochelatin content. The nitrate uptake by the root was substantially reduced at > or = 5 microM Cu(2+), coinciding with a decrease in biomass production. However, the activity of the sulfate transporters in the root was slightly enhanced at 2 and 5 microM Cu(2+), accompanied by enhanced expression of the Group 1 high affinity transporter Sultr1;2, and the Group 4 transporters Sultr4;1 and Sultr4;2. In the shoot, there was an induction of expression of Sultr4;2 at 5 and 10 microM Cu(2+). The expression of APS reductase was affected little in the root and shoot up to 10 microM Cu(2+). The upregulation of the sulfate transporters may be due not only to greater sulfur demand at higher Cu levels, but also the consequence of interference by Cu with the signal transduction pathway regulating the expression and activity of the sulfate transporters.

Sulfite reductase defines a newly discovered bottleneck for assimilatory sulfate reduction and is essential for growth and development in Arabidopsis thaliana

The role of sulfite reductase (SiR) in assimilatory reduction of inorganic sulfate to sulfide has long been regarded as insignificant for control of flux in this pathway. Two independent Arabidopsis thaliana T-DNA insertion lines (sir1-1 and sir1-2), each with an insertion in the promoter region of SiR, were isolated. sir1-2 seedlings had 14% SiR transcript levels compared with the wild type and were early seedling lethal. sir1-1 seedlings had 44% SiR transcript levels and were viable but strongly retarded in growth. In mature leaves of sir1-1 plants, the levels of SiR transcript, protein, and enzymatic activity ranged between 17 and 28% compared with the wild type. The 28-fold decrease of incorporation of (35)S label into Cys, glutathione, and protein in sir1-1 showed that the decreased activity of SiR generated a severe bottleneck in the assimilatory sulfate reduction pathway. Root sulfate uptake was strongly enhanced, and steady state levels of most of the sulfur-related metabolites, as well as the expression of many primary metabolism genes, were changed in leaves of sir1-1. Hexose and starch contents were decreased, while free amino acids increased. Inorganic carbon, nitrogen, and sulfur composition was also severely altered, demonstrating strong perturbations in metabolism that differed markedly from known sulfate deficiency responses. The results support that SiR is the only gene with this function in the Arabidopsis genome, that optimal activity of SiR is essential for normal growth, and that its downregulation causes severe adaptive reactions of primary and secondary metabolism.

Targeted knock-out of a gene encoding sulfite reductase in the moss Physcomitrella patens affects gametophytic and sporophytic development

A key step in sulfate assimilation into cysteine is the reduction of sulfite to sulfide by sulfite reductase (SiR). This enzyme is encoded by three genes in the moss Physcomitrella patens. To obtain a first insight into the roles of the individual isoforms, we deleted the gene encoding the SiR1 isoform in P. patens by homologous recombination and subsequently analysed the DeltaSiR1 mutants. While DeltaSiR1 mutants showed no obvious alteration in sulfur metabolism, their regeneration from protoplasts and their ability to produce mature spores was significantly affected, highlighting an unexpected link between moss sulfate assimilation and development, that is yet to be characterized.

Characterization of plant sulfiredoxin and role of sulphinic form of 2-Cys peroxiredoxin

The antioxidant function of 2-Cys peroxiredoxin (Prx) involves the oxidation of its conserved peroxidatic cysteine to sulphenic acid that is recycled by a reductor agent. In conditions of oxidative stress, the peroxidatic cysteine can be overoxidized to sulphinic acid inactivating the Prx. An enzyme recently discovered, named sulfiredoxin (Srx), reduces the sulphinic 2-Cys Prx (Prx-SO(2)H). To explore the physiological functions of Srx in plants we have cloned, expressed and purified to homogeneity a Srx from Arabidopsis thaliana (AtSrx), as well as five variants by site-directed mutagenesis on amino acids involved in its activity. The activity of sulfiredoxin, determined by a new method, is dependent on the concentration of the sulphinic form of Prx and the conserved Srx is capable of regenerating the functionality of both pea and Arabidopsis Prx-SO(2)H. Molecular modelling of AtSrx and the facts that the R28Q variant shows a partial inactivation, that the activity of the E76A variant is equivalent to that of the native enzyme and that the double mutation R28Q/E76A abolishes the enzymatic activity suggests that the pair His100-Glu76 may be involved in the activation of C72 in the absence of R28. The knock-out mutant plants without Srx or 2-Cys Prx exhibited phenotypical differences under growth conditions of 16 h light, probably due to the signalling role of the sulphinic form of Prx. These mutants showed more susceptibility to oxidative stress than wild-type plants. This work presents the first systematic biochemical characterization of the Srx/Prx system from plants and contributes to a better understanding of its physiological function.

Sulphur flux through the sulphate assimilation pathway is differently controlled by adenosine 5'-phosphosulphate reductase under stress and in transgenic poplar plants overexpressing gamma-ECS, SO, or APR

Sulphate assimilation provides reduced sulphur for the synthesis of cysteine, methionine, and numerous other essential metabolites and secondary compounds. The key step in the pathway is the reduction of activated sulphate, adenosine 5'-phosphosulphate (APS), to sulphite catalysed by APS reductase (APR). In the present study, [(35)S]sulphur flux from external sulphate into glutathione (GSH) and proteins was analysed to check whether APR controls the flux through the sulphate assimilation pathway in poplar roots under some stress conditions and in transgenic poplars. (i) O-Acetylserine (OAS) induced APR activity and the sulphur flux into GSH. (ii) The herbicide Acetochlor induced APR activity and results in a decline of GSH. Thereby the sulphur flux into GSH or protein remained unaffected. (iii) Cd treatment increased APR activity without any changes in sulphur flux but lowered sulphate uptake. Several transgenic poplar plants that were manipulated in sulphur metabolism were also analysed. (i) Transgenic poplar plants that overexpressed the gamma-glutamylcysteine synthetase (gamma-ECS) gene, the enzyme catalysing the key step in GSH formation, showed an increase in sulphur flux into GSH and sulphate uptake when gamma-ECS was targeted to the cytosol, while no changes in sulphur flux were observed when gamma-ECS was targeted to plastids. (ii) No effect on sulphur flux was observed when the sulphite oxidase (SO) gene from Arabidopsis thaliana, which catalyses the back reaction of APR, that is the reaction from sulphite to sulphate, was overexpressed. (iii) When Lemna minor APR was overexpressed in poplar, APR activity increased as expected, but no changes in sulphur flux were observed. For all of these experiments the flux control coefficient for APR was calculated. APR as a controlling step in sulphate assimilation seems obvious under OAS treatment, in gamma-ECS and SO overexpressing poplars. A possible loss of control under certain conditions, that is Cd treatment, Acetochlor treatment, and in APR overexpressing poplar, is discussed.

Increasing sulfur supply enhances tolerance to arsenic and its accumulation in Hydrilla verticillata (Lf.) Royle

The present study was aimed to analyze the effects of variable S supply on arsenic (As) accumulation potential of Hydrilla verticillata (Lf.) Royle. Plants were exposed to either arsenate (AsV; 50 microM) or arsenite (AsIII; 5 microM) for 4 h and 1 day while S supply was varied as deficient (2 microM, -S), normal (1 mM, +S) and excess (2 mM, +HS). The level of As accumulation (microg g(-1) dw) after 1 day was about 2-fold higher upon exposure to either AsV (30) or AsIII (50) in +HS plants than that being in +S (12 and 24) and -S (14 and 26) plants. The +HS plants showed a significant stimulation of the thiol metabolism upon As exposure. Besides, they did not experience significant toxicity, measured in terms of malondialdehyde accumulation; an indicator of oxidative stress. By contrast, -S plants suffered from oxidative stress probably due to negative impact to thiol metabolism. Variable S supply also modulated the activity of enzymes of glycine and serine biosynthesis indicating an interconnection between S and N metabolism. In conclusion, an improved supply of S to plants was found to augment their ability for As accumulation through stimulated thiol metabolism.

Increasing sulfur supply enhances tolerance to arsenic and its accumulation in Hydrilla verticillata (Lf.) Royle

The present study was aimed to analyze the effects of variable S supply on arsenic (As) accumulation potential of Hydrilla verticillata (Lf.) Royle. Plants were exposed to either arsenate (AsV; 50 microM) or arsenite (AsIII; 5 microM) for 4 h and 1 day while S supply was varied as deficient (2 microM, -S), normal (1 mM, +S) and excess (2 mM, +HS). The level of As accumulation (microg g(-1) dw) after 1 day was about 2-fold higher upon exposure to either AsV (30) or AsIII (50) in +HS plants than that being in +S (12 and 24) and -S (14 and 26) plants. The +HS plants showed a significant stimulation of the thiol metabolism upon As exposure. Besides, they did not experience significant toxicity, measured in terms of malondialdehyde accumulation; an indicator of oxidative stress. By contrast, -S plants suffered from oxidative stress probably due to negative impact to thiol metabolism. Variable S supply also modulated the activity of enzymes of glycine and serine biosynthesis indicating an interconnection between S and N metabolism. In conclusion, an improved supply of S to plants was found to augment their ability for As accumulation through stimulated thiol metabolism.

Oxidative stress and disruption of labile iron generate specific auxotrophic requirements in Salmonella enterica

The response of a cell to integrated stresses was investigated using environmental and/or genetic perturbations that disrupted labile iron homeostasis and increased oxidative stress. The effects of the perturbations were monitored as nutritional requirements, and were traced to specific enzymic targets. A yggX gshA cyaY mutant strain required exogenous thiamine and methionine for growth. The thiamine requirement, which had previously been linked to the Fe-S cluster proteins ThiH and ThiC, was responsive to oxidative stress and was not directly affected by manipulation of the iron pool. The methionine requirement was associated with the activity of sulfite reductase, an enzyme that appeared responsive to disruption of labile iron homeostasis. The results are incorporated in a model to suggest how the activity of iron-containing enzymes not directly sensitive to oxygen can be decreased by oxidation of the labile iron pool.

Nrf2-dependent sulfiredoxin-1 expression protects against cigarette smoke-induced oxidative stress in lungs

Oxidative stress results in protein oxidation and is involved in the pathogenesis of lung diseases such as chronic obstructive pulmonary disorder (COPD). Sulfiredoxin-1 (Srx1) catalyzes the reduction of cysteine sulfinic acid to sulfenic acid in oxidized proteins and protects them from inactivation. This study examined the mechanism of transcriptional regulation of Srx1 and its possible protective role during oxidative stress associated with COPD. Nrf2, a transcription factor known to influence susceptibility to pulmonary diseases, upregulates Srx1 expression during oxidative stress caused by cigarette smoke exposure in the lungs of mice. Disruption of Nrf2 signaling by genetic knockout in mice or RNAi in cells downregulated the expression of Srx1. In silico analysis of the 5'-promoter-flanking region of Srx1 identified multiple antioxidant-response elements (AREs) that are highly conserved. Reporter and chromatin-immunoprecipitation assays demonstrated that ARE1 at -228 is critical for the Nrf2-mediated response. Attenuation of Srx1 expression with RNAi potentiated the toxicity of hydrogen peroxide (H2O2), whereas overexpression of Srx1 protected against H2O2-mediated cell death in vitro. Immunoblot analysis revealed dramatic decreases in Srx1 expression in lungs from patients with COPD relative to nonemphysematous lungs together with a decline in Nrf2 protein. Thus, Srx1, a key Nrf2-regulated gene, contributes to protection against oxidative injury in the lung.

Expression and activity of sulfate transporters and APS reductase in curly kale in response to sulfate deprivation and re-supply

Both activity and expression of sulfate transporters and APS reductase in plants are modulated by the sulfur status of the plant. To examine the regulatory mechanisms in curly kale (Brassica oleracea L.), the sulfate supply was manipulated by the transfer of seedlings to sulfate-deprived conditions, which resulted in an up to 3-fold increase in the sulfate uptake capacity by the root, accompanied by an induction of transcript abundances of the Group 1 and 4 sulfate transporters in root and shoot. Upon sulfate re-supply, there was no correlation between the activity and expression of the sulfate transporters. Despite the decrease in the abundance of the sulfate transporter transcripts, especially at the onset of the sulfate re-supply, the sulfate uptake capacity was affected very little for up to 96h. There was no relationship between changes in the sulfate or thiol content and activity and expression of the sulfate transporters. Thus, their significance as regulatory signal compounds remains unresolved. The activity and expression of APS reductase, which was enhanced strongly only in the shoots of sulfate-deprived plants, and rapidly decreased again upon sulfate re-supply, corresponded with changes in thiol content, consistent with this pool having a role as a regulatory signal.

The cinnamon-derived Michael acceptor cinnamic aldehyde impairs melanoma cell proliferation, invasiveness, and tumor growth

Redox dysregulation in cancer cells represents a chemical vulnerability that can be targeted by pro-oxidant redox intervention. Dietary constituents that contain an electrophilic Michael acceptor pharmacophore may therefore display promising chemopreventive and chemotherapeutic anti-cancer activity. Here, we demonstrate that the cinnamon-derived dietary Michael acceptor trans-cinnamic aldehyde (CA) impairs melanoma cell proliferation and tumor growth. Feasibility of therapeutic intervention using high doses of CA (120 mg/kg, po, daily, 10 days) was demonstrated in a human A375 melanoma SCID mouse xenograft model. Low-micromolar concentrations (IC(50)< 10 microM) of CA, but not closely related CA derivatives devoid of Michael acceptor activity, suppressed proliferation of human metastatic melanoma cell lines (A375, G361, LOX) with G1 cell-cycle arrest, elevated intracellular ROS, and impaired invasiveness. Expression array analysis revealed that CA induced an oxidative stress response in A375 cells, up-regulating heme oxygenase 1, sulfiredoxin 1 homolog, thioredoxin reductase 1, and other genes, including the cell-cycle regulator and stress-responsive tumor suppressor gene cyclin-dependent kinase inhibitor 1A, a key mediator of G1-phase arrest. CA, but not Michael-inactive derivatives, inhibited NF-kappaB transcriptional activity and TNFalpha-induced IL-8 production in A375 cells. These findings support a previously unrecognized role of CA as a dietary Michael acceptor with potential anti-cancer activity.

Complex signaling network in regulation of adenosine 5'-phosphosulfate reductase by salt stress in Arabidopsis roots

Sulfur-containing compounds play an important role in plant stress defense; however, only a little is known about the molecular mechanisms of regulation of sulfate assimilation by stress. Using known Arabidopsis (Arabidopsis thaliana) mutants in signaling pathways, we analyzed regulation of the key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase (APR), by salt stress. APR activity and mRNA levels of all three APR isoforms increased 3-fold in roots after 5 h of treatment with 150 mm NaCl. The regulation of APR was not affected in mutants deficient in abscisic acid (ABA) synthesis and treatment of the plants with ABA did not affect the mRNA levels of APR isoforms, showing that APR is regulated by salt stress in an ABA-independent manner. In mutants deficient in jasmonate, salicylate, or ethylene signaling, APR mRNA levels were increased upon salt exposure similar to wild-type plants. Surprisingly, however, APR enzyme activity was not affected by salt in these plants. The same result was obtained in mutants affected in cytokinin and auxin signaling. Signaling via gibberellic acid, on the other hand, turned out to be essential for the increase in APR mRNA by salt treatment. These results demonstrate an extensive posttranscriptional regulation of plant APR and reveal that the sulfate assimilation pathway is controlled by a complex network of multiple signals on different regulatory levels.

Homology modeling of dissimilatory APS reductases (AprBA) of sulfur-oxidizing and sulfate-reducing prokaryotes

Background

The dissimilatory adenosine-5′-phosphosulfate (APS) reductase (cofactors flavin adenine dinucleotide, FAD, and two [4Fe-4S] centers) catalyzes the transformation of APS to sulfite and AMP in sulfate-reducing prokaryotes (SRP); in sulfur-oxidizing bacteria (SOB) it has been suggested to operate in the reverse direction. Recently, the three-dimensional structure of the Archaeoglobus fulgidus enzyme has been determined in different catalytically relevant states providing insights into its reaction cycle.

Methodology/Principal Findings

Full-length AprBA sequences from 20 phylogenetically distinct SRP and SOB species were used for homology modeling. In general, the average accuracy of the calculated models was sufficiently good to allow a structural and functional comparison between the beta- and alpha-subunit structures (78.8–99.3% and 89.5–96.8% of the AprB and AprA main chain atoms, respectively, had root mean square deviations below 1 Å with respect to the template structures). Besides their overall conformity, the SRP- and SOB-derived models revealed the existence of individual adaptations at the electron-transferring AprB protein surface presumably resulting from docking to different electron donor/acceptor proteins. These structural alterations correlated with the protein phylogeny (three major phylogenetic lineages: (1) SRP including LGT-affected Archaeoglobi and SOB of Apr lineage II, (2) crenarchaeal SRP Caldivirga and Pyrobaculum, and (3) SOB of the distinct Apr lineage I) and the presence of potential APS reductase-interacting redox complexes. The almost identical protein matrices surrounding both [4Fe-4S] clusters, the FAD cofactor, the active site channel and center within the AprB/A models of SRP and SOB point to a highly similar catalytic process of APS reduction/sulfite oxidation independent of the metabolism type the APS reductase is involved in and the species it has been originated from.

Conclusions

Based on the comparative models, there are no significant structural differences between dissimilatory APS reductases from SRP and SOB; this might be indicative for a similar catalytic process of APS reduction/sulfite oxidation.

The role of 5'-adenylylsulfate reductase in the sulfur assimilation pathway of soybean: molecular cloning, kinetic characterization, and gene expression

Soybean seeds are a major source of protein, but contain low levels of sulfur-containing amino acids. With the objective of studying the sulfur assimilation pathway of soybean, a full-length cDNA clone for 5'-adenylylsulfate reductase (APS reductase) was isolated and characterized. The cDNA clone contained an open reading frame of 1414 bp encoding a 52 kDa protein with a N-terminal chloroplast/plastid transit peptide. Southern blot analysis of genomic DNA indicated that the APS reductase in soybean is encoded by a small multigene family. Biochemical characterization of the heterologously expressed and purified protein shows that the clone encoded a functional APS reductase. Although expressed in tissues throughout the plant, these analyses established an abundant expression of the gene and activity of the encoded protein in the early developmental stages of soybean seed, which declined with seed maturity. Sulfur and phosphorus deprivation increased this expression level, while nitrogen starvation repressed APS reductase mRNA transcript and protein levels. Cold-treatment increased expression and the total activity of APS reductase in root tissues. This study provides insight into the sulfur assimilation pathway of this nutritionally important legume.

Differentiation of the roles of sulfide oxidase and rhodanese in the detoxification of sulfide by the colonic mucosa

PURPOSE

Identify the roles of sulfide oxidase and rhodanese in sulfide detoxification in rat colonic mucosa.

RESULTS

Gel filtration of colonic mucosa and purified bovine rhodanese showed that rhodanese and sulfide oxidizing activities resided in different proteins. In the presence of cyanide, rhodanese shifted the major mucosal metabolite of sulfide from thiosulfate to thiocyanate. The purported ability of purified rhodanese to metabolize sulfide reflects: (a) contamination with a sulfide oxidase, and (b) the spontaneous conversion of sulfide to thiosulfate during storage; rhodanese then catalyzes the conversion of this thiosulfate to thiocyanate.

CONCLUSIONS

Rhodanese does not metabolize sulfide. The rate-limiting step in sulfide detoxification is oxidation by a sulfide oxidase to thiosulfate. Rhodanese then converts this thiosulfate to thiocyanate, but this reaction does not increase the rate of sulfide detoxification. The recent use of rhodanese activity as a surrogate for the rate that colonic mucosa detoxifies sulfide is inappropriate.

Molecular analysis of the diversity of sulfate-reducing and sulfur-oxidizing prokaryotes in the environment, using aprA as functional marker gene

The dissimilatory adenosine-5'-phosphosulfate reductase is a key enzyme of the microbial sulfate reduction and sulfur oxidation processes. Because the alpha- and beta-subunit-encoding genes, aprBA, are highly conserved among sulfate-reducing and sulfur-oxidizing prokaryotes, they are most suitable for molecular profiling of the microbial community structure of the sulfur cycle in environment. In this study, a new aprA gene-targeting assay using a combination of PCR and denaturing gradient gel electrophoresis is presented. The screening of sulfate-reducing and sulfur-oxidizing reference strains as well as the analyses of environmental DNA from diverse habitats (e.g., microbial mats, invertebrate tissue, marine and estuarine sediments, and filtered hydrothermal water) by the new primer pair revealed an improved microbial diversity coverage and less-pronounced template-to-PCR product bias in direct comparison to those of the previously published primer set (B. Deplancke, K. R. Hristova, H. A. Oakley, V. J. McCracken, R. Aminov, R. I. Mackie, and H. R. Gaskins, Appl. Environ. Microbiol. 66:2166-2174, 2000). The concomitant molecular detection of sulfate-reducing and sulfur-oxidizing prokaryotes was confirmed. The new assay was applied in comparison with the 16S rRNA gene-based analysis to investigate the microbial diversity of the sulfur cycle in sediment, seawater, and manganese crust samples from four study sites in the area of the Lesser Antilles volcanic arc, Caribbean Sea (Caribflux project). The aprA gene-based approach revealed putative sulfur-oxidizing Alphaproteobacteria of chemolithoheterotrophic lifestyle to have been abundant in the nonhydrothermal sediment and water column. In contrast, the sulfur-based microbial community that inhabited the surface of the volcanic manganese crust was more complex, consisting predominantly of putative chemolithoautotrophic sulfur oxidizers of the Betaproteobacteria and Gammaproteobacteria.

Quantification of sulfate-reducing bacteria in industrial wastewater, by real-time polymerase chain reaction (PCR) using dsrA and apsA genes

Real-time polymerase chain reaction (PCR) is considered a highly sensitive method for the quantification of microbial organisms in environmental samples. This study was conducted to evaluate real-time PCR with SybrGreen detection as a quantification method for sulfate-reducing bacteria (SRB) in industrial wastewater produced by several chemical industries. We designed four sets of primers and developed standard curves based on genomic DNA of Desulfovibrio vulgaris from pure culture and on plasmids containing dissimilatory sulfate reductase (dsrA) or adenosine-5'-phosphosulfate reductase (apsA) genes of SRB. All the standard curves, two for dsrA and two for apsA genes, had a linear range between 0.95 x 10(2) and 9.5 x 10(6) copies/microL and between 1.2 x 10(3) and 1.2 x 10(7) copies/microL, respectively. The theoretical copy numbers of the tenfold dilutions of D. vulgaris genomic DNA were best estimated (between 2.7 to 10.5 times higher than theoretical numbers) by the standard curve with DSR1F and RH3-dsr-R primers. To mimic the effect of foreign DNA in environmental samples, serial dilutions of D. vulgaris genomic DNA were mixed with Escherichia coli chromosomal DNA (40 ng per assay). This influenced neither PCR amplification nor the quantification of target DNA. Industrial wastewater was sampled during a 15-month period and analyzed for the presence of SRB, based on dsrA gene amplification. SRB displayed a higher abundance during the summer (about 10(7)-10(8) targets mL(-1)) and lower during the winter (about 10(4)-10(5) targets mL(-1)). The results indicate that our real-time PCR approach can be used for detection of uncultured SRB and will provide valuable information related to the abundance of SRB in durable environmental samples, such as complex and saline industrial wastewaters.

Studies on the reducing systems for plant and animal thioredoxin-independent methionine sulfoxide reductases B

Two distinct stereospecific methionine sulfoxide reductases (Msr), MsrA and MsrB reduce the oxidized methionine (Met), methionine sulfoxide [Met(O)], back to Met. In this report, we examined the reducing systems required for the activities of two chloroplastic MsrB enzymes (NtMsrB1 and NtMsrB2) from tobacco (Nicotiana tabacum). We found that NtMrsB1, but not NtMsrB2, could use dithiothreitol as an efficient hydrogen donor. In contrast Escherichia coli thioredoxin (Trx) could serve as a reducing agent for NtMsrB2, but not for NtMsrB1. Similar to previously reported human Trx-independent hMsrB2 and hMsrB3, NtMsrB1 could also use bovine liver thionein and selenocysteamine as reducing agents. Furthermore, the unique plant Trx-like protein CDSP32 was shown to reduce NtMsrB1, hMsrB2 and hMsrB3. All these tested Trx-independent MsrB enzymes lack an additional cysteine (resolving cysteine) that is capable of forming a disulfide bond on the enzyme during the catalytic reaction. Our results indicate that plant and animal MsrB enzymes lacking a resolving cysteine likely share a similar reaction mechanism.

Innate immune recognition triggers secretion of lysosomal enzymes by macrophages

Gamma interferon-induced lysosomal thiolreductase (GILT) is expressed constitutively in antigen-presenting cells, where it reduces disulfide bonds to facilitate antigen presentation. GILT is synthesized as an enzymatically active precursor protein and is processed in early endosomes to yield the mature enzyme. The exposure of the promonocytic cell line THP-1 to Escherichia coli causes a differentiation-dependent induction of GILT expression in which the majority of precursor GILT is secreted as active enzyme. We confirm this result in cultured primary monocytes and macrophages, and demonstrate, as an in vivo correlate of the phenomenon, upregulation of precursor GILT levels in the serum of mice injected with lipopolysaccharide. We show that macrophage differentiation is accompanied by a transcriptional downregulation of mannose-6-phosphorylation, which likely prevents the recognition and proper sorting of soluble lysosomal enzymes by the mannose-6-phosphate receptors. We provide evidence for a mechanism of generalized soluble lysosomal enzyme secretion through the constitutive secretory pathway.

Chromate differentially affects the expression of a high-affinity sulfate transporter and isoforms of components of the sulfate assimilatory pathway in Zea mays (L.)

In this study the chromate accumulation and tolerance were investigated in ZEA MAYS L. in relation to sulfur availability since sulfate may interact with chromate for transport into the cells. Chromate inhibited sulfate uptake when supplied to plants for a short-term period, whereas phosphate uptake remained unchanged. Sulfate absorption was also reduced in S-starved (-S) and S-supplied (+S) plants treated for 2 d with 0.2 mM chromate and the concomitant repression of the root high-affinity sulfate root transporter ZMST1;1 transcript accumulation was observed. Conversely, the plasma membrane H (+)-ATPase MHA2 was unaffected by chromate in +S plants, allowing to exclude a general effect of chromate on the active membrane transport. As observed for sulfate uptake, chromate uptake was enhanced in -S condition and decreased in both +S and -S plants after 2 d of Cr treatment. Chromate reduced the concentration of sulfur and sulfate in +S plants to the basal level of -S plants, and maximum chromium accumulation was recorded in S-deprived plants. Analysis of transcript abundance of genes involved in sulfate assimilation revealed differential regulation by chromate, which was only partly related to sulfur availability and to the levels of thiols. This work shows for the first time that chromate specifically represses sulfate uptake, and such repression occurs without the implication of the candidate regulatory metabolites of the sulfate transport system in plants.

Interaction of sulfur and nitrogen nutrition in tobacco (Nicotiana tabacum) plants: significance of nitrogen source and root nitrate reductase

The significance of root nitrate reductase for sulfur assimilation was studied in tobacco (NICOTIANA TABACUM) plants. For this purpose, uptake, assimilation, and long-distance transport of sulfur were compared between wild-type tobacco and transformants lacking root nitrate reductase, cultivated either with nitrate or with ammonium nitrate. A recently developed empirical model of plant internal nitrogen cycling was adapted to sulfur and applied to characterise whole plant sulfur relations in wild-type tobacco and the transformant. Both transformation and nitrogen nutrition strongly affected sulfur pools and sulfur fluxes. Transformation decreased the rate of sulfate uptake in nitrate-grown plants and root sulfate and total sulfur contents in root biomass, irrespective of N nutrition. Nevertheless, glutathione levels were enhanced in the roots of transformed plants. This may be a consequence of enhanced APR activity in the leaves that also resulted in enhanced organic sulfur content in the leaves of the tranformants. The lack of nitrate reductase in the roots in the transformants caused regulatory changes in sulfur metabolism that resembled those observed under nitrogen deficiency. Nitrate nutrition reduced total sulfur content and all the major fractions analysed in the leaves, but not in the roots, compared to ammonium nitrate supply. The enhanced organic sulfur and glutathione levels in ammonium nitrate-fed plants corresponded well to elevated APR activity. But foliar sulfate contents also increased due to decreased re-allocation of sulfate into the phloem of ammonium nitrate-fed plants. Further studies will elucidate whether this decrease is achieved by downregulation of a specific sulfate transporter in vascular tissues.

Activation of the Heterodimeric Central Complex SoxYZ of Chemotrophic Sulfur Oxidation Is Linked to a Conformational Change and SoxY-Y Interprotein Disulfide Formation,

The central protein of the four component sulfur oxidizing (Sox) enzyme system of Paracoccus pantotrophus, SoxYZ, carries at the SoxY subunit the covalently bound sulfur substrate which the other three proteins bind, oxidize, and release as sulfate. SoxYZ of different preparations resulted in different specific thiosulfate-oxidizing activities of the reconstituted Sox enzyme system. From these preparations SoxYZ was activated up to 24-fold by different reductants with disodium sulfide being the most effective and yielded a uniform specific activity of the Sox system. The activation comprised the activities with hydrogen sulfide, thiosulfate, and sulfite. Sulfide-activation decreased the predominant beta-sheet character of SoxYZ by 4%, which caused a change in its conformation as determined by infrared spectroscopy. Activation of SoxYZ by sulfide exposed the thiol of the C-terminal Cys-138 of SoxY as evident from alkylation by 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid. Also, SoxYZ activation enhanced the formation of the Sox(YZ)2 heterotetramer as evident from density gradient gel electrophoresis. The tetramer was formed due to an interprotein disulfide between SoxY to yield a SoxY-Y dimer as determined by combined high pressure liquid chromatography and mass spectrometry. The significance of the conformational change of SoxYZ and the interprotein disulfide between SoxY-Y is discussed.

Complex formation between recombinant ATP sulfurylase and APS reductase of Allium cepa (L.)

Recombinant ATP sulfurylase (AcATPS1) and adenosine-5'-phosphosulfate reductase (AcAPR1) from Allium cepa have been used to determine if these enzymes form protein-protein complexes in vitro. Using a solid phase binding assay, AcAPR1 was shown to interact with AcATPS1. The AcAPR1 enzyme was also expressed in E. coli as the N-terminal reductase domain (AcAPR1-N) and the C-terminal glutaredoxin domain (AcAPR1-C), but neither of these truncated proteins interacted with AcATPS1. The solid-phase interactions were confirmed by immune-precipitation, where anti-AcATPS1 IgG precipitated the full-length AcAPR1 protein, but not AcAPR1-N and AcAPR1-C. Finally, using the ligand binding assay, full-length AcATPS1 has been shown to bind to membrane-localised full-length AcAPR1. The significance of an interaction between chloroplastidic ATPS and APR in A. cepa is evaluated with respect to the control of the reductive assimilation of sulfate.

Sulfite reduction in mycobacteria

Mycobacterium tuberculosis places an enormous burden on the welfare of humanity. Its ability to grow and its pathogenicity are linked to sulfur metabolism, which is considered a fertile area for the development of antibiotics, particularly because many of the sulfur acquisition steps in the bacterium are not found in the host. Sulfite reduction is one such mycobacterium-specific step and is the central focus of this paper. Sulfite reduction in Mycobacterium smegmatis was investigated using a combination of deletion mutagenesis, metabolite screening, complementation, and enzymology. The initial rate parameters for the purified sulfite reductase from M. tuberculosis were determined under strict anaerobic conditions [k(cat) = 1.0 (+/-0.1) electron consumed per second, and K(m(SO(3)(-2))) = 27 (+/-1) microM], and the enzyme exhibits no detectible turnover of nitrite, which need not be the case in the sulfite/nitrite reductase family. Deletion of sulfite reductase (sirA, originally misannotated nirA) reveals that it is essential for growth on sulfate or sulfite as the sole sulfur source and, further, that the nitrite-reducing activities of the cell are incapable of reducing sulfite at a rate sufficient to allow growth. Like their nitrite reductase counterparts, sulfite reductases require a siroheme cofactor for catalysis. Rv2393 (renamed che1) resides in the sulfur reduction operon and is shown for the first time to encode a ferrochelatase, a catalyst that inserts Fe(2+) into siroheme. Deletion of che1 causes cells to grow slowly on metabolites that require sulfite reductase activity. This slow-growth phenotype was ameliorated by optimizing growth conditions for nitrite assimilation, suggesting that nitrogen and sulfur assimilation overlap at the point of ferrochelatase synthesis and delivery.

An Arabidopsis quiescin-sulfhydryl oxidase regulates cation homeostasis at the root symplast-xylem interface

A genetic screen of Arabidopsis 'activation-tagging' mutant collection based on tolerance to norspermidine resulted in a dominant mutant (par1-1D) with increased expression of the QSO2 gene (At1g15020), encoding a member of the quiescin-sulfhydryl oxidase (QSO) family. The par1-1D mutant and transgenic plants overexpressing QSO2 cDNA grow better than wild-type Arabidopsis in media with toxic cations (polyamines, Li(+) and Na(+)) or reduced K(+) concentrations. This correlates with a decrease in the accumulation of toxic cations and an increase in the accumulation of K(+) in xylem sap and shoots. Conversely, three independent loss-of-function mutants of QSO2 exhibit phenotypes opposite to those of par1-1D. QSO2 is mostly expressed in roots and is upregulated by K(+) starvation. A QSO2Colon, two colonsGFP fusion ectopically expressed in leaf epidermis localized at the cell wall. The recombinant QSO2 protein, produced in yeast in secreted form, exhibits disulfhydryl oxidase activity. A plausible mechanism of QSO2 action consists on the activation of root systems loading K(+) into xylem, but different from the SKOR channel, which is not required for QSO2 action. These results uncover QSOs as novel regulators of ion homeostasis.

Insights into the metabolism of elemental sulfur by the hyperthermophilic archaeon Pyrococcus furiosus: characterization of a coenzyme A- dependent NAD(P)H sulfur oxidoreductase

The hyperthermophilic archaeon Pyrococcus furiosus uses carbohydrates as a carbon source and produces acetate, CO2, and H2 as end products. When S(0) is added to a growing culture, within 10 min the rate of H2 production rapidly decreases and H(2)S is detected. After 1 hour cells contain high NADPH- and coenzyme A-dependent S(0) reduction activity (0.7 units/mg, 85 degrees C) located in the cytoplasm. The enzyme responsible for this activity was purified to electrophoretic homogeneity (specific activity, 100 units/mg) and is termed NAD(P)H elemental sulfur oxidoreductase (NSR). NSR is a homodimeric flavoprotein (M(r), 100,000) and is encoded by PF1186. This designation was previously assigned to the gene encoding an enzyme that reduces coenzyme A disulfide, which is a side reaction of NSR. Whole-genome DNA microarray and quantitative PCR analyses showed that the expression of NSR is up-regulated up to sevenfold within 10 min of S(0) addition. This primary response to S(0) also involves the up-regulation (>16-fold) of a 13-gene cluster encoding a membrane-bound oxidoreductase (MBX). The cluster encoding MBX is proposed to replace the homologous 14-gene cluster that encodes the ferredoxin-oxidizing, H2-evolving membrane-bound hydrogenase (MBH), which is down-regulated >12-fold within 10 min of S(0) addition. Although an activity for MBX could not be demonstrated, it is proposed to conserve energy by oxidizing ferredoxin and reducing NADP, which is used by NSR to reduce S(0). A secondary response to S(0) is observed 30 min after S(0) addition and includes the up-regulation of genes encoding proteins involved in amino acid biosynthesis and iron metabolism, as well as two so-called sulfur-induced proteins termed SipA and SipB. This novel S(0)-reducing system involving NSR and MBX has been found so far only in the heterotrophic Thermococcales and is in contrast to the cytochrome- and quinone-based S(0)-reducing system in autotrophic archaea and bacteria.

Novel bacterial sulfur oxygenase reductases from bioreactors treating gold-bearing concentrates

The microbial community and sulfur oxygenase reductases of metagenomic DNA from bioreactors treating gold-bearing concentrates were studied by 16S rRNA library, real-time polymerase chain reaction (RT-PCR), conventional cultivation, and molecular cloning. Results indicated that major bacterial species were belonging to the genera Acidithiobacillus, Leptospirillum, Sulfobacillus, and Sphingomonas, accounting for 6.3, 66.7, 18.8, and 8.3%, respectively; the sole archaeal species was Ferroplasma sp. (100%). Quantitative RT-PCR revealed that the 16S rRNA gene copy numbers (per gram of concentrates) of bacteria and archaea were 4.59 x 10(9) and 6.68 x 10(5), respectively. Bacterial strains representing Acidithiobacillus, Leptospirillum, and Sulfobacillus were isolated from the bioreactors. To study sulfur oxidation in the reactors, pairs of new PCR primers were designed for the detection of sulfur oxygenase reductase (SOR) genes. Three sor-like genes, namely, sor (Fx), sor (SA), and sor (SB) were identified from metagenomic DNAs of the bioreactors. The sor (Fx) is an inactivated SOR gene and is identical to the pseudo-SOR gene of Ferroplasma acidarmanus. The sor (SA) and sor (SB) showed no significant identity to any genes in GenBank databases. The sor (SB) was cloned and expressed in Escherichia coli, and SOR activity was determined. Quantitative RT-PCR determination of the gene densities of sor (SA) and sor (SB) were 1,000 times higher than archaeal 16S rRNA gene copy numbers, indicating that these genes were mostly impossible from archaea. Furthermore, with primers specific to the sor (SB) gene, this gene was PCR-amplified from the newly isolated Acidithiobacillus sp. strain SM-1. So far as we know, this is the first time to determine SOR activity originating from bacteria and to document SOR gene in bioleaching reactors and Acidithiobacillus species.

The Arabidopsis thaliana sulfiredoxin is a plastidic cysteine-sulfinic acid reductase involved in the photooxidative stress response

The 2-cysteine peroxiredoxins (2-Cys-Prxs) are antioxidants that reduce peroxides through a thiol-based mechanism. During catalysis, these ubiquitous enzymes are occasionally inactivated by the substrate-dependent oxidation of the catalytic cysteine to the sulfinic acid (-SO2H) form, and are reactivated by reduction by sulfiredoxin (Srx), an enzyme recently identified in yeast and in mammal cells. In plants, 2-Cys-Prxs constitute the most abundant Prxs and are located in chloroplasts. Here we have characterized the unique Srx gene in Arabidopsis thaliana (AtSrx) from a functional point of view, and analyzed the phenotype of two AtSrx knockout (AtSrx-) mutant lines. AtSrx is a chloroplastic enzyme displaying sulfinic acid reductase activity, as shown by the ability of the recombinant AtSrx to reduce the overoxidized 2-Cys-Prx form in vitro, and by the accumulation of the overoxidized Prx in mutant lines lacking Srx in vivo. Furthermore, AtSrx mutants exhibit an increased tolerance to photooxidative stress generated by high light combined with low temperature. These data establish that, as in yeast and in mammals, plant 2-Cys-Prxs are subject to substrate-mediated inactivation reversed by Srx, and suggest that the 2-Cys-Prx redox status and sulfiredoxin are parts of a signaling mechanism participating in plant responses to oxidative stress.

Noncovalent complexes of APS reductase from M. tuberculosis: delineating a mechanistic model using ESI-FTICR MS

ESI-FTICR MS was utilized to characterize a 4Fe-4S containing protein Mycobacterium tuberculosis APS reductase. This enzyme catalyzes the reduction of APS to sulfite and AMP with reducing equivalents from the protein cofactor, thioredoxin. Under nondenaturing conditions, a distribution of the apoprotein, a 2Fe-2S intermediate, and the 4Fe-4S holoprotein were observed. Accurate mass measurements indicated an oxidation state of +2 for the 4Fe-4S cluster, with no disulfide bond in the holoenzyme. Gas-phase stability of the 4Fe-4S cluster was investigated using both in-source and collision induced dissociation, which provided information regarding the relative gas-phase binding strength of iron towards protein ligands and inorganic sulfides. Noncovalent complexes of the holoprotein with several ligands, including APS, thioredoxin, and AMP, were also investigated. Calculated values of dissociation constants for the complexes indicate that AMP binds with a higher affinity to the enzyme intermediate than to the free enzyme. The implications of the binary and ternary complexes observed by gas-phase noncovalent interactions in the mechanism of APS reduction are discussed.

Evidence for chemoautotrophic symbiosis in a Mediterranean cold seep clam (Bivalvia: Lucinidae): comparative sequence analysis of bacterial 16S rRNA, APS reductase and RubisCO genes

Symbioses between lucinid clams (Bivalvia: Lucinidae) and autotrophic sulphide-oxidizing bacteria have mainly been studied in shallow coastal species, and information regarding deep-sea species is scarce. Here we study the symbiosis of a clam, resembling Lucinoma kazani, which was recently collected in sediment cores from new cold-seep sites in the vicinity of the Nile deep-sea fan, eastern Mediterranean, at depths ranging from 507 to 1691 m. A dominant bacterial phylotype, related to the sulphide-oxidizing symbiont of Lucinoma aequizonata, was identified in gill tissue by comparative 16S rRNA gene sequence analysis. A second phylotype, related to spirochete sequences, was identified twice in a library of 94 clones. Comparative analyses of gene sequences encoding the APS reductase alpha subunit and ribulose-1,5-bisphosphate carboxylase oxygenase support the hypothesis that the dominant symbiont can perform sulphide oxidation and autotrophy. Transmission electron micrographs of gills confirmed the dominance of sulphide-oxidizing bacteria, which display typical vacuoles, and delta(13)C values measured in gill and foot tissue further support the hypothesis for a chemoautotrophic-sourced host carbon nutrition.

Adenosine 5'-phosphosulphate reductase is regulated differently in Allium cepa L. and Brassica oleracea L. upon exposure to H2S

The reduction of adenosine 5'-phosphosulphate (APS) by APS reductase (APR) is considered to be one of the rate-limiting steps in the assimilation of sulphur in plants. In order to identify the mechanisms of regulation of this enzyme, the impact of atmospheric H2S exposure on mRNA expression, protein level, and activity of APR was studied in two species (Allium cepa L. and Brassica oleracea L.) with different physiological responses to H2S exposure. As expected, H2S exposure resulted in a rapid increase in thiol compounds in the shoot of both species. There was a substantial increase in total sulphur content in shoots of A. cepa, whereas it was hardly affected or even slightly decreased in B. oleracea. Sulphate uptake was only marginally affected in A. cepa, whereas it was strongly decreased in B. oleracea upon H2S exposure. Furthermore, H2S exposure resulted in a down-regulation of APR activity in shoot and roots of both species, which was probably mediated by a transcriptional mechanism of regulation by thiols, since mRNA levels also decreased. However, in contrast to B. oleracea, APR protein level was not affected by H2S exposure in A. cepa. The reduction in APR activity in onion was therefore achieved by an additional as yet unknown post-translational regulation. These results demonstrate that not only the physiological response to H2S, but also the molecular mechanisms of regulation of APR differ in the two species.

The Two-Domain Structure of 5‘-Adenylylsulfate (APS) Reductase from Enteromorpha intestinalis Is a Requirement for Efficient APS Reductase Activity

5'-Adenylylsulfate (APS) reductase from Enteromorpha intestinalis (EiAPR) is composed of two domains that function together to reduce APS to sulfite. The carboxyl-terminal domain functions as a glutaredoxin that mediates the transfer of electrons from glutathione to the APS reduction site on the amino-terminal domain. To study the basis for the interdomain interaction, a heterologous system was constructed in which the C domain of EiAPR was fused to the carboxyl terminus of the APS reductase from Pseudomonas aeruginosa (PaAPR), an enzyme that normally uses thioredoxin as an electron donor and is incapable of using glutathione for this function. The hybrid enzyme, which retains the [4Fe-4S] cluster from PaAPR, was found to use both thioredoxin and glutathione as an electron donor for APS reduction. The ability to use glutathione was enhanced by the addition of Na2SO4 to the reaction buffer, a property that the hybrid enzyme shares with EiAPR. When the C domain was added as a separate component, it was much less efficient in conferring PaAPR with the ability to use glutathione as an electron donor, despite the fact that the separately expressed C domain functioned in two activities that are typical for glutaredoxins, hydroxyethyl disulfide reduction and electron donation to ribonucleotide reductase. These results suggest that the physical connection of the reductase and C domain on a single polypeptide is critical for the electron-transfer reaction. Moreover, the effect of Na2SO4 suggests that a water-ordering component of the reaction milieu is critical for the catalytic function of plant-type APS reductases by promoting the interdomain interaction.

Thiosulphate oxidation in the phototrophic sulphur bacterium Allochromatium vinosum

Two different pathways for thiosulphate oxidation are present in the purple sulphur bacterium Allochromatium vinosum: oxidation to tetrathionate and complete oxidation to sulphate with obligatory formation of sulphur globules as intermediates. The tetrathionate:sulphate ratio is strongly pH-dependent with tetrathionate formation being preferred under acidic conditions. Thiosulphate dehydrogenase, a constitutively expressed monomeric 30 kDa c-type cytochrome with a pH optimum at pH 4.2 catalyses tetrathionate formation. A periplasmic thiosulphate-oxidizing multienzyme complex (Sox) has been described to be responsible for formation of sulphate from thiosulphate in chemotrophic and phototrophic sulphur oxidizers that do not form sulphur deposits. In the sulphur-storing A. vinosum we identified five sox genes in two independent loci (soxBXA and soxYZ). For SoxA a thiosulphate-dependent induction of expression, above a low constitutive level, was observed. Three sox-encoded proteins were purified: the heterodimeric c-type cytochrome SoxXA, the monomeric SoxB and the heterodimeric SoxYZ. Gene inactivation and complementation experiments proved these proteins to be indispensable for thiosulphate oxidation to sulphate. The intermediary formation of sulphur globules in A. vinosum appears to be related to the lack of soxCD genes, the products of which are proposed to oxidize SoxY-bound sulphane sulphur. In their absence the latter is instead transferred to growing sulphur globules.

Catalytic and regulatory properties of sulphur metabolizing enzymes in cyanobacterium Synechococcus elongatus PCC 7942

Synechococcus elongatus PCC 7942 was able to grow with several S sources. The sulphur metabolizing enzymes viz. ATP sulphurylase, cysteine synthase, thiosulphate reductase and L- and D-cysteine desulphydrases were regulated by sulphur sources, particularly by sulphur amino acids and organic sulphate esters. Sulphur starvation reduced ATP sulphurylase and cysteine synthase whereas reduced glutathione appreciated Cys degradation activity. With partially purified enzymes apparent Km values for sulphate, ATP, D- and L-Cys, thiosulphate, sulphide and O-acetyl serine were in a range of 12-50 microM. p-Nitrophenyl sulphate inhibited ATP sulphurylase competitively. Met was a feedback inhibitor of several key enzymes.

Thermodynamics of unfolding of an integral membrane protein in mixed micelles

Quantitative studies of membrane protein folding and unfolding can be difficult because of difficulties with efficient refolding as well as a pronounced propensity to aggregate. However, mixed micelles, consisting of the anionic detergent sodium dodecyl sulfate and the nonionic detergent dodecyl maltoside facilitate reversible and quantitative unfolding and refolding. The 4-transmembrane helix protein DsbB from the inner membrane of Escherichia coli unfolds in mixed micelles according to a three-state mechanism involving an unfolding intermediate I. The temperature dependence of the kinetics of this reaction between 15 degrees and 45 degrees C supports that unfolding from I to the denatured state D is accompanied by a significant decrease in heat capacity. For water-soluble proteins, the heat capacity increases upon unfolding, and this is generally interpreted as the increased binding of water to the protein as it unfolds, exposing more surface area. The decrease in DsbB's heat capacity upon unfolding is confirmed by independent thermal scans. The decrease in heat capacity is not an artifact of the use of mixed micelles, since the water soluble protein S6 shows conventional heat-capacity changes in detergent. We speculate that it reflects the binding of SDS to parts of DsbB that are solvent-exposed in the native DM-bound state. This implies that the periplasmic loops of DsbB are relatively unstructured. This anomalous thermodynamic behavior has not been observed for beta-barrel membrane proteins, probably because they do not bind SDS so extensively. Thus the thermodynamic behavior of membrane proteins appears to be intimately connected to their detergent-binding properties.

Crystal structure and desulfurization mechanism of 2'-hydroxybiphenyl-2-sulfinic acid desulfinase

The desulfurization of dibenzothiophene in Rhodococcus erythropolis is catalyzed by two monooxygenases, DszA and DszC, and a desulfinase, DszB. In the last step of this pathway, DszB hydrolyzes 2'-hydroxybiphenyl-2-sulfinic acid into 2-hydroxybiphenyl and sulfite. We report on the crystal structures of DszB and an inactive mutant of DszB in complex with substrates at resolutions of 1.8A or better. The overall fold of DszB is similar to those of periplasmic substrate-binding proteins. In the substrate complexes, biphenyl rings of substrates are recognized by extensive hydrophobic interactions with the active site residues. Binding of substrates accompanies structural changes of the active site loops and recruits His(60) to the active site. The sulfinate group of bound substrates forms hydrogen bonds with side chains of Ser(27), His(60), and Arg(70), each of which is shown by site-directed mutagenesis to be essential for the activity. In our proposed reaction mechanism, Cys(27) functions as a nucleophile and seems to be activated by the sulfinate group of substrates, whereas His(60) and Arg(70) orient the syn orbital of sulfinate oxygen to the sulfhydryl hydrogen of Cys(27) and stabilize the negatively charged reaction intermediate. Cys, His, and Arg residues are conserved in putative proteins homologous to DszB, which are presumed to constitute a new family of desulfinases.

Siro(haem)amide inAllochromatium vinosumand relevance of DsrL and DsrN, a homolog of cobyrinic acida,c-diamide synthase, for sulphur oxidation

In the purple sulphur bacterium Allochromatium vinosum, the prosthetic group of dissimilatory sulphite reductase (DsrAB) was identified as siroamide, an amidated form of the classical sirohaem. The genes dsrAB are the first two of a large cluster of genes necessary for the oxidation of sulphur globules stored intracellularly during growth on sulphide and thiosulphate. DsrN is homologous to cobyrinic acid a,c diamide synthase and may therefore catalyze glutamine-dependent amidation of sirohaem. Indeed, an A. vinosumDeltadsrN in frame deletion mutant showed a significantly reduced sulphur oxidation rate that was fully restored upon complementation with dsrN in trans. Sulphite reductase was still present in the DeltadsrN mutant. DsrL is a homolog of the small subunits of bacterial glutamate synthases and was proposed to deliver glutamine for sirohaem amidation. However, recombinant DsrL does not exhibit glutamate synthase activity nor does the gene complement a glutamate synthase-deficient Escherichia coli strain. Deletion of dsrL showed that the encoded protein is absolutely essential for sulphur oxidation in A. vinosum.

Genetic and biochemical evidence for the involvement of a molybdenum-dependent enzyme in one of the selenite reduction pathways of Rhodobacter sphaeroides f. sp. denitrificans IL106

Selenite reduction in Rhodobacter sphaeroides f. sp. denitrificans was observed under photosynthetic conditions, following a 100-h lag period. This adaptation period was suppressed if the medium was inoculated with a culture previously grown in the presence of selenite, suggesting that selenite reduction involves an inducible enzymatic pathway. A transposon library was screened to isolate mutants affected in selenite reduction. Of the eight mutants isolated, two were affected in molybdenum cofactor synthesis. These moaA and mogA mutants showed an increased duration of the lag phase and a decreased rate of selenite reduction. When grown in the presence of tungstate, a well-known molybdenum-dependent enzyme (molybdoenzyme) inhibitor, the wild-type strain displayed the same phenotype. The addition of tungstate in the medium or the inactivation of the molybdocofactor synthesis induced a decrease of 40% in the rate of selenite reduction. These results suggest that several pathways are involved and that one of them involves a molybdoenzyme. Although addition of nitrate or dimethyl sulfoxide (DMSO) to the medium increased the selenite reduction activity of the culture, neither the periplasmic nitrate reductase NAP nor the DMSO reductase is the implicated molybdoenzyme, since the napA and dmsA mutants, with expression of nitrate reductase and DMSO reductase, respectively, eliminated, were not affected by selenite reduction. A role for the biotine sulfoxide reductase, another characterized molybdoenzyme, is unlikely, since its overexpression in a defective strain did not restore the selenite reduction activity.

Mutations in the FAD binding domain cause stress-induced misoxidation of the endoplasmic reticulum oxidoreductase Ero1beta

Disulfide bond catalysis is an essential component of protein biogenesis in the secretory pathway, from yeast through to man. In the endoplasmic reticulum (ER), protein-disulfide isomerase (PDI) catalyzes the oxidation and isomerization of disulfide bonds and is re-oxidized by an endoplasmic reticulum oxidoreductase (ERO). The elucidation of ERO function was greatly aided by the genetic analysis of two ero mutants, whose impairment results from point mutations in the FAD binding domain of the Ero protein. The ero1-1 and ero1-2 yeast strains have conditional and dithiothreitol-sensitive phenotypes, but the effects of the mutations on the behavior of Ero proteins has not been reported. Here, we show that these Gly to Ser and His to Tyr mutations do not prevent the dimerization of Ero1beta or the non-covalent interaction of Ero1beta with PDI. However, the Gly to Ser mutation abolishes disulfide-dependent PDI-Ero1beta heterodimers. Both the Gly to Ser and His to Tyr mutations make Ero1beta susceptible to misoxidation and aggregation, particularly during a temperature or redox stress. We conclude that the Ero FAD binding domain is critical for conformational stability, allowing Ero proteins to withstand stress conditions that cause client proteins to misfold.

Enzymic systems proposed to be involved in the dissimilatory reduction of selenite in the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodobacter capsulatus

Various enzymic systems, such as nitrite reductase, sulfite reductase and glutathione reductase, have been proposed for, or suspected to be involved in, the reduction of selenite in bacteria. As alphaproteobacteria have been shown to be highly tolerant to transition metal oxyanions, it seemed interesting to investigate the hypothetical involvement of these different enzymes in the reduction of selenite in the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodobacter capsulatus. The hypothetical involvement of nitrite reductase and sulfite reductase in the reduction of selenite in these bacteria was investigated by analysing the effects of nitrite and sulfite amendments on the growth and kinetics of selenite reduction. The reduction of selenite was not concomitant with that of either sulfite or nitrite in Rs. rubrum, suggesting that the reduction pathways operate independently. In Rb. capsulatus, strong interactions were observed between the nitrite reduction and selenite reduction pathways. However, in both organisms, selenite reduction took place during both the growth phase and the stationary phase, indicating that selenite metabolism is constitutively expressed. In contrast, neither nitrite nor sulfite was transformed during stationary phase, suggesting that the metabolism of both ions is induced, which implies that identical reduction pathways for selenite and nitrite or selenite and sulfite are excluded. Buthionine sulfoximine (BSO, S-n-butyl homocysteine sulfoximine), a specific inhibitor of glutathione synthesis, was used to depress the intracellular glutathione level. In stationary-phase cultures of both Rs. rubrum and Rb. capsulatus amended with BSO, the rate of reduction of selenite was slowed, indicating that glutathione may be involved in the dissimilatory reduction of selenite in these organisms. The analysis of the headspace gases of the cultures indicated that the synthesis of methylated selenium compounds was prevented in the presence of 3.0 mM BSO in both organisms, implying that glutathione is also involved in the transformation of selenite to volatile selenium compounds.