On the mechanism of photosynthetic sulfate reduction. An APS-sulfotransferase from Chlorella

HY5: a key regulator for light-mediated nutrient uptake and utilization by plants

ELONGATED HYPOCOTYL 5 (HY5), a bZIP-type transcription factor, is a master regulator of light-mediated responses. ELONGATED HYPOCOTYL 5 binds to the promoter of c. 3000 genes, thereby regulating various physiological and biological processes, including photomorphogenesis, flavonoid biosynthesis, root development, response to abiotic stress and nutrient homeostasis. In recent decades, it has become clear that light signaling plays a crucial role in promoting nutrient uptake and assimilation. Recent studies have revealed the molecular mechanisms underlying such encouraging effects and the crucial function of the transcription factor HY5, whose activity is regulated by many photoreceptors. The discovery that HY5 directly activates the expression of genes involved in nutrient uptake and utilization, including several nitrogen, iron, sulphur, phosphorus and copper uptake and assimilation-related genes, enhances our understanding of how light signaling regulates uptake and utilisation of multiple nutrients in plants. Here, we review recent advances in the role of HY5 in light-dependent nutrient uptake and utilization.

Changes in ATP Sulfurylase Activity in Response to Altered Cyanobacteria Growth Conditions

We investigated variations in cell growth and ATP Sulfurylase (ATPS) activity when two cyanobacterial strains-Synechocystis sp. PCC6803 and Synechococcus sp. WH7803-were grown in conventional media, and media with low ammonium, low sulfate and a high CO₂/low O₂ atmosphere. In both organisms, a transition and adaptation to the reconstructed environmental media resulted in a decrease in ATPS activity. This variation appears to be decoupled from growth rate, suggesting the enzyme is not rate-limiting in S assimilation and raising questions about the role of ATPS redox regulation in cell physiology and throughout Earth history.

Different parasite faunas in sympatric populations of sister hedgehog species in a secondary contact zone

Providing descriptive data on parasite diversity and load in sister species is a first step in addressing the role of host-parasite coevolution in the speciation process. In this study we compare the parasite faunas of the closely related hedgehog species Erinaceus europaeus and E. roumanicus from the Czech Republic where both occur in limited sympatry. We examined 109 hedgehogs from 21 localities within this secondary contact zone. Three species of ectoparasites and nine species of endoparasites were recorded. Significantly higher abundances and prevalences were found for Capillaria spp. and Brachylaemus erinacei in E. europaeus compared to E. roumanicus and higher mean infection rates and prevalences for Hymenolepis erinacei, Physaloptera clausa and Nephridiorhynchus major in E. roumanicus compared to E. europaeus. Divergence in the composition of the parasite fauna, except for Capillaria spp., which seem to be very unspecific, may be related to the complicated demography of their hosts connected with Pleistocene climate oscillations and consequent range dynamics. The fact that all parasite species with different abundances in E. europaeus and E. roumanicus belong to intestinal forms indicates a possible diversification of trophic niches between both sister hedgehog species.

Adenosine 5'-sulphatophosphate kinase activity in spinach leaf tissue

1. An F(-)-insensitive 3'-nucleotidase was purified from spinach leaf tissue; the enzyme hydrolysed 3'-AMP, 3'-CMP and adenosine 3'-phosphate 5'-sulphatophosphate but not adenosine 5'-nucleotides nor PP(i). The pH optimum of the enzyme was 7.5; K(m) (3'-AMP) was approx. 0.8mm and K(m) (3'-CMP) was approx. 3.3mm. 3'-Nucleotidase activity was not associated with chloroplasts. Purified Mg(2+)-dependent pyrophosphatase, free from F(-)-insensitive 3'-nucleotidase, catalysed some hydrolysis of 3'-AMP; this activity was F(-)-sensitive. 2. Adenosine 5'-sulphatophosphate kinase activity was demonstrated in crude spinach extracts supplied with 3'-AMP by the synthesis of the sulphate ester of 2-naphthol in the presence of purified phenol sulphotransferase; purified ATP sulphurylase and pyrophosphatase were also added to synthesize adenosine 5'-sulphatophosphate. Adenosine 5'-sulphatophosphate kinase activity was associated with chloroplasts and was released by sonication. 3. Isolated chloroplasts synthesized adenosine 3'-phosphate 5'-sulphatophosphate from sulphate and ATP in the presence of a 3'-nucleotide; the formation of adenosine 5'-sulphatophosphate was negligible. In the absence of a 3'-nucleotide the synthesis of adenosine 3'-phosphate 5'-sulphatophosphate was negligible, but the formation of adenosine 5'-sulphatophosphate was readily detected. Some properties of the synthesis of adenosine 3'-phosphate 5'-sulphatophosphate by isolated chloroplasts are described. 4. Adenosine 3'-phosphate 5'-sulphatophosphate, synthesized by isolated chloroplasts, was characterized by specific enzyme methods, electrophoresis and i.r. spectrophotometry. 5. Isolated chloroplasts catalysed the incorporation of sulphur from sulphate into cystine/cysteine; the incorporation was enhanced by 3'-AMP and l-serine. It was concluded that adenosine 3'-phosphate 5'-sulphatophosphate is an intermediate in the incorporation of sulphur from sulphate into cystine/cysteine.

Sulfate assimilation in eukaryotes: fusions, relocations and lateral transfers

Background

The sulfate assimilation pathway is present in photosynthetic organisms, fungi, and many bacteria, providing reduced sulfur for the synthesis of cysteine and methionine and a range of other metabolites. In photosynthetic eukaryotes sulfate is reduced in the plastids whereas in aplastidic eukaryotes the pathway is cytosolic. The only known exception is Euglena gracilis, where the pathway is localized in mitochondria. To obtain an insight into the evolution of the sulfate assimilation pathway in eukaryotes and relationships of the differently compartmentalized isoforms we determined the locations of the pathway in lineages for which this was unknown and performed detailed phylogenetic analyses of three enzymes involved in sulfate reduction: ATP sulfurylase (ATPS), adenosine 5'-phosphosulfate reductase (APR) and sulfite reductase (SiR).

Results

The inheritance of ATPS, APR and the related 3'-phosphoadenosine 5'-phosphosulfate reductase (PAPR) are remarkable, with multiple origins in the lineages that comprise the opisthokonts, different isoforms in chlorophytes and streptophytes, gene fusions with other enzymes of the pathway, evidence a eukaryote to prokaryote lateral gene transfer, changes in substrate specificity and two reversals of cellular location of host- and endosymbiont-originating enzymes. We also found that the ATPS and APR active in the mitochondria of Euglena were inherited from its secondary, green algal plastid.

Conclusion

Our results reveal a complex history for the enzymes of the sulfate assimilation pathway. Whilst they shed light on the origin of some characterised novelties, such as a recently described novel isoform of APR from Bryophytes and the origin of the pathway active in the mitochondria of Euglenids, the many distinct and novel isoforms identified here represent an excellent resource for detailed biochemical studies of the enzyme structure/function relationships.

Regulation of sulfate assimilation in Arabidopsis and beyond

BACKGROUND AND AIMS

Sulfate assimilation is a pathway used by prokaryotes, fungi and photosynthetic organisms to convert inorganic sulfate to sulfide, which is further incorporated into carbon skeletons of amino acids to form cysteine or homocysteine. The pathway is highly regulated in a demand-driven manner; however, this regulation is not necessarily identical in various plant species. Therefore, our knowledge of the regulation of sulfate assimilation is reviewed here in detail with emphasis on different plant species.

SCOPE

Although demand-driven control plays an essential role in regulation of sulfate assimilation in all plants, the molecular mechanisms of the regulation and the effects of various treatments on the individual enzymes and metabolites are often different. This review summarizes (1) the molecular regulation of sulfate assimilation in Arabidopsis thaliana, especially recent data derived from platform technologies and functional genomics, (2) the co-ordination of sulfate, nitrate and carbon assimilations in Lemna minor, (3) the role of sulfate assimilation and glutathione in plant-Rhizobia symbiosis, (4) the cell-specific distribution of sulfate reduction and glutathione synthesis in C(4) plants, (5) the regulation of glutathione biosynthesis in poplar, (6) the knock-out of the adenosine 5'phosphosulfate reductase gene in Physcomitrella patens and identification of 3'-phosphoadenosyl 5'-phosphosulfate reductase in plants, and (7) the sulfur sensing mechanism in green algae.

CONCLUSIONS

As the molecular mechanisms of regulation of the sulfate assimilation pathway are not known, the role of Arabidopsis as a model plant will be further strengthened. However, this review demonstrates that investigations of other plant species will still be necessary to address specific questions of regulation of sulfur nutrition.

Sulfur and phytoplankton: acquisition, metabolism and impact on the environment

Sulfur emission from marine phytoplankton has been recognized as an important factor for global climate and as an entry into the biogeochemical S cycle. Despite this significance, little is known about the cellular S metabolism in algae that forms the basis of this emission. Some biochemical and genetic evidence for regulation of S uptake and assimilation is available for the freshwater model alga Chlamydomonas. However, the marine environment is substantially different from most fresh waters, containing up to 50 times higher free sulfate concentrations and challenging the adaptive mechanisms of primary and secondary S metabolism in marine algae. This review intends to integrate ecological and physiological data to provide a comprehensive view of the role of S in the oceans.

Plant adenosine 5'-phosphosulfate reductase is a novel iron-sulfur protein

Adenosine 5'-phosphosulfate reductase (APR) catalyzes the two-electron reduction of adenosine 5'-phosphosulfate to sulfite and AMP, which represents the key step of sulfate assimilation in higher plants. Recombinant APRs from both Lemna minor and Arabidopsis thaliana were overexpressed in Escherichia coli and isolated as yellow-brown proteins. UV-visible spectra of these recombinant proteins indicated the presence of iron-sulfur centers, whereas flavin was absent. This result was confirmed by quantitative analysis of iron and acid-labile sulfide, suggesting a [4Fe-4S] cluster as the cofactor. EPR spectroscopy of freshly purified enzyme showed, however, only a minor signal at g = 2.01. Therefore, Mössbauer spectra of (57)Fe-enriched APR were obtained at 4.2 K in magnetic fields of up to 7 tesla, which were assigned to a diamagnetic [4Fe-4S](2+) cluster. This cluster was unusual because only three of the iron sites exhibited the same Mössbauer parameters. The fourth iron site gave, because of the bistability of the fit, a significantly smaller isomer shift or larger quadrupole splitting than the other three sites. Thus, plant assimilatory APR represents a novel type of adenosine 5'-phosphosulfate reductase with a [4Fe-4S] center as the sole cofactor, which is clearly different from the dissimilatory adenosine 5'-phosphosulfate reductases found in sulfate reducing bacteria.

Adenosine 5'-phosphosulfate sulfotransferase and adenosine 5'-phosphosulfate reductase are identical enzymes

Adenosine 5'-phosphosulfate (APS) sulfotransferase and APS reductase have been described as key enzymes of assimilatory sulfate reduction of plants catalyzing the reduction of APS to bound and free sulfite, respectively. APS sulfotransferase was purified to homogeneity from Lemna minor and compared with APS reductase previously obtained by functional complementation of a mutant strain of Escherichia coli with an Arabidopsis thaliana cDNA library. APS sulfotransferase was a homodimer with a monomer M(r) of 43,000. Its amino acid sequence was 73% identical with APS reductase. APS sulfotransferase purified from Lemna as well as the recombinant enzyme were yellow proteins, indicating the presence of a cofactor. Like recombinant APS reductase, recombinant APS sulfotransferase used APS (K(m) = 6.5 microM) and not adenosine 3'-phosphate 5'-phosphosulfate as sulfonyl donor. The V(max) of recombinant Lemna APS sulfotransferase (40 micromol min(-1) mg protein(-1)) was about 10 times higher than the previously published V(max) of APS reductase. The product of APS sulfotransferase from APS and GSH was almost exclusively SO(3)(2-). Bound sulfite in the form of S-sulfoglutathione was only appreciably formed when oxidized glutathione was added to the incubation mixture. Because SO(3)(2-) was the first reaction product of APS sulfotransferase, this enzyme should be renamed APS reductase.

Glutaredoxin function for the carboxyl-terminal domain of the plant-type 5'-adenylylsulfate reductase

5'-Adenylylsulfate (APS) reductase (EC 1.8.99.-) catalyzes the reduction of activated sulfate to sulfite in plants. The evidence presented here shows that a domain of the enzyme is a glutathione (GSH)-dependent reductase that functions similarly to the redox cofactor glutaredoxin. The APR1 cDNA encoding APS reductase from Arabidopsis thaliana is able to complement the cysteine auxotrophy of an Escherichia coli cysH [3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase] mutant, only if the E. coli strain produces glutathione. The purified recombinant enzyme (APR1p) can use GSH efficiently as a hydrogen donor in vitro, showing aKm[GSH] approximately of 0.6 mM. Gene dissection was used to express separately the regions of APR1p from amino acids 73-327 (the R domain), homologous with microbial PAPS reductase, and from amino acids 328-465 (the C domain), homologous with thioredoxin. The R and C domains alone are inactive in APS reduction, but the activity is partially restored by mixing the two domains. The C domain shows a number of activities that are typical of E. coli glutaredoxin rather than thioredoxin. Both the C domain and APR1p are highly active in GSH-dependent reduction of hydroxyethyldisulfide, cystine, and dehydroascorbate, showing a Km[GSH] in these assays of approximately 1 mM. The R domain does not show these activities. The C domain is active in GSH-dependent reduction of insulin disulfides and ribonucleotide reductase, whereas APR1p and R domain are inactive. The C domain can substitute for glutaredoxin in vivo as demonstrated by complementation of an E. coli mutant, underscoring the functional similarity between the two enzymes.

Redefining reductive sulfate assimilation in higher plants: a role for APS reductase, a new member of the thioredoxin superfamily?

The reaction steps leading from the intermediate adenosine 5'-phosphosulfate (APS) to sulfide within the higher plant reductive sulfate assimilation pathway are the subject of controversy. Two pathways have been proposed: a 'bound intermediate' pathway in which the sulfo group of APS is first transferred by APS sulfotransferase to a carrier molecule to form a bound sulfite intermediate and is then further reduced by thiosulfonate reductase to bound sulfide; and a 'free intermediate' pathway in which APS is further activated to 3'-phosphoadenosine 5'-phosphosulfate (PAPS) by APS kinase followed by reduction of the sulfo group to free sulfite by PAPS reductase. Sulfite is then reduced to free sulfide by sulfite reductase. Sulfide, either free or bound, is then incorporated into organic form (as cysteine) by the enzyme O-acetylserine (thiol) lyase. In order to better characterize the pathway we attempted to clone PAPS reductase cDNAs by functional complementation of an Escherichia coli cysH mutant to prototrophy. We found no evidence for PAPS reductase cDNAs but did identify cDNAs that encode a small family of novel, chloroplast-localized proteins with APS reductase activity that are new members of the thioredoxin superfamily. We show here that the thioredoxin domain of these proteins is functional. We speculate that rather than proceeding via either of the pathways proposed above, reductive sulfate assimilation proceeds via the reduction of APS to sulfite by APS reductase and the subsequent reduction of sulfite to sulfide by sulfite reductase. In this scheme the product of the APS kinase reaction, PAPS, is not a direct intermediate in the pathway but rather acts as a substrate for sulfotransferase action and perhaps as a store of activated sulfate that can be returned to the pathway as APS via phosphohydrolase action on PAPS. Interactions between enzyme isoforms within the chloroplast stroma may bring about substrate channeling of APS and contribute to the partitioning of APS between sulfotransferase reactions on the one hand and the synthesis of cysteine and related metabolites via the reductive sulfate assimilation pathway on the other.

Sulfate reduction in higher plants: molecular evidence for a novel 5'-adenylylsulfate reductase

Sulfate-assimilating organisms reduce inorganic sulfate for Cys biosynthesis. There are two leading hypotheses for the mechanism of sulfate reduction in higher plants. In one, adenosine 5'-phosphosulfate (APS) (5'-adenylysulfate) sulfotransferase carries out reductive transfer of sulfate from APS to reduced glutathione. Alternatively, the mechanism may be similar to that in bacteria in which the enzyme, 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase, catalyzes thioredoxin (Trx)-dependent reduction of PAPS. Three classes of cDNA were cloned from Arabidopsis thaliana termed APR1, -2, and -3, that functionally complement a cysH, PAPS reductase mutant strain of Escherichia coli. The coding sequence of the APR clones is homologous with PAPS reductases from microorganisms. In addition, a carboxyl-terminal domain is homologous with members of the Trx superfamily. Further genetic analysis showed that the APR clones can functionally complement a mutant strain of E. coli lacking Trx, and an APS kinase, cysC. mutant. These results suggest that the APR enzyme may be a Trx-independent APS reductase. Cell extracts of E. coli expressing APR showed Trx-independent sulfonucleotide reductase activity with a preference for APS over PAPS as a substrate. APR-mediated APS reduction is dependent on dithiothreitol, has a pH optimum of 8.5, is stimulated by high ionic strength, and is sensitive to inactivation by 5'-adenosinemonophosphate (5'-AMP). 2'-AMP, or 3'-phosphoadenosine-5'-phosphate (PAP), a competitive inhibitor of PAPS reductase, do not affect activity. The APR enzymes may be localized in different cellular compartments as evidenced by the presence of an amino-terminal transit peptide for plastid localization in APR1 and APR3 but not APR2. Southern blot analysis confirmed that the APR clones are members of a small gene family, possibly consisting of three members.

A cDNA for adenylyl sulphate (APS)-kinase from Arabidopsis thaliana

A cDNA clone with an open reading frame of 831 nucleotides was isolated from a lambda ZapII-library of Arabidopsis thaliana. The nucleotide sequence of the cDNA is homologous to the APS-kinase genes from enterobacteria, diazotrophic bacteria, and yeast: Escherichia coli (cys C: 53.2%), Rhizobium meliloti (nod Q: 52.6%), and Saccharomyces cerevisiae (met 14:57.1%). The polypeptide deduced from the plant APS-kinase cDNA is comprised of 276 amino acid residues with a molecular weight of 29,790. It contains an N-terminal extension of 77 amino acids. This extension includes a putative transit peptide of 37 residues separated from the core protein by a VRACV processing site for stromal peptidase; a molecular weight of 26,050 is predicted for the processed protein. The relatedness between bacterial, fungal and plant APS-kinase polypeptides ranges from 47.5% (E. coli), 55.4% (S. cerevisiae), 52.6% (R. meliloti), and 50.3% (Azospirillum brasilense). The plant polypeptide contains eight cysteine residues; two cysteines flank a conserved purine nucleotide binding domain: GxxxxGK. Also conserved are a serine-182 as a possible phosphate transferring group and a K/LARAGxxxxFTG motif described for PAPS dependent enzymes. The identity of the gene was confirmed by analyzing the function of the gene product. The putative transit peptide was deleted by PCR and the truncated gene was expressed in a pTac1 vector system. A polypeptide of MW 25761 could be induced by IPTG. The gene product was enzymatically active as APS-kinase. It produced PAPS from APS and ATP--the absence of ATP but supplemented with thiols, the APS-kinase reacted as APS-sulphotransferase. APS-sulphotransferase is not a separate enzyme but identical with APS-kinase.

On the role of S: Sulfotransferases in assimilatory sulfate reduction by plant cell suspension cultures

Cell suspension cultures of Catharanthus roseus (L.) G. Don were grown under S-auxotrophic (1.8 mM sulfate) and under S-heterotrophic (0.5 and 1.0 mM cysteine or methionine) conditions. The development of activity of the thiol sulfotransferases was followed during the complete growth period. Under auxotrophic growth, an NADPH-dependent S: sulfotransferase and a GSH-dependent S: sulfotransferase developed identically, whereas under heterotrophic growth, differences in the amount of enzymes and in the time course of their development occurred. The NADPH-dependent sulfotransferase appeared "repressed" by the S-amino acids but the GSH-dependent enzyme was "derepressed." In that phenomenon, the development of the GSH sulfotransferase paralleled the development of the ATP-sulfurylase (EC 2774) activity of the cells.

A factor-dependent sulfotransferase specific for 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in the Cyanobacterium Synechococcus 6301

A sulfotransferase isolated from the Cyanobacterium Synechococcus 6301 was found to be specific for 3'-phosphoadenosine-5'-phosphosulfate (PAPS). The molecular weight of this transferase has been estimated on a Sephadex-G-100 column to be about 58,000. The K m for PAPS was determined to be 20 μM. The pH optimum was 8.0. The thiol dithioerythritol was needed for activity; other thiols such as glutathione, cysteine, or mercaptoethanol did not catalyze this reaction. The transferase, however, could not react directly with the thiol. A heat-stable factor was needed in this reaction. This factor was purified by conventional techniques and its molecular weight was determined on a Sephadex-G-50 column to be about 11,500. The factor showed normal Michaelis-Menten behavior toward the PAPS-sulfotransferase. It has been identified as thioredoxin. The tranferase was inhibited by 3'-5'-ADP and 2'-5'-ADP; all other adenine-containing nucleotides such as 2'-AMP, 3'-AMP, 5'-AMP, ADP, and c-AMP did not influence this reaction.

Adenosine-5'-phosphosulfate (APS) as sulfate donor for assimilatory sulfate reduction in Rhodospirillum rubrum

Crude extracts of Rhodospirillum rubrum catalyzed the formation of acid-volatile radioactivity from (35S) sulfate, (35S) adenosine-5'-phosphosulfate, and (35S) 3'-phosphoadenosine-5'-phosphosulfate. An enzyme fraction similar to APS-sulfotransferases from plant sources was purified 228-fold from Rhodospirillum rubrum. It is suggested here that this enzyme is specific for adenosine-5'-phosphosulfate, because the purified enzyme fraction metabolized adenosine-5'-phosphosulfate; 3'-phosphoadenosine-5'-phosphosulfate, however, only at a rate of 1/10 of that with adenosine-5'-phosphosulfate. Further, the reaction with 3'-phosphoadenosine-5'-phosphosulfate was inhibited with 3'-phosphoadenosine-5'-phosphate whereas this nucleotide had no effect on the reaction with adenosine-5'-phosphosulfate. For this activity with adenosine-5'-phosphosulfate the name APS-sulfotransferase is suggested. This APS-sulfotransferase needs thiols for activity; good rates were obtained with either dithioerythritol or reduced glutathione; other thiols like cysteine, 2'-3'-dimercaptopropanol or mercaptoethanol are less effective. The electron donor methylviologen did not catalyze this reaction. The pH-optimum was about 9.0; the apparent Km for adenosine-5'phosphosulfate was determined to be 0.05 mM with this so far purified enzyme fraction. Enzyme activity was increased with K2SO4 and Na2SO4 and was inhibited by 5'-AMP. These properties are similar to assimilatory APS-sulfotransferases from spinach and Chlorella.

The formation of exchangeable sulphite from adenosine 3'-phosphate 5'-sulphatophosphate in yeast

A new low-molecular-weight bound sulphite was found in yeast enzyme reaction systems which convert the sulphur of 35S-labelled adenosine 3'-phosphate 5'-sulphatophosphate into exchangeable radioactive sulphite. This bound sulphite was separated from other components by paper electrophoresis and Sephadex G-25 chromatography, and shown to be a peptide with multiple thiol groups and an estimated mol.wt. of 1400. The labelled sulphur in this peptide is highly exchangeable with unlabelled sulphite, but exchangeability decreases with time and freeze-drying. The low-molecular-weight acceptor is tightly bound to enzyme B of the yeast system and, apparently, accepts the sulpho group of adenosine 3'-phosphate 5'-sulphatophosphate and is released as bound sulphite only in the presence of enzymically or chemically reduced fraction C. It is proposed that the low-molecular-weight acceptor is a carrier peptide which, after release of the reduced sulphur, becomes re-oxidized and returns to enzyme B. Fraction C appears to function as an obligatory reductant of the oxidized acceptor before it can accept another-SO-3-moiety from adenosine 3'-phosphate 5'-sulphatophosphate. These findings are consistent with mechanisms proposed for sulphate reduction in spinach and Chlorella, and suggest that fraction C is the natural thiol required in these systems. An improved column technique for the preparation of adenosine 3'-phosphate 5'-sulphatophosphate is described.

Sulfate-reducing pathway in Escherichia coli involving bound intermediates

Although a sulfate-reducing pathway in Escherichia coli involving free sulfite and sulfide has been suggested, it is shown that, as in Chlorella, a pathway involving bound intermediates is also present. E. coli extracts contained a sulfotransferase that transferred the sulfonyl group from a nucleosidephosphosulfate to an acceptor to form an organic thiosulfate. This enzyme was specific for adenosine 3'-phosphate 5'-phosphosulfate, did not utilize adenine 5'-phosphosulfate, and transferred to a carrier molecule that was identical with thioredoxin in molecular weight and amino acid composition. In the absence of thioredoxin, only very low levels of the transfer of the sulfo group to thiols was observed. As in Chlorella, thiosulfonate reductase activity that reduced glutathione-S-SO3- to bound sulfide could be detected. In E. coli, this enzyme used reduced nicotinamide adenine dinucleotide phosphate and Mg2+, but did not require the addition of ferredoxin or ferredoxin nicotinamide adenine dinucleotide phosphate reductase. Although in Chlorella the thiosulfonate reductase appears to be a different enzyme from the sulfite reductase, the E. coli thiosulfonate reductase and sulfite reductase may be activities of the same enzyme.

The adenosine-5'-phosphosulfate sulfotransferase from spinach (Spinacea oleracea L.). Stabilization, partial purification, and properties

Adenosine-5'-phosphosulfate (APS) sulfotransferase was purified 25-fold from spinach (Spinacea oleracea L.) leaves by Sephadex-G-200 gel filtration and chromatography on DEAE-cellulose. Enzyme activity was stabilized with 0.05 M Tris-HCl pH 8.0 containing 10 mM mercaptoethanol (ME), 10 mM MgCl2, and 30% glycerol. The molecular weight of the APS-sulfotransferase was estimated by gel filtration to be about 110,000 daltons. The enzyme is specific for the sulfonucleotide APS; PAPS is not a sulfur donor for this reaction. The apparent Km for APS was found to be 13 μM. The enzyme activity was determined with dithioerythritol (DTE) as acceptor, which has an apparent Km of 0.6 mM. Glutathione can substitute for DTE; other thiols such as mercaptoethanol and cysteine are less effective. The APS-sulfotransferase activity is inhibited by 5'-AMP, which increases the Km for APS but does not change Vmax, suggesting a competetive inhibition. Reduced methylviologen cannot substitute for a thiol in the spinach enzyme system. Thus it seems that assimilatory APS-sulfotransferase from spinach is different from the dissimilatory APS-reductase from Desulfovibrio or Thiobacillus, where methylviologen can be used as the electron donor.

Activation of sulphate in Anabaena cylindrica

Crude cell-free extracts of Anabaena cylindrica synthesized adenosine-5'-phosphosulphate (AP(35)S) and 3'-phosphoadenosine-5'-phosphosulphate (PAP(35)S) from (35)SO4 (2-) in the presence of Mg(2+), ATP and inorganic pyrophosphatase. Maximum AP(35)S and PAP(35)S were produced at pH 7.15 and 8.05, respectively. APS kinase was detected in the supernatant of crude cell-free extracts by a spectrophotometric procedure. ATP-Sulphurylase had an absolute requirement for Mg(2+) and less than 30% AP(35)S was formed when Mg(2+) was replaced by either Mn(2+) or Co(2+). Nucleotide triphosphates other than ATP and 2'-deoxyATP were ineffective in this reaction. Maximum enzyme activity was observed at equimolar concentrations of Mg(2+) and ATP and excess of either of these was inhibitory. Other nucleotide triphosphates, like GTP, UTP, CTP, TTP, ITP, or 2'-deoxyATP also inhibited the enzyme activity. Inhibition by GTP was competitive with respect to ATP. ATP-sulphurylase activity was not affected by cysteine, methionine or glutathione.

A sulfotransferase from spinach leaves using adenosine-5'-phosphosulfate

Active sulfotransferase can be extracted from spinach (Spinacea oleracea L.) leaves (and other higher plants) using a buffer system containing 0.1 M KCl and thiol reagents. This sulfotransferase is labile, it can, however, be stabilized by storage in 70% ammonium sulfate containing 10 mM mercaptoethanol. This extract will reduce labelled adenosine-5'-phosphosulfate (APS) and 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to acid-volatile radioactivity when dithioerythrol is added. The reduction from PAPS requires magnesium chloride and is inhibited by calcium chloride and sodium fluoride, whereas these chemicals have little effect on the APS-sulfotransferase activity. The reduction rates from both nucleotides are stimulated by increasing ionic strength and are inhibited by phosphate and cyanide. In the presence of non-labelled APS the acid-volatile radioactivity distilled from [(35)S] PAPS is drastically reduced, whereas the opposite experiment using [(35)S] APS in the presence of non-labelled PAPS has little effect. This indicates that APS is an obligatory intermediate in the conversion of [(35)S] PAPS to acid-volatile radioactivity. It is therefore concluded that the sulfotransferase from spinach is specific for APS. Activity with APS as sulfur-donor was found in 5 other plants in addition to spinach: Pennisetum, Zea (Gramineae); Brassica (Cruciferae); Helianthus (Compositae); and Vicia (Papilionaceae). These experiments demonstrate the use of APS for assimilatory sulfate reduction in higher plants. This has been shown previously for the green alga Chlorella.

Isolation of assimilatroy- and dissimilatory-type sulfite reductases from Desulfovibrio vulgaris

Bisulfite reductase (desulfoviridin) and an assimilatory sulfite reductase have been purified from extracts of Desulfovibrio vulgaris. The bisulfite reductase has absorption maxima at 628, 580, 408, 390, and 279 nm, and a molecular weight of 226,000 by sedimentation equilibrium, and was judged to be free of other proteins by disk electrophoresis and ultracentrifugation. On gels, purified bisulfite reductase exhibited two green bands which coincided with activity and protein. The enzyme appears to be a tetramer but was shown to have two different types of subunits having molecular weights of 42,000 and 50,000. The chromophore did not form an alkaline ferrohemochromogen, was not reduced with dithionite or borohydride, and did not form a spectrally visible complex with CO. The assimilatory sulfite reductase has absorption maxima at 590, 545, 405 and 275 nm and a molecular weight of 26,800, and appears to consist of a single polypeptide chain as it is not dissociated into subunits by sodium dodecyl sulfate. By disk electrophoresis, purified sulfite reductase exhibited a single greenish-brown band which coincided with activity and protein. The sole product of the reduction was sulfide, and the chromophore was reduced by borohydride in the presence of sulfite. Carbon monoxide reacted with the reduced chromophore but it did not form a typical pyridine ferrohemochromogen. Thiosulfate, trithionate, and tetrathionate were not reduced by either enzyme preparation. In the presence of 8 M urea, the spectrum of bisulfite reductase resembles that of the sulfite reductase, thus suggesting a chemical relationship between the two chromophores.