Sulphate activation in higher plants

Assimilatory sulfate reduction in the marine methanogen Methanothermococcus thermolithotrophicus

Methanothermococcus thermolithotrophicus is the only known methanogen that grows on sulfate as its sole sulfur source, uniquely uniting methanogenesis and sulfate reduction. Here we use physiological, biochemical and structural analyses to provide a snapshot of the complete sulfate reduction pathway of this methanogenic archaeon. We find that later steps in this pathway are catalysed by atypical enzymes. PAPS (3'-phosphoadenosine 5'-phosphosulfate) released by APS kinase is converted into sulfite and 3'-phosphoadenosine 5'-phosphate (PAP) by a PAPS reductase that is similar to the APS reductases of dissimilatory sulfate reduction. A non-canonical PAP phosphatase then hydrolyses PAP. Finally, the F₄₂₀-dependent sulfite reductase converts sulfite to sulfide for cellular assimilation. While metagenomic and metatranscriptomic studies suggest that the sulfate reduction pathway is present in several methanogens, the sulfate assimilation pathway in M. thermolithotrophicus is distinct. We propose that this pathway was 'mix-and-matched' through the acquisition of assimilatory and dissimilatory enzymes from other microorganisms and then repurposed to fill a unique metabolic role.

Localization of enzymes of assimilatory sulfate reduction in pea roots

The localization of enzymes of assimilatory sulfate reduction was examined in roots of 5-d-old pea (Pisum sativum L.) seedlings. During an 8-h period, roots of intact plants incorporated more label from (35)SO 4 (2-) in the nutrient solution into the amino-acid and protein fractions than shoots. Excised roots and roots of intact plants assimilated comparable amounts of radioactivity from (35)SO 4 (2-) into the amino-acid and protein fractions during a 1-h period, demonstrating that roots of pea seedlings at this stage of development were not completely dependent on the shoots for reduced sulfur compounds. Indeed, these roots contained activities of ATP-sulfurylase (EC 2.7.7.4), adenosine 5'-phosphosulfate sulfotransferase, sulfite reductase (EC 1.8.7.1) and O-acetyl-L-serine sulfhydrylase (EC 4.2.99.8) at levels of 50, 30, 120 and 100%, respectively, of that in shoots. Most of the extractable activity of adenosine 5'-phosphosulfate sulfotransferase was detected in the first centimeter of the root tip. Using sucrose density gradients for organelle separation from this part of the root showed that almost 40% of the activity of ATP-sulfurylase, adenosine 5'-phosphosulfate sulfotransferase and sulfite reductase banded with the marker enzyme for proplastids, whereas only approximately 7% of O-acetyl-L-serine sulfhydrylase activity was detected in these fractions. Because their distributions on the gradients were very similar to that of nitrite reductase, a proplastid enzyme, it is concluded that ATP-sulfurylase, adenosine 5'-phosphosulfate sulfotransferase and sulfite reductase are also exclusively or almost exclusively localized in the proplastids of pea roots. O-Acetyl-L-serine sulfhydrylase is predominantly present in the cytoplasm.

Phloem transport of sulfur in Ricinus

Mature leaves of Ricinus communis fed with (35)SO 4 (2-) in the light export labeled sulfate and reduced sulfur compounds by phloem transport. Only 1-2% of the absorbed radiosulfur is exported to the stem within 2-3 h, roughly 12% of (35)S recovered was in reduced form. The composition of phloem translocate moving down the stem toward the root was determined from phloem exudate: 20-40% of the (35)S moved in the form of organic sulfur compounds, however, the bulk of sulfur was transported as inorganic sulfate. The most important organic sulfur compound translocated was glutathione, carrying about 70% of the label present in the organic fraction. In addition, methionine and cysteine were involved in phloem sulfur transport and accounted for roughly 10%. Primarily, the reduced forms of both, glutathione and cysteine are prsent in the siever tubes.

Long-distance transport of sulfur in Nicotiana tabacum

Sulfur reduction in tobacco plants is a light-enhanced process that predominantly takes place in the leaves rather than the roots. The amount of sulfate reduced in mature leaves can exceed their own requirement and enables an export of reduced sulfur, both basipetal toward the roots as well as acropetal toward the growing parts of the stem. Evidence is presented that translocation of reduced sulfur toward the roots occurs in the phloem. TLC and paper chromatography reveal that glutathione is the main transport form of reduced sulfur in tobacco plants; 67-70% of reduced (35)S was confined to glutathione, 27-30% to methionine, and 2-8% to cysteine.

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.

Regulation of adenosine-5'-phosphosulfate sulfotransferase activity by H2S in Lemna minor L

When Lemna minor L. is transferred to an atmosphere with H2S, there is a rapid loss of extractable adenosine-5'-phosphosulfate sulfotransferase activity. The activity is restored within 24 h in an atmosphere without H2S. This restoration of activity is completely inhibited by cycloheximid but not by chloramphenicol. In vitro, S(2-) up to 5 mM and cysteine, methionine, and glutathione up to 50 mM do not inhibit the enzyme. The activities of ATP sulfurylase and O-acetyl-L-serine sulfhydrylase are not affected significantly by H2S. The physiological significance of the regulation of adenosine-5'-phosphosulfate sulfotransferase is discussed.

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.