The ferredoxin-thioredoxin reductase variable subunit gene from Anacystis nidulans

The ferredoxin-thioredoxin reductase variable subunit gene of Anacystis nidulans was cloned, and its nucleotide sequence was determined. A single-copy 219-bp open reading frame encoded a protein of 73 amino acid residues, with a calculated Mr of 8,400. The monocistronic transcripts were represented in a 400-base and a less abundant 300-base mRNA form.

Ferredoxin and ferredoxin-NADP reductase from photosynthetic and nonphotosynthetic tissues of tomato

Ferredoxin and ferredoxin-NADP+ oxidoreductase (FNR) were purified from leaves, roots, and red and green pericarp of tomato (Lycopersicon esculentum, cv VFNT and cv Momotaro). Four different ferredoxins were identified on the basis of N-terminal amino acid sequence and charge. Ferredoxins I and II were the most prevalent forms in leaves and green pericarp, and ferredoxin III was the most prevalent in roots. Red pericarp of the VFNT cv yielded variable amounts of ferredoxins II and III plus a unique form, ferredoxin IV. Red pericarp of the Momotaro cv contained ferredoxins I, II, and IV. This represents the first demonstration of ferredoxin in a chromoplast-containing tissue. There were no major differences among the tomato ferredoxins in absorption spectrum or cytochrome c reduction activity. Two forms of FNR were present in tomato as judged by anion exchange chromatography and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. FNR II had a lower apparent relative molecular weight, a slightly altered absorption spectrum, and a lower specific activity for cytochrome c reduction than FNR I. FNR II could be a partially degraded form of FNR I. The FNRs from the different tissues of tomato plants all showed diaphorase activity, with FNR II being more active than FNR I. The presence of ferredoxin and FNR in heterotrophic tissues of tomato is consistent with the existence of a nonphotosynthetic ferredoxin/FNR redox pathway to support the function of ferredoxin-dependent enzymes.

Identification of an NADP/thioredoxin system in Chlamydomonas reinhardtii

The protein components of the NADP/thioredoxin system, NADP-thioredoxin reductase (NTR) and thioredoxin h, have been purified and characterized from the green alga, Chlamydomonas reinhardtii. The analysis of this system confirms that photoautotrophic Chlamydomonas cells resemble leaves in having both an NADP- and ferrodoxin-linked thioredoxin redox system. Chlamydomonas thioredoxin h, which is smaller on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than thioredoxin m from the same source, cross-reacted with antisera to thioredoxin h from spinach (Spinacia oleracea L.) and wheat germ (Triticum vulgaris L.) but not with antisera to m or f thioredoxins. In these properties, the thioredoxin h resembled a thioredoxin from Chlamydomonas, designated Ch1, whose sequence was reported recently (P. Decottignies et al., 1991, Eur. J. Biochem. 198, 505-512). The differential reactivity of thioredoxin h with antisera was used to demonstrate that thioredoxin h is enriched outside the chloroplast. The NTR was purified from Chlamydomonas using thioredoxin h from the same source. Similar to its counterpart from other organisms, Chlamydomonas NTR had a subunit size of approx. 36 kDa and was specific for NADPH. Chlamydomonas NTR effectively reduced thioredoxin h from the same source but showed little activity with the other thioredoxins tested, including spinach thioredoxin h and Escherichia coli thioredoxin. Comparison of the reduction of Chlamydomonas thioredoxins m and h by each of the endogenous thioredoxin reductases, NTR and ferredoxin-thioredoxin reductase, revealed a differential specificity of each enzyme for thioredoxin. Thus, NTR showed increased activity with thioredoxin h and ferredoxin-thioredoxin reductase with thioredoxins m and f.

Novel forms of ferredoxin and ferredoxin-NADP reductase from spinach roots

Ferredoxin and the enzyme catalyzing its reduction by NADPH, ferredoxin-NADP reductase (ferredoxin-NADP+ oxidoreductase or FNR), were found to be present in roots of spinach (Spinacia oleracea). Localization experiments with endosperm of germinating castor beans (Ricinus communis), a classical nonphotosynthetic tissue for cell fractionation studies, confirmed that ferredoxin and FNR are localized in the plastid fraction. Both proteins were purified from spinach roots and found to resemble their leaf counterparts in activity, spectral properties, and complex formation, but to differ in amino acid composition and amino terminal sequence. The results indicate that the primary structures of the FNR and ferredoxin of spinach roots differ from that of the corresponding leaf proteins. Together with earlier findings, the present results provide evidence that nonphotosynthetic plastids, including those of roots, are capable of reducing ferredoxin with heterotrophically generated NADPH.

Pyrophosphate Fructose-6-P 1-Phosphotransferase from Tomato Fruit : Evidence for Change during Ripening

Three forms of pyrophosphate fructose-6-phosphate 1-phosphotransferase (PFP) were purified from both green and red tomato (Lycopersicon esculentum) fruit: (a) a classical form (designated Q(2)) containing alpha- (66 kilodalton) and beta- (60 kilodalton) subunits; (b) a form (Q(1)) containing a beta-doublet subunit; and (c) a form (Q(0)) that appeared to contain a beta-singlet subunit. Several lines of evidence suggested that the different forms occur under physiological conditions. Q(2) was purified to apparent electrophoretic homogeneity; Q(1) and Q(0) were highly purified, but not to homogeneity. The distribution of the PFP forms from red (versus green) tomato was: Q(2), 29% (90%); Q(1), 47% (6%); and Q(0), 24% (4%). The major difference distinguishing the red from the green tomato enzymes was the fructose-2,6-bisphosphate (Fru-2,6-P(2))-induced change in K(m) for fructose-6-phosphate (Fru-6-P), the ;green forms' showing markedly enhanced affinity on activation (K(m) decrease of 7-9-fold) and the ;red forms' showing either little change (Q(0), Q(1)) or a relatively small (2.5-fold) affinity increase (Q(2)). The results extend our earlier findings with carrot root to another tissue and indicate that forms of PFP showing low or no affinity increase for Fru 6-P on activation by Fru-2,6-P(2) (here Q(1) and Q(0)) are associated with sugar storage, whereas the classical form (Q(2)), which shows a pronounced affinity increase, is more important for starch storage.

Shikimate kinase from spinach chloroplasts : purification, characterization, and regulatory function in aromatic amino Acid biosynthesis

Shikimate kinase was purified to near homogenity from spinach Spinacia oleracea L. chloroplasts and found to consist of a single 31 kilodalton polypeptide. The purified enzyme was unstable, but could be stabilized by a variety of added proteins, including oxidized and reduced thioredoxins. Whereas the isolated enzyme was stimulated by mono- and dithiol reagents, the enzyme in intact chloroplasts was unaffected by added thiols and showed only minor response to dark/light transitions. These results indicate that the previously reported stimulation of shikimate kinase activity by reduced thioredoxins is due to enzyme stabilization rather than to activation. In the current study, the purified enzyme was inhibited by added ADP and showed a strong response to energy charge. When intact chloroplasts were incubated in the dark in presence of shikimate, phosphoenolpyruvate and a source of ATP (dihydroxyacetone phosphate or ATP itself under appropriate conditions), aromatic amino acids were formed: phenylalanine and tyrosine. The data indicate that energy charge plays a role in regulating shikimate kinase, thereby controlling the shikimate pathway. An unidentified enzyme of the latter part of the pathway, leading from shikimate-3-phosphate to phenylalanine, appears to be activated by light.

Contrasting evolutionary histories of chloroplast thioredoxins f and m

Fourteen thioredoxin sequences were used to construct a minimal phylogenetic tree by using parsimony. The bacterial thioredoxins clustered into three groups: one containing the photosynthetic purple bacteria, Escherichia and Corynebacterium; a second containing the photosynthetic green bacterium, Chlorobium; and a third containing cyanobacteria. These groupings are similar to those generated from earlier 16s RNA analyses. Animal thioredoxins formed a fourth group. The two thioredoxins of chloroplasts (f and m) showed contrasting phylogenetic patterns. As predicted from prior studies, spinach chloroplast thioredoxin m grouped with its counterparts from cyanobacteria and eukaryotic algae, but, unexpectedly, thioredoxin f grouped with the animal thioredoxins. The results indicate that, during evolution, thioredoxin m of contemporary photosynthetic eukaryotic cells was derived from a prokaryotic symbiont, whereas thioredoxin f descended from an ancestral eukaryote common to plants and animals. The findings illustrate the potential of thioredoxin as a phylogenetic marker and suggest a relationship between the animal and f-type thioredoxins.

The ferredoxin-thioredoxin system of a green alga, Chlamydomonas reinhardtii : Identification and characterization of thioredoxins and ferredoxin-thioredoxin reductase components

The components of the ferredoxin-thioredoxin (FT) system of Chlamydomonas reinhardtii have been purified and characterized. The system resembled that of higher plants in consisting of a ferredoxin-thioredoxin reductase (FTR) and two types of thioredoxin, a single f and two m species, m1 and m2. The Chlamydomonas m and f thioredoxins were antigenically similar to their higher-plant counterparts, but not to one another. The m thioredoxins were recognized by antibodies to both higher-plant m and bacterial thioredoxins, whereas the thioredoxin f was not. Chlamydomonas thioredoxin f reacted, although weakly, with the antibody to spinach thioredoxin f. The algal thioredoxin f differed from thioredoxins studied previously in behaving as a basic protein on ion-exchange columns. Purification revealed that the algal thioredoxins had molecular masses (Mrs) typical of thioredoxins from other sources, m1 and m2 being 10700 and f 11 500. Chlamydomonas FTR had two dissimilar subunits, a feature common to all FTRs studied thus far. One, the 13-kDa ("similar") subunit, resembled its counterpart from other sources in both size and antigenicity. The other, 10-kDa ("variable") sub-unit was not recognized by antibodies to any FTR tested. When combined with spinach, (Spinacia oleracea L.) thylakoid membranes, the components of the FT system functioned in the light activation of the standard target enzymes from chloroplasts, corn (Zea mays L.) NADP-malate dehydrogenase (EC 1.1.1.82) and spinach fructose 1,6-bisphosphatase (EC 3.1.3.11) as well as the chloroplast-type fructose 1,6-bisphosphatase from Chlamydomonas. Activity was greatest if ferredoxin and other components of the FT system were from Chlamydomonas. The capacity of the Chlamydomonas FT system to activate autologous FBPase indicates that light regulates the photosynthetic carbon metabolism of green algae as in other oxygenic photosynthetic organisms.

The ferredoxin-thioredoxin system of a green alga, Chlamydomonas reinhardtii: identification and characterization of thioredoxins and ferredoxin-thioredoxin reductase components

The components of the ferredoxin-thioredoxin (FT) system of Chlamydomonas reinhardtii have been purified and characterized. The system resembled that of higher plants in consisting of a ferredoxin-thioredoxin reductase (FTR) and two types of thioredoxin, a single f and two m species, m1 and m2. The Chlamydomonas m and f thioredoxins were antigenically similar to their higher-plant counterparts, but not to one another. The m thioredoxins were recognized by antibodies to both higher plant m and bacterial thioredoxins, whereas the thioredoxin f was not. Chlamydomonas thioredoxin f reacted, although weakly, with the antibody to spinach thioredoxin f. The algal thioredoxin f differed from thioredoxins studied previously in behaving as a basic protein on ion-exchange columns. Purification revealed that the algal thioredoxins had molecular masses (Mrs) typical of thioredoxins from other sources, m1 and m2 being 10700 and f 11500. Chlamydomonas FTR had two dissimilar subunits, a feature common to all FTRs studied thus far. One, the 13-kDa ("similar") subunit, resembled its counterpart from other sources in both size and antigenicity. The other, 10-kDa ("variable") subunit was not recognized by antibodies to any FTR tested. When combined with spinach, (Spinacia oleracea L.) thylakoid membranes, the components of the FT system functioned in the light activation of the standard target enzymes from chloroplasts, corn (Zea mays L.) NADP-malate dehydrogenase (EC 1.1.1.82) and spinach fructose 1,6-bisphosphatase (EC 3.1.3.11) as well as the chloroplast-type fructose 1,6-bisphosphatase from Chlamydomonas. Activity was greatest if ferredoxin and other components of the FT system were from Chlamydomonas. The capacity of the Chlamydomonas FT system to activate autologous FBPase indicates that light regulates the photosynthetic carbon metabolism of green algae as in other oxygenic photosynthetic organisms.

A reverse KREBS cycle in photosynthesis: consensus at last

The Krebs cycle (citric acid or tricarboxylic acid cycle), the final common pathway in aerobic metabolism for the oxidation of carbohydrates, fatty acids and amino acids, is known to be irreversible. It liberates CO2 and generates NADH whose aerobic oxidation yields ATP but it does not operate in reverse as a biosynthetic pathway for CO2 assimilation. In 1966, our laboratory described a cyclic pathway for CO2 assimilation (Evans, Buchanan and Arnon 1966) that was unusual in two respects: (i) it provided the first instance of an obligate photoautotroph that assimilated CO2 by a pathway different from Calvin's reductive pentose phosphate cycle (Calvin 1962) and (ii) in its overall effect the new cycle was a reversal of the Krebs cycle. Named the 'reductive carboxylic acid cycle' (sometimes also called the reductive tricarboxylic acid cycle) the new cycle appeared to be the sole CO2 assimilation pathway in Chlorobium thiosulfatophilum (Evans et al. 1966) (now known as Chlorobium limicola forma thiosulfatophilum). Chlorobium is a photosynthetic green sulfur bacterium that grows anaerobically in an inorganic medium with sulfide and thiosulfate as electron donors and CO2 as an obligatory carbon source. In the ensuing years, the new cycle was viewed with skepticism. Not only was it in conflict with the prevailing doctrine that the 'one important property ... shared by all (our emphasis) autotrophic species is the assimilation of CO2 via the Calvin cycle' (McFadden 1973) but also some of its experimental underpinnings were challenged. It is only now that in the words of one of its early skeptics (Tabita 1988) 'a long and tortuous controversy' has ended with general acceptance of the reductive carboxylic acid cycle as a photosynthetic CO2 assimilation pathway distinct from the pentose cycle. (Henceforth, to minimize repetitiveness, the reductive pentose phosphate cycle will often be referred to as the pentose cycle and the reductive carboxylic acid cycle as the carboxylic acid cycle.) Aside from photosynthetic pathways which are the focus of this article, CO2 assimilation is also known to sustain autotrophic growth via the acetyl-CoA pathway (Wood et al. 1986). Our aim here is to discuss (i) the findings that led our group to the discovery of the reductive carboxylic acid cycle, (ii) the nature and resolution of the controversy that followed, and (iii) the possible evolutionary implications of the cycle as an ancient mechanism for photosynthetic CO2 assimilation that preceded the pentose cycle and served as a precursor of the Krebs cycle in aerobic metabolism.

Identification of two forms of PFK and a fructose-2,6-bisphosphate independent form of PFP in a green alga

Cell-free preparations from the green alga, Chlorella pyrenoidosa, contained two forms of phosphofructokinase (PFK), designated PFK I and PFK II. This represents the first evidence for a second form of PFK in green algae. A pyrophosphate D-fructose-6-phosphate, 1-phosphotransferase (PFP) activity, that was unaffected by the regulatory metabolite, fructose-2,6-bisphosphate, co-purified with PFK II through several steps. The data suggest that Chlorella pyrenoidosa resembles higher plants in containing two forms of PFK, but differs in containing an atypical form of PFP.

Evidence for function of the ferredoxin/thioredoxin system in the reductive activation of target enzymes of isolated intact chloroplasts

Results obtained with isolated intact chloroplasts maintained aerobically under light and dark conditions confirm earlier findings with reconstituted enzyme assays and indicate that the ferredoxin/thioredoxin system functions as a light-mediated regulatory thiol chain. The results were obtained by application of a newly devised procedure in which a membrane-permeable thiol labeling reagent, monobromobimane (mBBr), reacts with sulfhydryl groups and renders the derivatized protein fluorescent. The mBBr-labeled protein in question is isolated individually from chloroplasts by immunoprecipitation and its thiol redox status is determined quantitatively by combining sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorescence measurements. The findings indicate that each member of the ferredoxin/thioredoxin system containing a catalytically active thiol group is reduced in isolated intact chloroplasts after a 2-min illumination. The extents of reduction were FTR, 38%; thioredoxin m, 75% (11-kDa form) and 87% (13-kDa form); thioredoxin f, 95%. Reduction of each of these components was negligible both in the dark and when chloroplasts were transferred from light to dark conditions. The target enzyme, NADP-malate dehydrogenase, also underwent net reduction in illuminated intact chloroplasts. Fructose-1,6-bisphosphatase showed increased mBBr labeling under these conditions, but due to interfering gamma globulin proteins it was not possible to determine whether this was a result of net reduction as is known to take place in reconstituted assays. Related experiments demonstrated that mBBr, as well as N-ethylmaleimide, stabilized photoactivated NADP-malate dehydrogenase and fructose-1,6-bisphosphatase so that they remained active in the dark. By contrast, phosphoribulokinase, another thioredoxin-linked enzyme, was immediately deactivated following mBBr addition. These latter results provide new information on the relation between the regulatory and active sites of these enzymes.

Activation of a chloroplast type of fructose bisphosphatase from Chlamydomonas reinhardtii by light-mediated agents

A chloroplast type of fructose-1,6-bisphosphatase, a central regulatory enzyme of photosynthetic carbon metabolism, has been partially purified from Chlamydomonas reinhardtii. Unlike its counterpart from spinach chloroplasts, the algal FBPase showed a strict requirement for a dithiol reductant irrespective of Mg2+ concentration. The enzymes from the two sources resembled each other immunologically, in subunit molecular mass and response to pH. In the presence of dithiothreitol, the pH optimum for both the algal and spinach enzymes shifted from 8.5 to a more physiologic value of 8.0 as the Mg2+ concentration was increased from 1 to 16 mM. At 1 mM Mg2+, a concentration estimated to be close to physiological, the Chlamydomonas FBPase was active only in the presence of reduced thioredoxin and was most active with Chlamydomonas thioredoxin f. Under these conditions, the enzyme showed a pH optimum of 8.0. The data suggest that the Chlamydomonas enzyme resembles its spinach counterpart in most respects, but it has a stricter requirement for reduction and less strict reductant specificity. A comparison of the properties of the FBPases from Chlamydomonas and spinach will be helpful for elucidating the mechanism of the reductive activation of this enzyme.

Purification and characterization of fructose-2,6-bisphosphatase, a substrate-specific cytosolic enzyme from leaves

Fructose-2,6-bisphosphatase (EC 3.1.3.46), which hydrolyzes fructose 2,6-bisphosphate to fructose 6-phosphate and Pi, has been purified to apparent homogeneity from spinach leaves and found to be devoid of fructose-6-phosphate,2-kinase activity. The isolated enzyme is a dimer (76 kDa determined by gel filtration) composed of two 33-kDa subunits. The enzyme is highly specific and displays hyperbolic kinetics with its fructose 2,6-bisphosphate substrate (Km = 32 microM). The products of the reaction, fructose 6-phosphate and Pi, along with AMP and Mg2+ are inhibitors of the enzyme. Nonaqueous cell fractionation revealed that, like the fructose 2,6-bisphosphate substrate, fructose-2,6-bisphosphatase as well as fructose-6-phosphate,2-kinase occur in the cytosol of spinach leaves.

Studies on the entry of fructose-2,6-bisphosphate into chloroplasts

The regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P(2)) has an important function in controlling the intermediary carbon metabolism of leaves. Fru-2,6-P(2) controls two cytosolic enzymes involved in the interconversion of fructose-6-phosphate and fructose-1,6-bisphosphate (fructose-1,6-bisphosphatase and pyrophosphate, fructose-6-phosphate 1-phosphotransferase) and thereby controls the partitioning of photosynthate between sucrose and starch. It has been demonstrated that Fru-2,6-P(2) is present mainly in the cytosol. Here we present evidence that Fru-2,6-P(2) can be taken up by isolated intact chloroplasts but at a very slow rate (about 0.01 micromoles per milligram of chlorophyll per hour). This uptake is time and concentration dependent and is inhibited by PPi. When provided a physiological concentration of Fru-2,6-P(2) (10 micromolar), chloroplasts accumulated up to 0.6 micromolar Fru-2,6-P(2) in the stroma. Elevated plastid Fru-2,6-P(2) levels had no effect on overall photosynthetic rates of isolated chloroplasts. The results indicate that, while Fru-2,6-P(2) enters isolated chloroplasts at a sluggish rate, caution should be exercised in ascribing physiological importance to effects of Fru-2,6-P(2) on chloroplast enzymes.

Thioredoxin is essential for photosynthetic growth. The thioredoxin m gene of Anacystis nidulans

We have taken advantage of the transformation properties of the cyanobacterium Anacystis nidulans R2 to investigate the importance of thioredoxin for photosynthetic growth. The gene encoding thioredoxin m, designated trxM, was cloned from A. nidulans using a synthetic oligonucleotide probe. Based on the nucleotide sequence, thioredoxin m of A. nidulans is composed of 107 amino acids and shares 84, 48, and 48% sequence identity with thioredoxins from Anabaena, spinach, and Escherichia coli, respectively. The trxM gene is single copy and is transcribed on a 510-nucleotide mRNA. We demonstrate that disruption of the trxM gene with a kanamycin resistance gene cartridge is a lethal mutation. Although dispensable in E. coli, thioredoxin is essential for the photosynthetic growth of A. nidulans.

An NADP/thioredoxin system in leaves: purification and characterization of NADP-thioredoxin reductase and thioredoxin h from spinach

An NADP/thioredoxin system, consisting of NADPH, NADP-thioredoxin reductase (NTR), and its thioredoxin, thioredoxin h, has been previously described for heterotrophic plant tissues, i.e., wheat seeds and cultured carrot cells. Until now there was no evidence for this system in green leaves. Here, we report the identification of protein components of the NADP/thioredoxin system in leaves of several species. Thioredoxin h and NTR, which were both recovered in the extrachloroplastic fraction, were purified to apparent homogeneity from spinach leaves. This represents the first time that NTR has been characterized from a plant source. Similar to that from bacterial and mammalian sources, spinach leaf NTR was a flavoprotein (Mr 68,000) composed of two subunits of identical molecular mass (Mr 33,000) that resembled Escherichia coli NTR immunologically. Spinach thioredoxin h existed in two forms (Mr of 13,500 and 12,000) and was highly specific for plant NTR. Thioredoxin h and NTR partially purified from spinach roots showed properties similar to their counterparts from leaves. Spinach cytosolic thioredoxin h differed from chloroplast thioredoxin m or f from the same source but was similar to thioredoxin h from wheat seed in immunological properties.

Thioredoxin from Rhodospirillum rubrum: primary structure and relation to thioredoxins from other photosynthetic bacteria

Thioredoxin was isolated from a photosynthetic purple nonsulfur bacterium, Rhodospirillum rubrum, and its primary structure was determined by high-performance tandem mass spectrometry. The sequence identity of R. rubrum thioredoxin to Escherichia coli thioredoxin was intermediate to those of the Chlorobium thiosulfatophilum and Chromatium vinosum proteins. The results indicate that R. rubrum has an NADP-thioredoxin system similar to that of other photosynthetic purple bacteria.

Reduction of purothionin by the wheat seed thioredoxin system

Thioredoxin h, the thioredoxin characteristic of heterotrophic plant tissues, was purified to homogeneity from wheat endosperm (flour) and found to resemble its counterpart from carrot cell cultures. In the presence of NADPH, homogeneous thioredoxin h and partially purified wheat endosperm thioredoxin reductase (NADPH), (EC 1.6.4.5), purothionin promoted the activation of chloroplast fructose-1,6-bisphosphatase (EC 3.1.3.11). Under these conditions, NADPH provided the reducing equivalents for a series of thiol reactions in which (a) thioredoxin reductase reduced thioredoxin h thereby converting it from disulfide (S-S) to sulfhydryl (SH) form; (b) the sulfhydryl form of thioredoxin h reduced the disulfide form of purothionin-a 5 kilodalton seed storage protein with 4 S-S bridges; and (c) the sulfhydryl form of purothionin reductively activated fructose-1,6-bisphosphatase. The results show that, since thioredoxin h does not react effectively with fructose-1,6-bisphosphatase, the thioredoxin system can activate an enzyme through purothionin by secondary thiol redox control. In a related type reaction, purothionin, inhibited the activity of either Escherichia coli or calf thymus ribonucleotide reductase with reduced thioredoxin as hydrogen donor. The results suggest that purothionin competes with ribonucleotide reductase for reducing equivalents from thioredoxin. Thus, inhibition of deoxyribonucleotide synthesis should be considered a possible mechanism when examining the toxic effects of purothionin on mammalian cells in S-phase.

Ferredoxin-thioredoxin reductase: a catalytically active dithiol group links photoreduced ferredoxin to thioredoxin functional in photosynthetic enzyme regulation

The mechanism by which the ferredoxin-thioredoxin system activates the target enzyme, NADP-malate dehydrogenase, was investigated by analyzing the sulfhydryl status of individual protein components with [14C]iodoacetate and monobromobimane. The data indicate that ferredoxin-thioredoxin reductase (FTR)--an iron-sulfur enzyme present in oxygenic photosynthetic organisms--is the first member of a thiol chain that links light to enzyme regulation. FTR possesses a catalytically active dithiol group localized on the 13 kDa (similar) subunit, that occurs in all species investigated and accepts reducing equivalents from photoreduced ferredoxin and transfers them stoichiometrically to the disulfide form of thioredoxin m. The reduced thioredoxin m, in turn, reduces NADP-malate dehydrogenase, thereby converting it from an inactive (S-S) to an active (SH) form. The means by which FTR is able to combine electrons (from photoreduced ferredoxin) with protons (from the medium) to reduce its active disulfide group remains to be determined.

Mass spectrometrically derived amino acid sequence of thioredoxin from Chlorobium, an evolutionarily prominent photosynthetic bacterium

The amino acid sequence of the thioredoxin isolated from the photosynthetic green sulfur bacterium Chlorobium thiosulfatophilum was determined chiefly by fast atom bombardment mass spectrometry combined with Edman degradation and tandem mass spectrometry. For this purpose, the protein was digested with trypsin, alpha-chymotrypsin, thermolysin, and Staphylococcus aureus protease or combinations thereof. Chemical cleavage with cyanogen bromide was also used alone or in combination with trypsin. The resulting sequence of 108 amino acids is as follows: Ala-Gly- Lys-Tyr-Phe-Glu-Ala-Thr-Asp-Lys-Asn-Phe-Gln- Thr-Glu-Xle-Xle-Asp-Ser-Asp-Lys-(Ala-Val)-Xle- Val-Asp-Phe-Trp-Ala-Ser-Trp-Cys-Gly-(Pro-Cys)- Met-Met-Xle-Gly-Pro-Val-Xle-Glu-Gln-Xle-Ala-Asp- Asp-Tyr-Glu-Gly-Lys-Ala-Xle-Xle-Ala-Lys-Xle-Asn- Val-Asp-Glu-Asn-Pro-Asn-Xle-Ala-Gly-Gln-Tyr-Gly- Xle-Arg-Ser-Xle-Pro-Thr-Met-Xle-Xle-Xle-Ly s- (Gly-Gly-Lys)-Val-Val-Asp-Gln-Met-Val-Gly-Ala- Xle-Pro-Lys-Asn-Met-Xle-Ala-Lys-Lys-Xle-Asp-Glu-His-Il e-Gly (where Xle represents leucine or isoleucine; sequences in parentheses are based on homology considerations). It exhibits less than 53% homology with Escherichia coli thioredoxin.