Purification and functional analysis of the Mycobacterium leprae thioredoxin/thioredoxin reductase hybrid protein

Dithiol disulphide exchange in redox regulation of chloroplast enzymes in response to evolutionary and structural constraints

Redox regulation of chloroplast enzymes via disulphide reduction is believed to control the rates of CO₂ fixation. The study of the thioredoxin reduction pathways and of various target enzymes lead to the following guidelines.

Analysis of antigens of Mycobacterium leprae by interaction to sera IgG, IgM, and IgA response to improve diagnosis of leprosy

Till 2010, several countries have declared less than one leprosy patient among population of 10,000 and themselves feeling as eliminated from leprosy cases. However, new leprosy cases are still appearing from all these countries. In this situation one has to be confident to diagnose leprosy. This review paper highlighted already explored antigens for diagnosis purposes and finally suggested better combinations of protein antigens of M. leprae versus immunoglobulin as detector antibody to be useful for leprosy diagnosis.

Insights into the specificity of thioredoxin reductase-thioredoxin interactions. A structural and functional investigation of the yeast thioredoxin system

The enzymatic activity of thioredoxin reductase enzymes is endowed by at least two redox centers: a flavin and a dithiol/disulfide CXXC motif. The interaction between thioredoxin reductase and thioredoxin is generally species-specific, but the molecular aspects related to this phenomenon remain elusive. Here, we investigated the yeast cytosolic thioredoxin system, which is composed of NADPH, thioredoxin reductase (ScTrxR1), and thioredoxin 1 (ScTrx1) or thioredoxin 2 (ScTrx2). We showed that ScTrxR1 was able to efficiently reduce yeast thioredoxins (mitochondrial and cytosolic) but failed to reduce the human and Escherichia coli thioredoxin counterparts. To gain insights into this specificity, the crystallographic structure of oxidized ScTrxR1 was solved at 2.4 A resolution. The protein topology of the redox centers indicated the necessity of a large structural rearrangement for FAD and thioredoxin reduction using NADPH. Therefore, we modeled a large structural rotation between the two ScTrxR1 domains (based on the previously described crystal structure, PDB code 1F6M ). Employing diverse approaches including enzymatic assays, site-directed mutagenesis, amino acid sequence alignment, and structure comparisons, insights were obtained about the features involved in the species-specificity phenomenon, such as complementary electronic parameters between the surfaces of ScTrxR1 and yeast thioredoxin enzymes and loops and residues (such as Ser(72) in ScTrx2). Finally, structural comparisons and amino acid alignments led us to propose a new classification that includes a larger number of enzymes with thioredoxin reductase activity, neglected in the low/high molecular weight classification.

NTRC new ways of using NADPH in the chloroplast

Despite being the primary source of energy in the biosphere, photosynthesis is a process that inevitably produces reactive oxygen species. Chloroplasts are a major source of hydrogen peroxide production in plant cells; therefore, different systems for peroxide reduction, such as ascorbate peroxidase and peroxiredoxins (Prxs), are found in this organelle. Most of the reducing power required for hydrogen peroxide reduction by these systems is provided by Fd reduced by the photosynthetic electron transport chain; hence, the function of these systems is highly dependent on light. Recently, it was described a novel plastidial enzyme, stated NTRC, formed by a thioredoxin reductase (NTR) domain at the N-terminus and a thioredoxin (Trx) domain at the C-terminus. NTRC is able to conjugate both NTR and Trx activities to efficiently reduce 2-Cys Prx using NADPH as a source of reducing power. Based on these results, it was proposed that NTRC is a new pathway to transfer reducing power to the chloroplast detoxification system, allowing the use of NADPH, besides reduced Fd, for such function. In this article, the most important features of NTRC are summarized and the implications of this novel activity in the context of chloroplast protection against oxidative damage are discussed.

A novel NADPH thioredoxin reductase, localized in the chloroplast, which deficiency causes hypersensitivity to abiotic stress in Arabidopsis thaliana

Plants contain three thioredoxin systems. Chloroplast thioredoxins are reduced by ferredoxin-thioredoxin reductase, whereas the cytosolic and mitochondrial thioredoxins are reduced by NADPH thioredoxin reductase (NTR). There is high similarity among NTRs from plants, lower eukaryotes, and bacteria, which are different from mammal NTR. Here we describe the OsNTRC gene from rice encoding a novel NTR with a thioredoxin-like domain at the C terminus, hence, a putative NTR/thioredoxin system in a single polypeptide. Orthologous genes were found in other plants and cyanobacteria, but not in bacteria, yeast, or mammals. Full-length OsNTRC and constructs with truncated NTR and thioredoxin domains were expressed in Escherichia coli as His-tagged polypeptides, and a polyclonal antibody specifically cross-reacting with the OsNTRC enzyme was raised. An in vitro activity assay showed that OsNTRC is a bifunctional enzyme with both NTR and thioredoxin activity but is not an NTR/thioredoxin system. Although the OsNTRC gene was expressed in roots and shoots of rice seedlings, the protein was exclusively found in shoots and mature leaves. Moreover, fractionation experiments showed that OsNTRC is localized to the chloroplast. An Arabidopsis NTRC knock-out mutant showed growth inhibition and hypersensitivity to methyl viologen, drought, and salt stress. These results suggest that the NTRC gene is involved in plant protection against oxidative stress.

The thioredoxin system?From science to clinic

The thioredoxin system-formed by thioredoxin reductase and its characteristic substrate thioredoxin-is an important constituent of the intracellular redox milieu. Interactions with many different metabolic pathways such as DNA-synthesis, selenium metabolism, and the antioxidative network as well as significant species differences render this system an attractive target for chemotherapeutic approaches in many fields of medicine-ranging from infectious diseases to cancer therapy. In this review we will present and evaluate the preclinical and clinical results available today. Current trends in drug development are emphasized.

Isolation and characterization of a new peroxiredoxin from poplar sieve tubes that uses either glutaredoxin or thioredoxin as a proton donor

A sequence coding for a peroxiredoxin (Prx) was isolated from a xylem/phloem cDNA library from Populus trichocarpa and subsequently inserted into an expression plasmid yielding the construction pET-Prx. The recombinant protein was produced in Escherichia coli cells and purified to homogeneity with a high yield. The poplar Prx is composed of 162 residues, a property that makes it the shortest plant Prx sequence isolated so far. It was shown that the protein is monomeric and possesses two conserved cysteines (Cys). The Prx degrades hydrogen peroxide and alkyl hydroperoxides in the presence of an exogenous proton donor that can be either thioredoxin or glutaredoxin (Grx). Based on this finding, we propose that the poplar protein represents a new type of Prx that differs from the so-called 2-Cys and 1-Cys Prx, a suggestion supported by the existence of natural fusion sequences constituted of a Prx motif coupled to a Grx motif. The protein was shown to be highly expressed in sieve tubes where thioredoxin h and Grx are also major proteins.

Presence of human T-cell responses to the Mycobacterium leprae 45-kilodalton antigen reflects infection with or exposure to M. leprae

The ability of the 45-kDa serine-rich Mycobacterium leprae antigen to stimulate peripheral blood mononuclear cell (PBMC) proliferation and gamma interferon (IFN-gamma) production was measured in leprosy patients, household contacts, and healthy controls from areas of endemicity in Mexico. Almost all the tuberculoid leprosy patients gave strong PBMC proliferation responses to the M. leprae 45-kDa antigen (92.8%; n = 14). Responses were lower in lepromatous leprosy patients (60.6%; n = 34), but some responses to the 45-kDa antigen were detected in patients unresponsive to M. leprae sonicate. The proportion of positive responses to the M. leprae 45-kDa antigen was much higher in leprosy contacts (88%; n = 17) than in controls from areas of endemicity (10%; n = 20). None of 15 patients with pulmonary tuberculosis gave a positive proliferation response to the 45-kDa antigen. The 45-kDa antigen induced IFN-gamma secretion similar to that induced by the native Mycobacterium tuberculosis 30/31-kDa antigen in tuberculoid leprosy patients and higher responses than those induced by the other recombinant antigens (M. leprae 10- and 65-kDa antigens, thioredoxin, and thioredoxin reductase); in patients with pulmonary tuberculosis it induced lower IFN-gamma secretion than the other recombinant antigens. These results suggest that the M. leprae 45-kDa antigen is a potent T-cell antigen which is M. leprae specific in these Mexican donors. This antigen may therefore have diagnostic potential as a new skin test reagent or as an antigen in a simple whole-blood cytokine test.

Transmembrane electron transfer by the membrane protein DsbD occurs via a disulfide bond cascade

The cytoplasmic membrane protein DsbD transfers electrons from the cytoplasm to the periplasm of E. coli, where its reducing power is used to maintain cysteines in certain proteins in the reduced state. We split DsbD into three structural domains, each containing two essential cysteines. Remarkably, when coexpressed, these truncated proteins restore DsbD function. Utilizing this three piece system, we were able to determine a pathway of the electrons through DsbD. Our findings strongly suggest that the pathway is based on a series of multistep redox reactions that include direct interactions between thioredoxin and DsbD, and between DsbD and its periplasmic substrates. A thioredoxin-fold domain in DsbD appears to have the novel role of intramolecular electron shuttle.

Thioredoxin reductase as a pathophysiological factor and drug target

Human cytosolic thioredoxin reductase (TrxR), a homodimeric protein containing 1 selenocysteine and 1 FAD per subunit of 55 kDa, catalyses the NADPH-dependent reduction of thioredoxin disulfide and of numerous other oxidized cell constituents. As a general reducing enzyme with little substrate specificity, it also contributes to redox homeostasis and is involved in prevention, intervention and repair of damage caused by H2O2-based oxidative stress. Being a selenite-reducing enzyme as well as a selenol-containing enzyme, human TrxR plays a central role in selenium (patho)physiology. Both dietary selenium deficiency and selenium oversupplementation, a lifestyle phenomenon of our time, appear to interfere with the activity of TrxR. Selenocysteine 496 of human TrxR is a major target of the anti-rheumatic gold-containing drug auranofin, the formal Ki for the stoichiometric inhibition being 4 nM. The hypothesis that TrxR and extracellular thioredoxin play a pathophysiologic role in chronic diseases such as rheumatoid arthritis, Sjögren's syndrom, AIDS, and certain malignancies, is substantiated by biochemical, virological, and clinical evidence. Reduced thioredoxin acts as an autocrine growth factor in various tumour diseases, as a chemoattractant, and it synergises with interleukins 1 and 2. The effects of anti-tumour drugs such as carmustine and cisplatin can be explained in part by the inhibition of TrxR. Consistently, high levels of the enzyme can support drug resistance. TrxRs from different organisms such as Escherichia coli, Mycobacterium leprae, Plasmodium falciparum, Drosophila melanogaster, and man show a surprising diversity in their chemical mechanism of thioredoxin reduction. This is the basis for attempts to develop specific TrxR inhibitors as drugs against bacterial infections like leprosy and parasitic diseases like amebiasis and malaria.

Attachment of the N-terminal domain of Salmonella typhimurium AhpF to Escherichia coli thioredoxin reductase confers AhpC reductase activity but does not affect thioredoxin reductase activity

AhpF of Salmonella typhimurium, the flavoprotein reductase required for catalytic turnover of AhpC with hydroperoxide substrates in the alkyl hydroperoxide reductase system, is a 57 kDa protein with homology to thioredoxin reductase (TrR) from Escherichia coli. Like TrR, AhpF employs tightly bound FAD and redox-active disulfide center(s) in catalyzing electron transfer from reduced pyridine nucleotides to the disulfide bond of its protein substrate. Homology of AhpF to the smaller (35 kDa) TrR protein occurs in the C-terminal part of AhpF; a stretch of about 200 amino acids at the N-terminus of AhpF contains an additional redox-active disulfide center and is required for catalysis of AhpC reduction. We have demonstrated that fusion of the N-terminal 207 amino acids of AhpF to full-length TrR results in a chimeric protein (Nt-TrR) with essentially the same catalytic efficiency (k(cat)/K(m)) as AhpF in AhpC reductase assays; both k(cat) and the K(m) for AhpC are decreased about 3-4-fold for Nt-TrR compared with AhpF. In addition, Nt-TrR retains essentially full TrR activity. Based on results from two mutants of Nt-TrR (C129, 132S and C342,345S), AhpC reductase activity requires both centers while TrR activity requires only the C-terminal-most disulfide center in Nt-TrR. The high catalytic efficiency with which Nt-TrR can reduce thioredoxin implies that the attached N-terminal domain does not block access of thioredoxin to the TrR-derived Cys342-Cys345 center of Nt-TrR nor does it impede the putative conformational changes that this part of Nt-TrR is proposed to undergo during catalysis. These studies indicate that the C-terminal part of AhpF and bacterial TrR have very similar mechanistic properties. These findings also confirm that the N-terminal domain of AhpF plays a direct role in AhpC reduction.

The AhpC and AhpD antioxidant defense system of Mycobacterium tuberculosis

The peroxiredoxin AhpC from Mycobacterium tuberculosis has been expressed, purified, and characterized. It differs from other well characterized AhpC proteins in that it has three rather than one or two cysteine residues. Mutagenesis studies show that all three cysteine residues are important for catalytic activity. Analysis of the M. tuberculosis genome identified a second protein, AhpD, which has no sequence identity with AhpC but is under the control of the same promoter. This protein has also been cloned, expressed, purified, and characterized. AhpD, which has only been identified in the genomes of mycobacteria and Streptomyces viridosporus, is shown here to also be an alkylhydroperoxidase. The endogenous electron donor for catalytic turnover of the two proteins is not known, but both can be turned over with AhpF from Salmonella typhimurium or, particularly in the case of AhpC, with dithiothreitol. AhpC and AhpD reduce alkylhydroperoxides more effectively than H(2)O(2) but do not appear to interact with each other. These two proteins appear to be critical elements of the antioxidant defense system of M. tuberculosis and may be suitable targets for the development of novel anti-tuberculosis strategies.

AhpF can be dissected into two functional units: tandem repeats of two thioredoxin-like folds in the N-terminus mediate electron transfer from the thioredoxin reductase-like C-terminus to AhpC

AhpF, the flavin-containing component of the Salmonella typhimurium alkyl hydroperoxide reductase system, catalyzes the NADH-dependent reduction of an active-site disulfide bond in the other component, AhpC, which in turn reduces hydroperoxide substrates. The amino acid sequence of the C-terminus of AhpF is 35% identical to that of thioredoxin reductase (TrR) from Escherichia coli. AhpF contains an additional 200-residue N-terminal domain possessing a second redox-active disulfide center also required for AhpC reduction. Our studies indicate that this N-terminus contains a tandem repeat of two thioredoxin (Tr)-like folds, the second of which contains the disulfide redox center. Structural and catalytic properties of independently expressed fragments of AhpF corresponding to the TrR-like C-terminus (F[208-521]) and the 2Tr-like N-terminal domain (F[1-202]) have been addressed. Enzymatic assays, reductive titrations, and circular dichroism studies of the fragments indicate that each folds properly and retains many functional properties. Electron transfer between F[208-521] and F[1-202] is, however, relatively slow (4 x 10(4) M(-)(1) s(-)(1) at 25 degrees C) and nonsaturable up to 100 microM F[1-202]. TrR is nearly as efficient at F[1-202] reduction as is F[208-521], although neither the latter fragment, nor intact AhpF, can reduce Tr. An engineered mutant AhpC substrate with a fluorophore attached via a disulfide bond has been used to demonstrate that only F[1-202], and not F[208-521], is capable of electron transfer to AhpC, thereby establishing the direct role this N-terminal domain plays in mediating electron transfer between the TrR-like part of AhpF and AhpC.

The capsule of Mycobacterium tuberculosis and its implications for pathogenicity

Mycobacterium tuberculosis, one of the most prevalent causes of death worldwide, is a facultative intracellular parasite that invades and persists within the macrophages. Within host cells, the bacterium is surrounded by a capsule which is electron-transparent in EM sections, outside the bacterial wall and plasma membrane. Although conventional processing of samples for microscopy studies failed to demonstrate this structure around in vitro-grown bacilli, the application of new microscopy techniques to mycobacteria allows the visualization of a thick capsule in specimen from axenic cultures of mycobacteria. Gentle mechanical treatment and detergent extraction remove the outermost components of this capsule which consist primarily of polysaccharide and protein, with small amounts of lipid. Being at the interface between the bacterium and host cells, the capsule and its constituents would be expected to be involved in bacterial pathogenicity and past work supports this concept. Recent studies have identified several capsular substances potentially involved in the key steps of pathogenicity. In this respect, some of the capsular glycans have been shown to mediate the adhesion to and the penetration of bacilli into the host's cells; of related interest, secreted and/or surface-exposed enzymes and transporters probably involved in intracellular multiplication have been characterized in short-term culture filtrates of M. tuberculosis. In addition, the presence of inducible proteases and lipases has been shown. The capsule would also represent a passive barrier by impeding the diffusion of macromolecules towards the inner parts of the envelope; furthermore, secreted enzymes potentially involved in the detoxification of reactive oxygen intermediates have been identified, notably catalase/peroxidase and superoxide dismutase, which may participate to the active resistance of the bacterium to the host's microbicidal mechanisms. Finally, toxic lipids and contact-dependent lytic substances, as well as constituents that inhibit both macrophage-priming and lymphoproliferation, have been found in the capsule, thereby explaining part of the immunopathology of tuberculosis.

Reduction of peroxides and dinitrobenzenes by Mycobacterium tuberculosis thioredoxin and thioredoxin reductase

The thioredoxin (Trx) and thioredoxin reductase (TR) of Mycobacterium tuberculosis have been expressed in Escherichia coli and shown to reduce peroxides and dinitrobenzenes. The reduction of H2O2 requires both Trx and TR and is more efficient under anaerobic than aerobic conditions. In contrast, cumene hydroperoxide is reduced to cumyl alcohol and acetophenone in a process that requires NADPH and TR but not Trx. Cumene hydroperoxide reduction is partially inhibited by chelation of trace metals in the medium. The reduction of cumene hydroperoxide by TR is more effective under anaerobic than aerobic conditions due to a competing oxidase reaction in which electrons are transferred from TR to O2. Under anaerobic conditions, dinitrobenzenes also serve as electron acceptors and are reduced by TR to nitroanilines, but the enzyme does not reduce mononitrobenzenes or mononitroimidazoles such as metronidazole. The reductive activity of the Trx-TR system may modify the antioxidant defenses of M. tuberculosis.

Thioredoxin reductase-thioredoxin fusion enzyme from Mycobacterium leprae: comparison with the separately expressed thioredoxin reductase

Thioredoxin reductase (TrxR) catalyzes the reduction of thioredoxin (Trx) by NADPH. A unique gene organization of TrxR and Trx has been found in Mycobacterium leprae, where TrxR and Trx are encoded by a single gene and, therefore, are expressed as a fusion protein (MlTrxR-Trx). This fusion enzyme is able to catalyze the reduction of thioredoxin or 5,5'-dithiobis(2-nitrobenzoic acid) or 1, 4-naphthoquinone by NADPH, though the activity is much lower than that of Escherichia coli TrxR. It has been proposed that a large conformational change is required in catalysis of E. coli TrxR. Because the reductase portion of the enzyme from M. leprae shows significant primary structure similarity with E. coli TrxR, it is possible that MlTrxR-Trx may require a similar conformational change and that the change in conformation may be affected by the tethered Trx. The reductase has been expressed without Trx attached (MlTrxR). As reported here, comparison of the steady-state and pre-steady-state kinetics of MlTrxR-Trx with those of MlTrxR suggests that the low reductase activity of the fusion enzyme is an inherent property of the reductase, and that any steric limitation caused by the attached thioredoxin in the fusion protein makes only a minor contribution to the low activity. Titration of MlTrxR-Trx and MlTrxR with 3-aminopyridine adenine dinucleotide phosphate (AADP+), an NADP(H) analogue, results in only slight quenching of FAD fluorescence, suggesting an enzyme conformation in which the binding site of AADP+ is not close to the FAD, as in one of the conformations of E. coli TrxR.

A three-domain iron-sulfur flavoprotein obtained through gene fusion of ferredoxin and ferredoxin-NADP+ reductase from spinach leaves

Ferredoxin and ferredoxin-NADP+ reductase are the two last partners of the photosynthetic electron-transfer chain, responsible for the final reduction of NADP+ to NADPH. Herein, we report the engineering and characterization of a novel protein molecule in which the electron-carrier protein (ferredoxin I) and the reductase (a flavoprotein) were covalently linked in a single polypeptide chain by gene fusion. The gene was obtained by joining the cDNAs encoding the respective proteins and subsequently by deleting the intervening sequence between them by site-directed mutagenesis. No extra amino acid residues were introduced between the C-terminus of ferredoxin I and the N-terminus of the flavoenzyme. The chimera was purified to homogeneity and characterized. The M(r) of the chimera apoprotein was 45,800 as determined by mass spectrometry, in agreement with the expected value of 45,846. Both flavin and iron-sulfur cluster were in 1:1 ratio with respect to the apoprotein. The chimera was found active as a diaphorase and, more interestingly, highly efficient as a cytochrome c reductase, without need for free ferredoxin addition in the assay medium. Several lines of evidence indicate that the ferredoxin and the reductase in the chimera assume a configuration quite similar to that in the dissociable physiological complex. Thus, the fusion protein could be a useful tool for studying the mechanism of protein-protein recognition and electron transfer in the ferredoxin-ferredoxin-NADP+ reductase system.

Disruption of the Pseudomonas aeruginosa dipZ gene, encoding a putative protein-disulfide reductase, leads to partial pleiotropic deficiency in c-type cytochrome biogenesis

The Pseudomonas aeruginosa dipZ gene has been cloned and sequenced. Whereas disruption of Escherichia coli dipZ (dsbD), the hydrophilic C-terminal domain of which has been deduced to be periplasmic and to function as a protein-disulfide reductase, leads to the absence of c-type cytochromes, disruption of P. aeruginosa dipZ attenuated, but did not abolish, holo-c-type cytochrome biosynthesis. Comparison of the P. aeruginosa DipZ sequence with three other DipZ sequences indicated that there are not only two conserved cysteine residues in the C-terminal hydrophilic domain, but also two more in the central highly hydrophobic domain. The latter would be located toward the centre of two of the eight membrane-spanning alpha-helices predicted to compose the hydrophobic central domain of DipZ. Both these cysteine residues, plus other transmembrane helix residues, notably prolines and glycines, are also conserved in a group of membrane proteins, related to Bacillus subtilis CcdA, which lack the N- and C-terminal hydrophilic domains of the DipZ proteins. It is proposed that DipZ of P. aeruginosa and other organisms transfers reducing power from the cytoplasm to the periplasm through reduction and reoxidation of an intramembrane disulfide bond, or other mechanism involving these cysteine residues, and that this function can also be performed by B. subtilis CcdA and other CcdA-like proteins. The failure of dipZ disruption to abolish c-type cytochrome synthesis in P. aeruginosa suggests that, in contrast to the situation in E. coli, the absence of DipZ can be compensated for by one or more other proteins, for example a CcdA-like protein acting in tandem with one or more thioredoxin-like proteins.

The 58 kDa mouse selenoprotein is a BCNU-sensitive thioredoxin reductase

The flavoprotein thioredoxin reductase [EC 1.6.4.5] (NADPH + H+ + thioredoxin-S2 --> NADP+ + thioredoxin-(SH)2) was isolated from mouse Ehrlich ascites tumour (EAT) cells. Like the counterpart from human placenta but unlike the known thioredoxin reductases from non-vertebrate organisms, the mouse enzyme was found to contain 1 equivalent of selenium per subunit of 58 kDa. The K(M) values were 4.5 microM for NADPH, 480 microM for DTNB and 36 microM for Escherichia coli thioredoxin, the turnover number with DTNB being approximately 40 s(-1). As mouse is a standard animal model in cancer and malaria research, thioredoxin reductase and glutathione reductase [EC 1.6.4.2] from EAT cells were compared with each other. While both enzymes in their 2-electron reduced form are targets of the cytostatic drug carmustine (BCNU), no immunologic cross-reactivity between the two mouse disulfide reductases was observed.

Increased intracellular survival of Mycobacterium smegmatis containing the Mycobacterium leprae thioredoxin-thioredoxin reductase gene

The thioredoxin (Trx) system of Mycobacterium leprae is expressed as a single hybrid protein containing thioredoxin reductase (TR) at its N terminus and Trx at its C terminus. This hybrid Trx system is unique to M. leprae, since in all other organisms studied to date, including other mycobacteria, both TR and Trx are expressed as two separate proteins. Because Trx has been shown to scavenge reactive oxygen species, we have investigated whether the TR-Trx gene product can inhibit oxygen-dependent killing of mycobacteria by human mononuclear phagocytes and as such could contribute to mycobacterial virulence. The gene encoding M. leprae TR-Trx was cloned into the apathogenic, fast-growing bacterium Mycobacterium smegmatis. Recombinant M. smegmatis containing the gene encoding TR-Trx was killed to a significantly lesser extent than M. smegmatis containing the identical vector with either no insert or a control M. leprae construct unrelated to TR-Trx. Upon phagocytosis, M. smegmatis was shown to be killed predominantly by oxygen-dependent macrophage-killing mechanisms. Coinfection of M. smegmatis expressing the gene encoding TR-Trx together with Staphylococcus aureus, which is known to be killed via oxygen-dependent microbicidal mechanisms, revealed that the TR-Trx gene product interferes with the intracellular killing of this bacterium. A similar coinfection with Streptococcus pyogenes, known to be killed by oxygen-independent mechanisms, showed that the TR-Trx gene product did not influence the oxygen-independent killing pathway. The data obtained in this study suggest that the Trx system of M. leprae can inhibit oxygen-dependent killing of intracellular bacteria and thus may represent one of the mechanisms by which M. leprae can deal with oxidative stress within human mononuclear phagocytes.

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.