Substrate and inhibitor specificities differ between human cytosolic and mitochondrial thioredoxin reductases: Implications for development of specific inhibitors

The cytosolic and mitochondrial thioredoxin reductases (TrxR1 and TrxR2) and thioredoxins (Trx1 and Trx2) are key components of the mammalian thioredoxin system, which is important for antioxidant defense and redox regulation of cell function. TrxR1 and TrxR2 are selenoproteins generally considered to have comparable properties, but to be functionally separated by their different compartments. To compare their properties we expressed recombinant human TrxR1 and TrxR2 and determined their substrate specificities and inhibition by metal compounds. TrxR2 preferred its endogenous substrate Trx2 over Trx1, whereas TrxR1 efficiently reduced both Trx1 and Trx2. TrxR2 displayed strikingly lower activity with dithionitrobenzoic acid (DTNB), lipoamide, and the quinone substrate juglone compared to TrxR1, and TrxR2 could not reduce lipoic acid. However, Sec-deficient two-amino-acid-truncated TrxR2 was almost as efficient as full-length TrxR2 in the reduction of DTNB. We found that the gold(I) compound auranofin efficiently inhibited both full-length TrxR1 and TrxR2 and truncated TrxR2. In contrast, some newly synthesized gold(I) compounds and cisplatin inhibited only full-length TrxR1 or TrxR2 and not truncated TrxR2. Surprisingly, one gold(I) compound, [Au(d2pype)(2)]Cl, was a better inhibitor of TrxR1, whereas another, [(iPr(2)Im)(2)Au]Cl, mainly inhibited TrxR2. These compounds also inhibited TrxR activity in the cytoplasm and mitochondria of cells, but their cytotoxicity was not always dependent on the proapoptotic proteins Bax and Bak. In conclusion, this study reveals significant differences between human TrxR1 and TrxR2 in substrate specificity and metal compound inhibition in vitro and in cells, which may be exploited for development of specific TrxR1- or TrxR2-targeting drugs.

LeERF-1, a novel AP2/ERF family gene within the B3 subcluster, is down-regulated by light signals in Lithospermum erythrorhizon

We previously showed that ethylene might be involved in the process of shikonin biosynthesis regulated by light signals. Here, we cloned a full-length cDNA of LeERF-1, a putative ethylene response factor gene, from Lithospermum erythrorhizon using the RACE (rapid amplification of cDNA ends) method. Phylogenetic analysis revealed that LeERF-1 was classified in the B3 subfamily, together with ERF1 and ORA59 of Arabidopsis. Heterologous expression of LeERF-1 in Arabidopsis showed that LeERF-1:eGFP fusion protein was precisely localised to the nucleus, implying that it might function as a transcription factor. Detailed expression analysis with real-time PCR showed that LeERF-1 was significantly down-regulated by white, blue and red light, although the inhibitory effect of red light was relatively weak compared to other light conditions. Tissue-specific expression analysis also indicated that LeERF-1 was dominantly expressed in the roots, which grow in soil in darkness. These patterns are all consistent with the effects of different light signals on regulating formation of shikonin and its derivatives, indicating that LeERF-1 might be a crucial positive regulator, like other B3 subfamily proteins (such as ORCA3 and ORA59), in regulating biosynthesis of secondary metabolites.

[Function of the granaticin biosynthetic gene orf20 from Streptomyces vietnamensis]

OBJECTIVE

The transcription factor SoxR, which serves as a cellular redox sensor, is a global activator of antioxidative stress in Escherichia coli. The granaticin biosynthetic gene orf20 from Streptomyces vietnamensis was found to be a soxR-like gene. This study was carried out towards understanding the physiological function of this gene.

METHODS

The orf20 gene was cloned and expressed in E. coli. An orf20 deletion mutant of S. vietnamensis was constructed by using a modified PCR-targeting disruption method. The growth curves of the recombinant E. coli strains and the mutant S. vietnamensis at various paraquat concentrations were assayed to resolve any changes of resistance levels.

RESULTS

Soluble expression of orf20 gene in E. coli was achieved, and the recombinant ORF20 protein was tagged by seven histidines at the carboxylic end. An orJ20 deletion mutant of S. vietnamensis DMR20 was successfully constructed. The DMR20 mutant showed no growth changes to the wild, and the ability of sporulation was retained, too. However, the production of granaticin was improved for more than three folds. Compared to the wild type, the DMR20 mutant had no visible changes of resistance to paraquat, however, E. coli carrying the recombinant plasmid pET28b-orf20 received an elevated resistance to paraquat.

CONCLUSION

The orf20 gene can complement the soxR gene in E. coli, but is not involved in the regulation of anti-oxidative stress response in S. vietnamensis. Instead, it imposes a negative effect on granaticin production.

Effect of different strains of Agrobacterium rhizogenes and nature of explants on Plumbago indica hairy root culture with special emphasis on root biomass and plumbagin production

The aim of the present study was to determine the effect of three strains of Agrobacterium rhizogenes (ATCC 15834, A4 and LBA 9402) and the nature of explants (leaf and stem) on hairy root induction, growth and plumbagin production in Plumbago indica. The first appearance of hairy roots, the transformation frequency, dry root biomass and plumbagin accumulation were found to be maximum in hairy roots induced in leaf explants infected with A. rhizogenes ATCC 15834 as compared with the other two bacterial strains. The hairy roots generated from stem explants infected with all three strains were not found to be productive in terms of the selected parameters. Finally, the insertion of the rolB gene of A. rhizogenes ATCC 15834 in hairy roots of P. indica derived from leaf explants was confirmed by PCR analysis.

Expression of 3-hydroxy-3-methylglutaryl-CoA reductase, p-hydroxybenzoate-m-geranyltransferase and genes of phenylpropanoid pathway exhibits positive correlation with shikonins content in arnebia [Arnebia euchroma (Royle) Johnston]

BACKGROUND

Geranyl pyrophosphate (GPP) and p-hydroxybenzoate (PHB) are the basic precursors involved in shikonins biosynthesis. GPP is derived from mevalonate (MVA) and/or 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway(s), depending upon the metabolite and the plant system under consideration. PHB, however, is synthesized by only phenylpropanoid (PP) pathway. GPP and PHB are central moieties to yield shikonins through the synthesis of m-geranyl-p-hydroxybenzoate (GHB). Enzyme p-hydroxybenzoate-m-geranyltransferase (PGT) catalyses the coupling of GPP and PHB to yield GHB. The present research was carried out in shikonins yielding plant arnebia [Arnebia euchroma (Royle) Johnston], wherein no molecular work has been reported so far. The objective of the work was to identify the preferred GPP synthesizing pathway for shikonins biosynthesis, and to determine the regulatory genes involved in the biosynthesis of GPP, PHB and GHB.

RESULTS

A cell suspension culture-based, low and high shikonins production systems were developed to facilitate pathway identification and finding the regulatory gene. Studies with mevinolin and fosmidomycin, inhibitors of MVA and MEP pathway, respectively suggested MVA as a preferred route of GPP supply for shikonins biosynthesis in arnebia. Accordingly, genes of MVA pathway (eight genes), PP pathway (three genes), and GHB biosynthesis were cloned. Expression studies showed down-regulation of all the genes in response to mevinolin treatment, whereas gene expression was not influenced by fosmidomycin. Expression of all the twelve genes vis-à-vis shikonins content in low and high shikonins production system, over a period of twelve days at frequent intervals, identified critical genes of shikonins biosynthesis in arnebia.

CONCLUSION

A positive correlation between shikonins content and expression of 3-hydroxy-3-methylglutaryl-CoA reductase (AeHMGR) and AePGT suggested critical role played by these genes in shikonins biosynthesis. Higher expression of genes of PP pathway was a general feature for higher shikonins biosynthesis.

Induced production of antifungal naphthoquinones in the pitchers of the carnivorous plant Nepenthes khasiana

Nepenthes spp. are carnivorous plants that have developed insect capturing traps, evolved by specific modification of the leaf tips, and are able to utilize insect degradation products as nutritional precursors. A chitin-induced antifungal ability, based on the production and secretion to the trap liquid of droserone and 5-O-methyldroserone, is described here. Such specific secretion uniquely occurred when chitin injection was used as the eliciting agent and probably reflects a certain kind of defence mechanism that has been evolved for protecting the carnivory-based provision of nutritional precursors. The pitcher liquid containing droserone and 5-O-methyldroserone at 3:1 or 4:1 molar ratio, as well as the purified naphthoquinones, exerted an antifungal effect on a wide range of plant and human fungal pathogens. When tested against Candida and Aspergillus spp., the concentrations required for achieving inhibitory and fungicidal effects were significantly lower than those causing cytotoxicity in cells of the human embryonic kidney cell line, 293T. These naturally secreted 1,4-naphthoquinone derivatives, that are assumed to act via semiquinone enhancement of free radical production, may offer a new lead to develop alternative antifungal drugs with reduced selectable pressure for potentially evolved resistance.

Topical buparvaquone formulations for the treatment of cutaneous leishmaniasis

As the part of a study to develop buparvaquone (BPQ) formulations for the treatment of cutaneous leishmaniasis, the topical delivery of BPQ and one of its prodrugs from a range of formulations was evaluated. In previous studies, BPQ and its prodrugs were shown to be potent antileishmanials in-vitro, with ED50 values in the nanomolar range. 3-Phosphono-oxymethyl-buparvaquone (3-POM-BPQ) was the most potent antileishmanial and was chosen, together with the parent drug, for further investigation. The ability of the parent and prodrug formulations to cross human and murine skin was tested in-vitro using the Franz diffusion cells. Formulations intended for topical application containing either BPQ or 3-POM-BPQ were developed using excipients that were either acceptable for topical use (GRAS or FDA inactive ingredients) or currently going through the regulatory process. BPQ was shown to penetrate both human epidermal membranes and full thickness BALB/c skin from a range of formulations (gels, emulsions). Similarly, 3-POM-BPQ penetrated full-thickness BALB/c skin from several gel formulations. In-vitro binding studies showed that BPQ bound melanin in a dose-dependent manner and preferably bound to delipidized skin over untreated BALB/c skin (on a weight to weight basis). The results confirm that BPQ and its prodrug 3-POM-BPQ can penetrate the skin from several formulations, making them potentially interesting candidates for further investigation of topical formulations using in-vivo models of cutaneous leishmaniasis.

Biodegradation of 2-naphthol and its metabolites by coupling Aspergillus niger with Bacillus subtilis

To explore biodegradation of 2-naphthol and its metabolites accumulated in wastewater treatment, a series of bio-degradation experiments were conducted. Two main metabolites of 2-naphthol, 1,2-naphthalene-diol and 1,2-naphthoquinone, were identified by high-performance liquid chromatography with standards. Combining fungus Aspergillus niger with bacterium Bacillus subtilis in the treatment enhanced 2-naphthol degradation efficiency, lowered the accumulation of the two toxic metabolites. There were two main phases during the degradation process by the kinetic analysis: 2-naphthol was first partly degraded by the fungus, producing labile and easily accumulated metabolites, and then the metabolites were mainly degraded by the bacterium, attested by the degradation processes of 1,2-naphthalene-diol and 1,2-naphthoquinone as sole source of carbon and energy. Sodium succinate, as a co-metabolic substrate, was the most suitable compound for the continuous degradation. The optimum concentration of 2-naphthol was 50 mg/L. The overall 2-naphthol degradation rate was 92%, and the CODcr removal rate was 80% on day 10. These results indicated that high degradation rate of 2-naphthol should not be considered as the sole desirable criterion for the bioremediation of 2-naphthol-contaminated soils/wastewater.

Iron and its complexation by phenolic cellular metabolites: from oxidative stress to chemical weapons

Iron is a transition metal that forms chelates and complexes with various organic compounds, also with phenolic plant secondary metabolites. The ligands of iron affect the redox potential of iron. Electrons may be transferred either to hydroxyl radicals, hydrogen peroxide or molecular oxygen. In the first case, oxidative stress is decreased, in the latter two cases, oxidative stress is increased. This milieu-dependent mode of action may explain the non-linear mode of action of juglone and other secondary metabolites. Attention to this phenomenon may help to explain idiosyncratic and often nonlinear effects that result in biological assays. Current chemical assays are discussed that help to explore these aspects of redox chemistry.

Nitric oxide regulates shikonin formation in suspension-cultured Onosma paniculatum cells

Endogenously occurring nitric oxide (NO) is involved in the regulation of shikonin formation in Onosma paniculatum cells. NO generated after cells were inoculated into shikonin production medium reached the highest level after 2 d of culture, which was 16 times that at the beginning of the experiment, and maintained a high level for 6 d. A nitric oxide synthase (NOS) inhibitor, N(omega)-nitro-L-arginine (L-NNA), and a nitrate reductase (NR) inhibitor, sodium azide (SoA), consistent with their inhibition of NO biosynthesis, decreased shikonin formation significantly. This reduction could be alleviated or even abolished by exogenous NO supplied by sodium nitroprusside (SNP), suggesting that the inhibition of NO biosynthesis resulted in decreased shikonin formation. However, when endogenous NO biosynthesis was up-regulated by the elicitor from Rhizoctonia cerealis, shikonin production was enhanced further, showing a dependence on the elicitor-induced NO burst. Real-time PCR analysis showed that NO could significantly up-regulate the expression of PAL, PGT and HMGR, which encode key enzymes involved in shikonin biosynthesis. These results demonstrated that NO plays a critical role in shikonin formation in O. paniculatum cells.

In vitro metabolism of beta-lapachone (ARQ 501) in mammalian hepatocytes and cultured human cells

ARQ 501 (3,4-dihydro-2,2-dimethyl-2H-naphthol[1,2-b]pyran-5,6-dione, beta-lapachone) is an anticancer agent, currently in multiple phase II clinical trials as monotherapy and in combination with other cytotoxic drugs. This study focuses on in vitro metabolism in cryopreserved hepatocytes from mice, rats, dogs and humans using [(14)C]-labeled ARQ 501. Metabolite profiles were characterized using liquid chromatography/mass spectrometry combined with an accurate radioactivity counter. Ion trap mass spectrometry was employed for further structural elucidation. A total of twelve metabolites were detected in the mammalian hepatocytes studied; all of which but one were generated from phase II conjugation reactions. Ten of the observed metabolites were produced by conjugations occurring at the reduced ortho-quinone carbonyl groups of ARQ 501. The metabolite profiles revealed that glucuronidation was the major biotransformation pathway in mouse and human hepatocytes. Monosulfation was the major pathway in dog, while, in rat, it appears glucuronidation and sulfation pathways contributed equally. Three major metabolites were found in rats: monoglucuronide M1, monosulfate M6, and glucuronide-sulfate M9. Two types of diconjugation metabolites were formed by attachment of the second glycone to an adjacent hydroxyl or to an existing glycone. Of the diconjugation metabolites, glucosylsulfate M10, diglucuronide M5, and glucuronide-glucoside M11 represent rarely observed phase II metabolites in mammals. The only unconjugated metabolite was generated through hydrolysis and was observed in rat, dog and human hepatocytes. ARQ 501 appeared less stable in human hepatocytes than in those of other species. To further elucidate the metabolism of ARQ 501 in extrahepatic sites, its metabolism in human kidney, lung and intestine cells was also studied, and only monoglucuronide M1 was observed in all the cell types examined.

Expression analysis of light-regulated genes isolated from a full-length-enriched cDNA library of Onosma paniculatum cell cultures

Shikonin and its derivatives are formed in large amounts in dark-cultured Onosma paniculatum cells. In order to isolate and identify the genes regulating shikonin biosynthesis, we constructed and characterized a full-length-enriched cDNA library of dark-cultured cells by using the SMART (Switching Mechanism At 5'-end of RNA Transcript) cDNA synthesis and LD-PCR (long-distance PCR) strategies. The titer of the primary cDNA library was 1.04 x 10(6)pfu/mL with a recombination rate of 99.60%. Most of the cDNA inserts ranged from 1.0 to 2.5 kb, and 78.33% of the 76 randomly selected clones contained full-length coding regions. Expression analysis of randomly selected genes by small scale microarray revealed that 23 genes were down-regulated, including 17 genes with known functions, 2 genes with putative functions, and 4 novel genes, and that 3 genes were up-regulated (two-fold) in cells cultured under white light as compared with those cultured in the dark. Interestingly, two of the down-regulated genes, encoding aci-reductone dioxygenase (ARD)-like protein and ethylene responsive factor (ERF), are involved in ethylene biosynthesis and signal transduction, implying that ethylene might play an important role as a signal molecule in light-regulated shikonin formation. These data contribute to a better understanding of light-involvement in regulating the formation of plant secondary metabolites.

Vinyl ketone reduction by three distinct Gluconobacter oxydans 621H enzymes

Three cytosolic NADPH-dependent flavin-associated proteins (Gox2107, Gox0502, and Gox2684) from Gluconobacter oxydans 621H were overproduced in Escherichia coli, and the recombinant enzymes were purified and characterized. Apparent native molecular masses of 65.2, 78.2, and 78.4 kDa were observed for Gox2107, Gox0502, and Gox2684, corresponding to a trimeric structure for Gox2107 and dimers for Gox0502 and Gox2684. Analysis of flavin content revealed Gox2107 was flavin adenine dinucleotide dependent, whereas Gox0502 and Gox2684 contained flavin mononucleotide. The enzymes were able to reduce vinyl ketones and quinones, reducing the olefinic bond of vinyl ketones as shown by (1)H nuclear magnetic resonance. Additionally, Gox0502 and Gox2684 stereospecifically reduced 5S-(+)-carvone to 2R,5S-dihydrocarvone. All enzymes displayed highest activities with 3-butene-2-one and 1,4-naphthoquinone. Gox0502 and Gox2684 displayed a broader substrate spectrum also reducing short-chain alpha-diketones, whereas Gox2107 was most catalytically efficient.

Naphthoquinone-dependent generation of superoxide radicals by quinone reductase isolated from the plasma membrane of soybean

Using a tetrazolium-based assay, a NAD(P)H oxidoreductase was purified from plasma membranes prepared from soybean (Glycine max) hypocotyls. The enzyme, a tetramer of 85 kD, produces O2(.-) by a reaction that depended on menadione or several other 1,4-naphthoquinones, in apparent agreement with a classification as a one-electron-transferring flavoenzyme producing semiquinone radicals. However, the enzyme displayed catalytic and molecular properties of obligatory two-electron-transferring quinone reductases of the DT-diaphorase type, including insensitivity to inhibition by diphenyleneiodonium. This apparent discrepancy was clarified by investigating the pH-dependent reactivity of menadionehydroquinone toward O2 and identifying the protein by mass spectrometry and immunological techniques. The enzyme turned out to be a classical NAD(P)H:quinone-acceptor oxidoreductase (EC 1.6.5.2, formerly 1.6.99.2) that reduces menadione to menadionehydroquinone and subsequently undergoes autoxidation at pH > or = 6.5. Autoxidation involves the production of the semiquinone as an intermediate, creating the conditions for one-electron reduction of O2. The possible function of this enzyme in the generation of O2(.-) and H2O2 at the plasma membrane of plants in vivo is discussed.

Crystallization and preliminary characterization of a novel haem-binding protein of Streptomyces reticuli

Streptomyces reticuli is a soil-growing Gram-positive bacteria that has been shown to secrete a novel haem-binding protein known as HbpS. Sequence analysis reveals that homologues of HbpS are found in a wide variety of bacteria, including different Actinobacteria and the Gram-negative Vibrio cholera and Klebsiella pneumoniae. The in vivo production of HbpS is greatly increased when S. reticuli is cultured in the presence of the natural antibiotic haemin (Fe3+ oxidized form of haem). Mutational analysis demonstrated that HbpS significantly increases the resistance of S. reticuli to toxic concentrations of haemin. Previous data show that the presence of the newly identified two-component sensor system SenS-SenR also considerably enhances the resistance of S. reticuli to haemin and the redox-cycling compound plumbagin, suggesting a role in the sensing of redox changes. Specific interaction between HbpS and SenS-SenR, which regulates the expression of the catalase-peroxidase CpeB, as well as HbpS, has been demonstrated in vitro. HbpS has been recombinantly overexpressed, purified and crystallized in space group P2(1)3, with a cell edge of 152.5 A. Diffraction data were recorded to a maximal resolution of 2.25 A and phases were obtained using the SAD method from crystals briefly soaked in high concentrations of sodium bromide.

Biosynthetic investigations of lactonamycin and lactonamycin z: cloning of the biosynthetic gene clusters and discovery of an unusual starter unit

The antibiotics lactonamycin and lactonamycin Z provide attractive leads for antibacterial drug development. Both antibiotics contain a novel aglycone core called lactonamycinone. To gain insight into lactonamycinone biosynthesis, cloning and precursor incorporation experiments were undertaken. The lactonamycin gene cluster was initially cloned from Streptomyces rishiriensis. Sequencing of ca. 61 kb of S. rishiriensis DNA revealed the presence of 57 open reading frames. These included genes coding for the biosynthesis of l-rhodinose, the sugar found in lactonamycin, and genes similar to those in the tetracenomycin biosynthetic gene cluster. Since lactonamycin production by S. rishiriensis could not be sustained, additional proof for the identity of the S. rishiriensis cluster was obtained by cloning the lactonamycin Z gene cluster from Streptomyces sanglieri. Partial sequencing of the S. sanglieri cluster revealed 15 genes that exhibited a very high degree of similarity to genes within the lactonamycin cluster, as well as an identical organization. Double-crossover disruption of one gene in the S. sanglieri cluster abolished lactonamycin Z production, and production was restored by complementation. These results confirm the identity of the genetic locus cloned from S. sanglieri and indicate that the highly similar locus in S. rishiriensis encodes lactonamycin biosynthetic genes. Precursor incorporation experiments with S. sanglieri revealed that lactonamycinone is biosynthesized in an unusual manner whereby glycine or a glycine derivative serves as a starter unit that is extended by nine acetate units. Analysis of the gene clusters and of the precursor incorporation data suggested a hypothetical scheme for lactonamycinone biosynthesis.

The influence of water quality on the toxicity and degradation of juglone (5-hydroxy 1,4-naphthoquinone)

This study was part of a broader investigation of low molecular weight quinones under consideration as biocides for the control of aquatic nuisance species (ANS). Preliminary investigations identified the 2-ring naphthoquinones as broad spectrum biocides controlling a wide range of aquatic organisms. All biocides were relatively short-lived in saline waters, with half-lives between 5 and 30h. Juglone (5-hydroxy 1,4-naphthoquinone) and plumbagin (5-hydroxy-2-methyl-1,4- naphthoquinone) showed the greatest toxicity against most aquatic organisms. These qualities formed the basis for a patent focusing on these two compounds as biocides for ANS control, with juglone identified as the more cost-effective of the two. Although juglone has been extensively studied as a plant toxin and reducing agent, remarkably little information exists on its use as an aquatic biocide. We describe the toxicity of juglone over the range of water quality parameters likely to be encountered in ballast water, a major vector for ANS. Tests indicated that its molecular stability was enhanced in freshwater and particularly under neutral to acid conditions. This was supported by results of bioassays on the freshwater cladoceran Daphnia magna that indicated enhanced juglone toxicity at pHs of < or =6.7. A low octanol:water partition coefficient for juglone indicated little capacity for these compounds to be adsorbed by suspended particulates and for bioaccumulation. These properties together with their relatively rapid degradation (t1/2 < or =30h), particularly in the marine environment, indicated a low the risk of residual toxicity associated with the release of juglone-treated water.

Involvement of the osmosensor histidine kinase and osmotic stress-activated protein kinases in the regulation of secondary metabolism in Fusarium graminearum

Fusarium graminearum produces trichothecenes in aerial hyphae, a process which is markedly suppressed by NaCl without a significant effect on fungal growth. Here we report on the involvement of kinases of the two-component osmotic signal transduction pathway in the regulation of secondary metabolism in F. graminearum. While a deletion null mutant of FgOs1 (encoding the osmosensor histidine kinase) (deltaFgOs1) produced a reduced amount of the red pigment aurofusarin and was unaltered in its ability to produce trichothecenes, deletion null mutants of FgOs4 (encoding mitogen-activated protein kinase kinase kinase; MAPKKK), FgOs5 (MAPKK), and FgOs2 (MAPK) showed markedly enhanced pigmentation and failed to produce trichothecenes in aerial hyphae. Also, the transcript levels of PKS12 and GIP2 (aurofusarin biosynthetic pathway and regulatory genes, respectively) were significantly enhanced in the deltaFgOs4, deltaFgOs5, and deltaFgOs2 mutants and were reduced in the deltaFgOs1 mutant. In addition, expression of Tri4 and Tri6 (trichothecene biosynthetic pathway and regulatory genes) and production of trichothecenes in rice medium were markedly reduced in the former three protein kinase mutants. This is the first report demonstrating the involvement of a MAPK in the regulation of secondary metabolism.

RETRACTED: A computational docking study for prediction of binding mode of diospyrin and derivatives: Inhibitors of human and leishmanial DNA topoisomerase-I

A computational approach was utilized to study the relative binding modes of diospyrin (bisnaphthoquinonoid) with the crystal structure of human DNA-TopoI and the recently reported Leishmania donavani DNA-TopoI. Additionally, the binding site interactions of amino derivatives of diospyrin with human TopoI were studied extensively. Based on the docking results, binding modes of diospyrin with the human and leishmanial TopoI catalytic core were predicted. The parallel use of two efficient and predictive docking programs, GOLD and Ligandfit, allowed mutual validation of the predicted binding poses. A reasonably good correlation coefficient between the calculated docking scores and the experimentally determined cytotoxicity helped in validating the docking method. Furthermore, a structure-based pharmacophore model was developed for L. donavani DNA-TopoI inhibition which helped in elucidating the topological and spatial requirements of the ligand-receptor interactions. This study provides an understanding of the structural basis of ligand binding to the topoisomerase receptor, which may be used for the structure-based design of potent and novel ligands for anticancer and antileishmanial therapy. To our knowledge, this is the first report of a binding mode exploration study for diospyrin and its derivatives as inhibitors of the leishmanial and human TopoI enzymes.

Biosynthesis of pentangular polyphenols: deductions from the benastatin and griseorhodin pathways

The benastatins, pradimicins, fredericamycins, and members of the griseorhodin/rubromycin family represent a structurally and functionally diverse group of long-chain polyphenols from actinomycetes. Comparison of their biosynthetic gene clusters (ben, prm, fdm, grh, rub) revealed that all loci harbor genes coding for a similar, yet uncharacterized, type of ketoreductases. In a phylogenetic survey of representative KRs involved in type II PKS systems, we found that it is generally possible to deduce the KR regiospecificity (C-9, C-15, C17) from the amino acid sequence and thus to predict the nature of the aromatic polyketide (e.g., angucycline, anthracycline, benzoisochromanequinones). We hypothezised that the new clade of KRs is characteristic for biosynthesis of polyphenols with an extended angular architecture we termed "pentangular". To test this hypothesis, we demonstrated the biogenetic relationship between benastatin and the structurally unrelated spiro ketal griseorhodin by generating a mutant producing collinone, a pentangular pathway intermediate. The benastatin pathway served as a model to characterize the KR. Gene inactivation of benL resulted in the formation of a series of 19-hydroxy benastatin and bequinostatin derivatives (e.g., benastatin K and benastatin L). These results clearly showed that BenL functions as a C-19 KR in pentangular pathways.

Different Binding Modes of Two Flaviolin Substrate Molecules in Cytochrome P450 158A1 (CYP158A1) Compared to CYP158A2,

Cytochrome P450 158A2 (CYP158A2) has been shown to catalyze an unusual oxidative C-C coupling reaction to polymerize flaviolin and form highly conjugated pigments (three isomers of biflaviolin and one triflaviolin) in Streptomyces coelicolor A3(2) which protect the soil bacterium from deleterious effects of UV irradiation (Zhao B. et al. (2005) J. Biol. Chem. 280, 11599-11607). The present studies demonstrate that the subfamily partner CYP158A1, sharing 61% amino acid identity with CYP158A2, can also catalyze the same flaviolin dimerization reactions, but it generates just two of the three isomers of biflaviolin that CYP158A2 produces. Furthermore, the two CYP158A1 products have very different molar ratios compared with the corresponding CYP158A2 products, indicating that each enzyme maintains its own stereo- and regiospecificity. To find an explanation for these differences, three CYP158A1 structures have been solved by X-ray crystallography and have been compared with those for CYP158A2. The structures reveal surprising differences. Particularly, only one flaviolin molecule is present close to the heme iron in CYP158A1, and the second flaviolin molecule binds at the entrance of the putative substrate access channel on the protein distal surface 9 A away. Our work describes two members of the same P450 subfamily, which produce the same products by oxidative C-C coupling yet show very different structural orientations of substrate molecules in the active site.

Initiation of actinorhodin export in Streptomyces coelicolor

Many microorganisms produce molecules having antibiotic activity and expel them into the environment, presumably enhancing their ability to compete with their neighbours. Given that these molecules are often toxic to the producer, mechanisms must exist to ensure that the assembly of the export apparatus accompanies or precedes biosynthesis. Streptomyces coelicolor produces the polyketide antibiotic actinorhodin in a multistep pathway involving enzymes encoded by genes that are clustered together. Embedded within the cluster are genes for actinorhodin export, two of which, actR and actA resemble the classic tetR and tetA repressor/efflux pump-encoding gene pairs that confer resistance to tetracycline. Like TetR, which represses tetA, ActR is a repressor of actA. We have identified several molecules that can relieve repression by ActR. Importantly (S)-DNPA (an intermediate in the actinorhodin biosynthetic pathway) and kalafungin (a molecule related to the intermediate dihydrokalafungin), are especially potent ActR ligands. This suggests that along with the mature antibiotic(s), intermediates in the biosynthetic pathway might activate expression of the export genes thereby coupling export to biosynthesis. We suggest that this could be a common feature in the production of many bioactive natural products.

Biosynthesis of the Isoprenoid Moieties of Furanonaphthoquinone I and Endophenazine A in Streptomyces cinnamonensis DSM 1042

Streptomyces cinnamonensis DSM 1042 produces the polyketide-isoprenoid compound furanonaphthoquinone I (FNQ I) and isoprenylated phenazines, predominantly endophenazine A. However, the recently identified biosynthetic gene cluster for these compounds only contains a single gene for a mevalonate pathway enzyme, that is, a putative mevalonate kinase gene. This is in strong contrast to all Streptomyces strains examined so far, where all six genes encoding the mevalonate pathway enzymes are clustered in a single operon of 6.8 kb and, thus, raised the question about the biosynthetic origin of the isoprenoid moieties of FNQ I and endophenazine A. In this study, we investigated the incorporation of [13C2]acetate and [2-13C]glycerol into FNQ I and endophenazine A. The results unequivocally prove that the isoprenoid building blocks of both compounds are predominantly formed via the mevalonate pathway (approximately 80%) but that the MEP pathway (approximately 20%) contributes to the biosynthesis of these molecules, too. In actinomycetes, this is the first experimentally proven example of the utilization of both biosynthetic routes for the formation of one single secondary metabolite. The incorporation pattern of [2-13C]glycerol was consistent with a "reverse" prenyl transfer, that is, with the formation of a C-C bond from C-3 of GPP to the polyketide nucleus of FNQ I.

Occurrence and fate of the phytotoxin juglone in alley soils under black walnut trees

Juglone (5-hydroxy-1,4-napthoquinone) is a chemical released by walnut trees, which can be toxic at various levels to several plant species. A balance among competing source and sink mechanisms and rates will ultimately determine whether juglone is capable of attaining sufficient levels to be allelopathic to intercrops in a walnut tree agroforestry system. In this study, juglone's release, accumulation, and decline in soil are explored using data from soil beneath a black walnut tree (Juglans nigra L) alley cropping system, greenhouse pot studies, and laboratory sorption/degradation studies. Juglone pore water concentrations estimated from extracts of surficial soil from beneath the alley cropping system exceeded the lowest solution culture toxicity levels reported for some plants of 10(-7) M, but did not exceed the inhibition threshold reported for typical intercrops such as maize and soybeans 10(-5) M. Further assessment of the likely persistence of juglone in soils indicated that juglone is both microbially and abiotically degraded, and that it will be particularly short-lived in soils supporting microbial activity. However, walnut seedlings planted in sand-filled pots clearly showed that juglone is released in measurable quantities to the soil's rhizosphere. Therefore, juglone accumulation in low fertility soils is plausible, and may still be worthy of consideration in management of alley agroforestry systems.

A gene cluster for prenylated naphthoquinone and prenylated phenazine biosynthesis in Streptomyces cinnamonensis DSM 1042

Streptomyces cinnamonensis DSM 1042 produces two classes of secondary metabolites of mixed isoprenoid/nonisoprenoid origin: the polyketide-isoprenoid compound furanonaphthoquinone I (FNQ I) and several prenylated phenazines, predominantly endophenazine A. We now report the cloning and sequence analysis of a 55 kb gene cluster required for the biosynthesis of these compounds. Several inactivation experiments confirmed the involvement of this gene cluster in the biosynthesis of FNQ I and endophenazine A. The six identified genes for endophenazine biosynthesis showed close similarity to phenazine biosynthetic genes from Pseudomonas. Of the 28 open reading frames identified in the adjacent FNQ I cluster, 13 showed close similarity to genes contained in the cluster for furaquinocin-a structurally similar metabolite from another Streptomyces strain. These genes included a type III polyketide synthase sequence, a momA-like monooxygenase gene, and two cloQ-like prenyltransferase genes designated fnq26 and fnq28. Inactivation experiments confirmed the involvement of fnq26 in FNQ I biosynthesis, whereas no change in secondary-metabolite formation was observed after fnq28 inactivation. The FNQ I cluster contains a contiguous group of five genes, which together encode all the enzymatic functions required for the recycling of S-adenosylhomocysteine (SAH) to S-adenosylmethionine (SAM). Two SAM-dependent methyltransferases are encoded within the cluster. Inactivation experiments showed that fnq9 is responsible for the 7-O-methylation and fnq27 for the 6-C-methylation reaction in FNQ I biosynthesis.

Molecular basis for chloronium-mediated meroterpene cyclization: cloning, sequencing, and heterologous expression of the napyradiomycin biosynthetic gene cluster

Structural inspection of the bacterial meroterpenoid antibiotics belonging to the napyradiomycin family of chlorinated dihydroquinones suggests that the biosynthetic cyclization of their terpenoid subunits is initiated via a chloronium ion. The vanadium-dependent haloperoxidases that catalyze such reactions are distributed in fungi and marine algae and have yet to be characterized from bacteria. The cloning and sequence analysis of the 43-kb napyradiomycin biosynthetic cluster (nap) from Streptomyces aculeolatus NRRL 18422 and from the undescribed marine sediment-derived Streptomyces sp. CNQ-525 revealed 33 open reading frames, three of which putatively encode vanadium-dependent chloroperoxidases. Heterologous expression of the CNQ-525-based nap biosynthetic cluster in Streptomyces albus produced at least seven napyradiomycins, including the new analog 2-deschloro-2-hydroxy-A80915C. These data not only revealed the molecular basis behind the biosynthesis of these novel meroterpenoid natural products but also resulted in the first in vivo verification of vanadium-dependent haloperoxidases.

Deficiency in Phylloquinone (Vitamin K1) Methylation Affects Prenyl Quinone Distribution, Photosystem I Abundance, and Anthocyanin Accumulation in the Arabidopsis AtmenG Mutant

Phylloquinone (vitamin K(1)) is synthesized in cyanobacteria and in chloroplasts of plants, where it serves as electron carrier of photosystem I. The last step of phylloquinone synthesis in cyanobacteria is the methylation of 2-phytyl-1,4-naphthoquinone by the menG gene product. Here, we report that the uncharacterized Arabidopsis gene At1g23360, which shows sequence similarity to menG, functionally complements the Synechocystis menG mutant. An Arabidopsis mutant, AtmenG, carrying a T-DNA insertion in the gene At1g23360 is devoid of phylloquinone, but contains an increased amount of 2-phytyl-1,4-naphthoquinone. Phylloquinone and 2-phytyl-1,4-naphthoquinone in thylakoid membranes of wild type and AtmenG, respectively, predominantly localize to photosystem I, whereas excess amounts of prenyl quinones are stored in plastoglobules. Photosystem I reaction centers are decreased in AtmenG plants under high light, as revealed by immunoblot and spectroscopic measurements. Anthocyanin accumulation and chalcone synthase (CHS1) transcription are affected during high light exposure, indicating that alterations in photosynthesis in AtmenG affect gene expression in the nucleus. Photosystem II quantum yield is decreased under high light. Therefore, the loss of phylloquinone methylation affects photosystem I stability or turnover, and the limitation in functional photosystem I complexes results in overreduction of photosystem II under high light.

Functional characterization of ketoreductase (rubN6) and aminotransferase (rubN4) genes in the gene cluster of Streptomyces achromogenes var. rubradiris

ORF's for rubN6 and rubN4 have been annotated as thymidine diphosphate glucose 4-ketoreductase and thymidine diphosphate glucose 3-aminotransferase by sequence analysis of the rubradirin biosynthetic gene cluster cloned from Streptomyces achromogenes var. rubradiris NRRL 3061. Both ORFs were heterologously expressed in Escherichia coli as His-tagged fusion proteins. The functionalities of TDP-glucose 4-ketoreductase and TDP-glucose 3-aminotransferase were verified by in vitro enzyme assay, and a biosynthetic pathway for TDP-D: -rubranitrose is proposed.

SH-7, a new synthesized shikonin derivative, exerting its potent antitumor activities as a topoisomerase inhibitor

1-(1,4-dihydro-5,8-dihydroxy-1,4-dioxonaphthalen-2-yl)-4-methylpent-3-enylfuran-2-caroxylate (SH-7), a new naphthoquinone compound, derived from shikonin, exhibited obvious inhibitory actions on topoisomerase II (Topo II) and topoisomerase I (Topo I), which were stronger than its mother compound shikonin. Notably, the SH-7's inhibitory potency on Topo II was much stronger than that on Topo I. In addition, SH-7 significantly stabilized Topo II-DNA cleavable complex and elevated the expression of phosphorylated-H2AX. The in vitro cell-based investigation demonstrated that SH-7 displayed wide cytotoxicity in diversified cancer cell lines with the mean IC(50) value of 7.75 microM. One important finding is SH-7 displayed significant cytotoxicity in the 3 MDR cell lines, with an average IC(50) value nearly equivalent to that of the corresponding parental cell lines. The average resistance factor (RF) of SH-7 was 1.74, which was much lower than those of reference drugs VP-16 (RF 145.92), ADR (RF 105.97) and VCR (RF 197.39). Further studies illustrated that SH-7 had the marked apoptosis-inducing function on leukemia HL-60 cells, which was validated to be of mitochondria-dependence. The in vivo experiments showed that SH-7 had inhibitory effects on S-180 sarcoma implanted to mice, SMMC-7721, BEL-7402 human hepatocellular carcinoma and PC-3 human prostate cancer implanted to nude mice. Taken together, these results suggest that SH-7 induces DSBs as a Topo II inhibitor, which was crucial to activate the apoptotic process, and subsequently accounts for its both in vitro and in vivo antitumor activities. The well-defined Topo II inhibitory activity, antitumor effects particularly with its obvious anti-MDR action, better solubility and less toxicity make SH-7 as a potential antitumor drug candidate for further research and development.

Salvicine functions as novel topoisomerase II poison by binding to ATP pocket

Salvicine, a structurally modified diterpenoid quinone derived from Salvia prionitis, is a nonintercalative topoisomerase II (topo II) poison. The compound possesses potent in vitro and in vivo antitumor activity with a broad spectrum of anti-multidrug resistance activity and is currently in phase II clinical trials. To elucidate the distinct antitumor properties of salvicine and obtain valuable structural information of salvicine-topo II interactions, we characterized the effects of salvicine on human topo IIalpha (htopo IIalpha), including possible binding sites and molecular interactions. The enzymatic assays disclosed that salvicine mainly inhibits the catalytic activity with weak DNA cleavage action, in contrast to the classic topo II poison etoposide (VP16). Molecular modeling studies predicted that salvicine binds to the ATP pocket in the ATPase domain and superimposes on the phosphate and ribose groups. In a surface plasmon resonance binding assay, salvicine exhibited higher affinity for the ATPase domain of htopo IIalpha than ATP and ADP. Competitive inhibition tests demonstrated that ATP competitively and dose-dependently blocked the interactions between salvicine and ATPase domain of htopo IIalpha. The data illustrate that salvicine shares a common binding site with ATP and functions as an ATP competitor. To our knowledge, this is the first report to identify an ATP-binding pocket as the structural binding motif for a nonintercalative eukaryotic topo II poison. These findings collectively support the potential value of an ATP competitor of htopo IIalpha in tumor chemotherapy.

The biosynthetic pathway for aurofusarin in Fusarium graminearum reveals a close link between the naphthoquinones and naphthopyrones

Fungal polyketide biosynthesis typically involves multiple enzymatic steps and the encoding genes are often found in gene clusters. A gene cluster containing PKS12, the polyketide synthase gene responsible for the synthesis of the pigment aurofusarin, was analysed by gene replacement using Agrobacterium tumefaciens-mediated transformation to determine the biosynthesis pathway of aurofusarin. Replacement of aurR1 with hygB shows that it encodes a positively acting transcription factor that is required for the full expression of PKS12, aurJ, aurF, gip1 and FG02329.1, which belong to the gene cluster. AurR1 and PKS12 deletion mutants are unable to produce aurofusarin and rubrofusarin. Bio- and chemoinformatics combined with chemical analysis of replacement mutants (DeltaaurJ, DeltaaurF, Deltagip1, DeltaaurO and DeltaPKS12) indicate a five-step enzyme catalysed pathway for the biosynthesis of aurofusarin, with rubrofusarin as an intermediate. This links the biosynthesis of naphthopyrones and naphthoquinones together. Replacement of the putative transcription factor aurR2 results in an increased level of rubrofusarin relative to aurofusarin. Gip1, a putative laccase, is proposed to be responsible for the dimerization of two oxidized rubrofusarin molecules resulting in the formation of aurofusarin.

A Fluoro Analogue of the Menadione Derivative 6-[2‘-(3‘-Methyl)-1‘,4‘-naphthoquinolyl]hexanoic Acid Is a Suicide Substrate of Glutathione Reductase. Crystal Structure of the Alkylated Human Enzyme†

Glutathione reductase is an important housekeeping enzyme for redox homeostasis both in human cells and in the causative agent of tropical malaria, Plasmodium falciparum. Glutathione reductase inhibitors were shown to have anticancer and antimalarial activity per se and to contribute to the reversal of drug resistance. The development of menadione chemistry has led to the selection of 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl]hexanoic acid, called M(5), as a potent reversible and uncompetitive inhibitor of both human and P. falciparum glutathione reductases. Here we describe the synthesis and kinetic characterization of a fluoromethyl-M(5) analogue that acts as a mechanism-based inhibitor of both enzymes. In the course of enzymatic catalysis, the suicide substrate is activated by one- or two-electron reduction, and then a highly reactive quinone methide is generated upon elimination of the fluorine. Accordingly the human enzyme was found to be irreversibly inactivated with a k(inact) value of 0.4 +/- 0.2 min(-1). The crystal structure of the alkylated enzyme was solved at 1.7 A resolution. It showed the inhibitor to bind covalently to the active site Cys58 and to interact noncovalently with His467', Arg347, Arg37, and Tyr114. On the basis of the crystal structure of the inactivated human enzyme and stopped-flow kinetic studies with two- and four-electron-reduced forms of the unreacted P. falciparum enzyme, a mechanism is proposed which explains naphthoquinone reduction at the flavin of glutathione reductase.

Ligand-assisted inhibition in cytochrome P450 158A2 from Streptomyces coelicolor A3(2)

Cytochrome P450 158A2 (CYP158A2) can polymerize flaviolin to red-brown pigments, which may afford physical protection to the organism, possibly against the deleterious effects of UV radiation. We have found that the small molecule malonic acid enables cocrystallization of this mixed function oxidase with the azole inhibitor 4-phenylimidazole. The presence of malonate molecules affects the behavior of the binding of 4-phenylimidazole to CYP158A2 and increases inhibition potency up to 2-fold compared to 4-phenylimidazole alone. We report here the crystal structure of the 4-phenylimidazole/malonate complex of CYP158A2 at 1.5 A. Two molecules of malonate used in crystallization are found above the single inhibitor molecule in the active site. Those two molecules are linked between the BC loop and beta 1-4/beta 6-1 strands via hydrogen bond interactions to stabilize the conformational changes of the BC loop and beta strands that take place upon inhibitor binding compared to the ligand-free structure we have reported previously. 4-Phenylimidazole can launch an extensive hydrogen-bonding network in the region of the F/G helices which may stabilize the conformational changes. Our findings clearly show that two molecules of malonate assist the inhibitor 4-phenylimidazole to assume a specific location producing more inhibition in the enzyme catalytic activity.

[Naphthoquinones--biosynthesis, occurrence and metabolism in plants]

Naphthoquinones are relatively widely occurring natural substances, products of secondary metabolism of some actinomycetes, fungi, lichens, and higher plants. The importance of these substances for the producers proper is, due to their wide biological activity, still discussed. In most cases they act as phytoalexines. In the case of fungi, they may play a significant role in the pathogenicity of moulds--naphthoquinones interact with mitochondria, microsomes and cytoplasmic proteins, in the form of radicals they are bound to DNA and RNA, and they do damage to them. Naphthoquinones are highly cytotoxic substances; their antimicrobial, antifungal, antiviral and antiparasitic effects have been observed. In traditional medicines, particularly in some parts of Asia (China) and South America, naphthoquinones-containing plants are widely used primarily in the treatment of various tumoral and parasitic diseases.

Auxofuran, a novel metabolite that stimulates the growth of fly agaric, is produced by the mycorrhiza helper bacterium Streptomyces strain AcH 505

The mycorrhiza helper bacterium Streptomyces strain AcH 505 improves mycelial growth of ectomycorrhizal fungi and formation of ectomycorrhizas between Amanita muscaria and spruce but suppresses the growth of plant-pathogenic fungi, suggesting that it produces both fungal growth-stimulating and -suppressing compounds. The dominant fungal-growth-promoting substance produced by strain AcH 505, auxofuran, was isolated, and its effect on the levels of gene expression of A. muscaria was investigated. Auxofuran and its synthetic analogue 7-dehydroxy-auxofuran were most effective at a concentration of 15 microM, and application of these compounds led to increased lipid metabolism-related gene expression. Cocultivation of strain AcH 505 and A. muscaria stimulated auxofuran production by the streptomycete. The antifungal substances produced by strain AcH 505 were identified as the antibiotics WS-5995 B and C. WS-5995 B completely blocked mycelial growth at a concentration of 60 microM and caused a cell stress-related gene expression response in A. muscaria. Characterization of these compounds provides the foundation for molecular analysis of the fungus-bacterium interaction in the ectomycorrhizal symbiosis between fly agaric and spruce.

Cell Growth and Shikonin Production of Arnebia euchroma in a Periodically Submerged Airlift Bioreactor

Arnebia euchroma was grown in a 2-l periodically submerged, airlift bioreactor (PSAB) in which the non-submerged (immobilization culture) and submerged (suspension culture) operations were controlled automatically. PSAB had advantages in improving cell growth, shikonin content, shikonin production and cell aggregation compared with suspension culture. Under the optimal submerged/non-submerged period of 10 min/15 h, the shikonin content (4.6%, w/w) and, cell dry mass (16.8 g/l) were 229 and 26% higher than those in suspension culture.

An unusual reduction on the quinonoid ring of 5-amino-8-hydroxy-1,4-naphthoquinone by Staphylococcus aureus

AIMS

The objective of this study was to investigate the interactions between 5-amino-8-hydroxy-1,4-naphthoquinone (ANQ) and Staphylococcus aureus.

METHODS AND RESULTS

The compound ANQ display antimicrobial activity against S. aureus. During incubation with 50 microg ml(-1) of ANQ, an unusual reduction reaction takes place and leads to the isolation of 2,3-dihydro-5-amino-8-hydroxy-1,4-naphthoquinone (ANQ-H(2)), fully characterized by means of (13)C-NMR and (1)H-NMR, plus infrared, UV-visible and mass spectroscopy. Oxygen uptake by S. aureus cells was inhibited by ANQ, but in a significantly minor extent by ANQ-H(2).

CONCLUSIONS

The ability of S. aureus to reduce the double bond at C2-C3 of the ANQ is a unusual behaviour for biological transformation of naphthoquinones.

SIGNIFICANCE AND IMPACT OF THE STUDY

This uncommon reaction may provide valuable understanding of the S. aureus regarding to the antimicrobial effect and the acquisition of resistance to naphthoquinones.

Culture conditions improving the production of jadomycin B

The jadomycins are a unique family of benzoxazolophenanthridine antibiotics produced by Streptomyces venezuelae ISP5230 following heat or ethanol shock or phage infection. We have modified the culture conditions by altering the carbon source, buffer, inoculum size, and timing of ethanol shock, thereby reducing growing times and improving jadomycin B production. Our optimized conditions use glucose as the carbon source, MOPS as buffer, low concentrations of phosphate, a defined inoculum concentration and an immediate ethanol shock to induce jadomycin B production; results that contrast previous studies. The altered media will facilitate the isolation of related jadomycin B congeners.

Functional analysis of the polyketide synthase genes in the filamentous fungus Gibberella zeae (anamorph Fusarium graminearum)

Polyketides are a class of secondary metabolites that exhibit a vast diversity of form and function. In fungi, these compounds are produced by large, multidomain enzymes classified as type I polyketide synthases (PKSs). In this study we identified and functionally disrupted 15 PKS genes from the genome of the filamentous fungus Gibberella zeae. Five of these genes are responsible for producing the mycotoxins zearalenone, aurofusarin, and fusarin C and the black perithecial pigment. A comprehensive expression analysis of the 15 genes revealed diverse expression patterns during grain colonization, plant colonization, sexual development, and mycelial growth. Expression of one of the PKS genes was not detected under any of 18 conditions tested. This is the first study to genetically characterize a complete set of PKS genes from a single organism.

Interactions of Nitroaromatic Compounds with the Mammalian Selenoprotein Thioredoxin Reductase and the Relation to Induction of Apoptosis in Human Cancer Cells

Here we described novel interactions of the mammalian selenoprotein thioredoxin reductase (TrxR) with nitroaromatic environmental pollutants and drugs. We found that TrxR could catalyze nitroreductase reactions with either one- or two-electron reduction, using its selenocysteine-containing active site and another redox active center, presumably the FAD. Tetryl and p-dinitrobenzene were the most efficient nitroaromatic substrates with a k(cat) of 1.8 and 2.8 s(-1), respectively, at pH 7.0 and 25 degrees C using 50 muM NADPH. As a nitroreductase, TrxR cycled between four- and two-electron-reduced states. The one-electron reactions led to superoxide formation as detected by cytochrome c reduction and, interestingly, reductive N-denitration of tetryl or 2,4-dinitrophenyl-N-methylnitramine, resulting in the release of nitrite. Most nitroaromatics were uncompetitive and noncompetitive inhibitors with regard to NADPH and the disulfide substrate 5,5'-dithiobis(2-nitrobenzoic acid), respectively. Tetryl and 4,6-dinitrobenzofuroxan were, however, competitive inhibitors with respect to 5,5'-dithiobis(2-nitrobenzoic acid) and were clearly substrates for the selenolthiol motif of the enzyme. Furthermore, tetryl and 4,6-dinitrobenzofuroxan efficiently inactivated TrxR, likely by alkylation of the selenolthiol motif as in the inhibition of TrxR by 1-chloro-2,4-dinitrobenzene/dinitrochlorobenzene (DNCB) or juglone. The latter compounds were the most efficient inhibitors of TrxR activity in a cellular context. DNCB, juglone, and tetryl were highly cytotoxic and induced caspase-3/7 activation in HeLa cells. Furthermore, DNCB and juglone were potent inducers of apoptosis also in Bcl2 overexpressing HeLa cells or in A549 cells. Based on these findings, we suggested that targeting of intracellular TrxR by alkylating nitroaromatic or quinone compounds may contribute to the induction of apoptosis in exposed human cancer cells.

[Mycological examination of domestic unpolished rice and mycotoxin production by isolated Penicillium islandicum]

Fungi growing on domestic rice were examined from April to June, 2003. One hundred samples of rice, which had been harvested in the autumn of 2002, were collected from the local market, and 15 samples of stored rice, which had been harvested in 2001 and stored in warehouses under government control, were used as samples. From each sample, 50 grains (100 grains in total) were plated on potato-dextrose agar (PDA) and malt yeast 40% sucrose agar (M40YA) containing chloramphenicol after being washed with sterile distilled water to remove any microorganisms on the surface, and incubated at 25 degrees C for a week. For most of the rice samples harvested in the preceding year, the proportion of grains infected with fungi was less than 20% of the total grains tested. In about half the samples of rice stored for one and half years, more than 80% of the grains were infected with fungi that grew on M40YA. The major genera of fungi isolated from the rice harvested in the preceding year were Penicillium and Alternaria, and those from the rice stored for one and a half years were Aspergillus, Penicillium and Eurotium. P. islandicum, A. versicolor, A. ochraceus and others were isolated as possible mycotoxin-producers in the mycoflora of domestic rice. P. islandicum was isolated from 3 samples, and 82% of the grains were infected with this fungus in one sample. All three isolates from these samples appeared to produce luteoskyrin on Czapek yeast extract agar, based on TLC and HPLC analysis.

2-[(R-phenyl)amine]-1,4-naphthalendiones as photosystem I electron acceptors: structure-activity relationship of m- and p-PAN compounds with QSAR analysis

Nineteen 2-[(R-phenyl)amine]-1,4-naphthalendione derivatives (PAN) were tested on spinach thylakoids for their activity as electron acceptors. These molecules act as photosystem I electron acceptors in the micromolar range. AC(50) values varied from 5 nM to 24 microM. QSAR analysis revealed a linear correlation of the m-PAN derivative log [1/AC(50)] with the energy difference of the LUMO and HOMO orbitals. The biological activity of p-PAN derivatives correlates linearly with structural parameters. Electron affinity is being the most important. The half wave I potential values (E(1/2)) of PAN compounds (from -213 to -569 mV vs. NHE) match with the mid-point potentials of the A(0) to F(X) niche of PSI electron transport carriers. The logP values of PAN derivatives were 3.35 and 3.88, indicating that they are hydrophobic compounds. Therefore PAN compounds accept electrons at the hydrophobic A(0) to F(X) niche of PSI.

GIP2, a putative transcription factor that regulates the aurofusarin biosynthetic gene cluster in Gibberella zeae

Gibberella zeae (anamorph: Fusarium graminearum) is an important pathogen of maize, wheat, and rice. Colonies of G. zeae produce yellow-to-tan mycelia with the white-to-carmine red margins. In this study, we focused on nine putative open reading frames (ORFs) closely linked to PKS12 and GIP1, which are required for aurofusarin biosynthesis in G. zeae. Among them is an ORF designated GIP2 (for Gibberella zeae pigment gene 2), which encodes a putative protein of 398 amino acids that carries a Zn(II)2Cys6 binuclear cluster DNA-binding domain commonly found in transcription factors of yeasts and filamentous fungi. Targeted gene deletion and complementation analyses confirmed that GIP2 is required for aurofusarin biosynthesis. Expression of GIP2 in carrot medium correlated with aurofusarin production by G. zeae and was restricted to vegetative mycelia. Inactivation of the 10 contiguous genes in the DeltaGIP2 strain delineates an aurofusarin biosynthetic gene cluster. Overexpression of GIP2 in both the DeltaGIP2 and the wild-type strains increases aurofusarin production and reduces mycelial growth. Thus, GIP2 is a putative positive regulator of the aurofusarin biosynthetic gene cluster, and aurofusarin production is negatively correlated with vegetative growth by G. zeae.

The structure–activity relationships of mansonone F, a potent anti-MRSA sesquiterpenoid quinone: SAR studies on the C6 and C9 analogs

For the systematic SAR study on mansonone F, a series of C6 and C9 analogs of mansonone F have been synthesized and their anti-MRSA activities were evaluated. Most of the analogs exhibited good or excellent anti-MRSA activities. In particular, the 6-n-butylmansonone F showed fourfold higher antibacterial activities compared to that of vancomycin.

Functional studies on a ketoreductase gene from Streptomyces sp. AM-7161 to control the stereochemistry in medermycin biosynthesis

Medermycin shows the same trans (3S,15R) configuration as actinorhodin in the pyran ring crucial for its bioactivity. One medermycin biosynthetic gene, med-ORF12, is assumed to be involved in the stereochemical control at C-3. Functional complementation suggested that it plays a similar role as actVI-ORF1 previously proved to determine the stereospecificity at C-3 in actinorhodin biosynthesis. Co-expression of med-ORF12 with actinorhodin early biosynthetic genes further demonstrated that med-ORF12 encodes a ketoreductase responsible for the enantioselective reduction at C-3 in the formation of the pyran ring.