Genetic and biosynthetic studies of the fungal prenylated xanthone shamixanthone and related metabolites in Aspergillus spp. revisited

Anthrol reductases: discovery, role in biosynthesis and applications in natural product syntheses

Covering: up to 2023Short-chain dehydrogenase/reductases (SDR) are known to catalyze the regio- and stereoselective reduction of a variety of substrate types. Investigations of the deoxygenation of emodin to chrysophanol has led to the discovery of the anthrol reductase activity of an SDR, MdpC involved in monodictyphenone biosynthesis of Aspergillus nidulans and provided access to (R)-dihydroanthracenone, a putative biosynthetic intermediate. This facilitated the identification of several MdpC-related enzymes involved in the biosynthesis of aflatoxins B1, cladofulvin, neosartorin, agnestins and bisanthraquinones. Because of their ability to catalyze the reduction of hydroanthraquinone (anthrols) using NADPH, they were named anthrol reductases. This review provides a comprehensive summary of all the anthrol reductases that have been identified and characterized in the last decade along with their role in the biosynthesis of natural products. In addition, the applications of these enzymes towards the chemoenzymatic synthesis of flavoskyrins, modified bisanthraquinones, 3-deoxy anthraquinones, chiral cycloketones and β-halohydrins have been discussed.

Recent advances in the identification of biosynthetic genes and gene clusters of the polyketide-derived pathways for anthraquinone biosynthesis and biotechnological applications

Natural anthraquinones are represented by a large group of compounds. Some of them are widespread across the kingdoms, especially in bacteria, fungi and plants, while the others are restricted to certain groups of organisms. Despite the significant pharmacological potential of several anthraquinones (hypericin, skyrin and emodin), their biosynthetic pathways and candidate genes coding for key enzymes have not been experimentally validated. Understanding the genetic and epigenetic regulation of the anthraquinone biosynthetic gene clusters in fungal endophytes would help not only understand their pathways in plants, which ensure their commercial availability, but also favor them as promising systems for prospective biotechnological production.

The 'emodin family' of fungal natural products-amalgamating a century of research with recent genomics-based advances

Covering: up to 2022A very large group of biosynthetically linked fungal secondary metabolites are formed via the key intermediate emodin and its corresponding anthrone. The group includes anthraquinones such as chrysophanol and cladofulvin, the grisandienes geodin and trypacidin, the diphenyl ether pestheic acid, benzophenones such as monodictyphenone and various xanthones including the prenylated shamixanthones, the agnestins and dimeric xanthones such as the ergochromes, cryptosporioptides and neosartorin. Such compounds exhibit a wide range of bioactivities and as such have been utilised in traditional medicine for centuries, as well as garnering more recent interest from the pharmaceutical sector. Additional interest comes from industries such as textiles and cosmetics due to their use as natural colourants. A variety of biosynthetic routes and mechanisms have been proposed for this family of compounds, being altered and updated as new biosynthetic methods develop and new results emerge. After nearly 100 years of such research, this review aims to provide a comprehensive overview of what is currently known about the biosynthesis of this important family, amalgamating the early chemical and biosynthetic studies with the more recent genetics-based advances and comparative bioinformatics.

Structural analysis of an anthrol reductase inspires enantioselective synthesis of enantiopure hydroxycycloketones and β-halohydrins

Asymmetric reduction of prochiral ketones, particularly, reductive desymmetrization of 2,2-disubstituted prochiral 1,3-cyclodiketones to produce enantiopure chiral alcohols is challenging. Herein, an anthrol reductase CbAR with the ability to accommodate diverse bulky substrates, like emodin, for asymmetric reduction is identified. We firstly solve crystal structures of CbAR and CbAR-Emodin complex. It reveals that Tyr210 is critical for emodin recognition and binding, as it forms a hydrogen-bond interaction with His162 and π-π stacking interactions with emodin. This ensures the correct orientation for the stereoselectivity. Then, through structure-guided engineering, variant CbAR-H162F can convert various 2,2-disubstituted 1,3-cyclodiketones and α-haloacetophenones to optically pure (2S, 3S)-ketols and (R)-β-halohydrins, respectively. More importantly, their stereoselectivity mechanisms are also well explained by the respective crystal structures of CbAR-H162F-substrate complex. Therefore, this study demonstrates that an in-depth understanding of catalytic mechanism is valuable for exploiting the promiscuity of anthrol reductases to prepare diverse enantiopure chiral alcohols.

Dothideomins A-D, Antibacterial Polycyclic Bisanthraquinones from the Endophytic Fungus Dothideomycetes sp. BMC-101

Four new bisanthraquinones, dothideomins A-D (1-4), were identified from Dothideomycetes sp. BMC-101, an endophytic fungus isolated from Magnolia grandiflora L. leaves. Their chemical structures were established by NMR analysis, single-crystal X-ray crystallography, and ECD analysis. Dothideomins A-D (1-4) were characterized by an unusual 6/6/6/5/6/3/6/6 octocyclic scaffold (1 and 2) and a 6/6/6/5/6/6/6 heptacyclic scaffold (3 and 4), respectively. All compounds, especially 1 and 3, exhibited potent antibacterial activity with MIC values ranging from 0.4 to 0.8 μg/mL.

Halogenase-Targeted Genome Mining Leads to the Discovery of (±) Pestalachlorides A1a, A2a, and Their Atropisomers

Genome mining has become an important tool for discovering new natural products and identifying the cryptic biosynthesis gene clusters. Here, we utilized the flavin-dependent halogenase GedL as the probe in combination with characteristic halogen isotope patterns to mine new halogenated secondary metabolites from our in-house fungal database. As a result, two pairs of atropisomers, pestalachlorides A1a (1a)/A1b (1b) and A2a (2a)/A2b (2b), along with known compounds pestalachloride A (3) and SB87-H (4), were identified from Pestalotiopsis rhododendri LF-19-12. A plausible biosynthetic assembly line for pestalachlorides involving a putative free-standing phenol flavin-dependent halogenase was proposed based on bioinformatics analysis. Pestalachlorides exhibited antibacterial activity against sensitive and drug-resistant S. aureus and E. faecium with MIC values ranging from 4 μg/mL to 32 μg/mL. This study indicates that halogenase-targeted genome mining is an efficient strategy for discovering halogenated compounds and their corresponding halogenases.

Biochemical and structural insights of multifunctional flavin-dependent monooxygenase FlsO1-catalyzed unexpected xanthone formation

Xanthone-containing natural products display diverse pharmacological properties. The biosynthetic mechanisms of the xanthone formation have not been well documented. Here we show that the flavoprotein monooxygenase FlsO1 in the biosynthesis of fluostatins not only functionally compensates for the monooxygenase FlsO2 in converting prejadomycin to dehydrorabelomycin, but also unexpectedly converts prejadomycin to xanthone-containing products by catalyzing three successive oxidations including hydroxylation, epoxidation and Baeyer-Villiger oxidation. We also provide biochemical evidence to support the physiological role of FlsO1 as the benzo[b]-fluorene C5-hydrolase by using nenestatin C as a substrate mimic. Finally, we resolve the crystal structure of FlsO1 in complex with the cofactor flavin adenine dinucleotide close to the “in” conformation to enable the construction of reactive substrate-docking models to understand the basis of a single enzyme-catalyzed multiple oxidations. This study highlights a mechanistic perspective for the enzymatic xanthone formation in actinomycetes and sets an example for the versatile functions of flavoproteins.

The biosynthesis of xanthones has not been well documented. Here, the authors report that monooxygenase FlsO1 catalyzes three successive oxidations – hydroxylation, epoxidation and Baeyer–Villiger oxidation—to form the xanthone scaffold in actinomycetes.

Chemical diversity, medicinal potentialities, biosynthesis, and pharmacokinetics of anthraquinones and their congeners derived from marine fungi: a comprehensive update

Marine fungi receive excessive attention as prolific producers of structurally unique secondary metabolites, offering promising potential as substitutes or conjugates for current therapeutics, whereas existing research has only scratched the surface in terms of secondary metabolite diversity and potential industrial applications as only a small share of bioactive natural products have been identified from marine-derived fungi thus far. Anthraquinones derived from filamentous fungi are a distinct large group of polyketides containing compounds which feature a common 9,10-dioxoanthracene core, while their derivatives are generated through enzymatic reactions such as methylation, oxidation, or dimerization to produce a large variety of anthraquinone derivatives. A considerable number of reported anthraquinones and their derivatives have shown significant biological activities as well as highly economical, commercial, and biomedical potentialities such as anticancer, antimicrobial, antioxidant, and anti-inflammatory activities. Accordingly, and in this context, this review comprehensively covers the state-of-art over 20 years of about 208 structurally diverse anthraquinones and their derivatives isolated from different species of marine-derived fungal genera along with their reported bioactivity wherever applicable. Also, in this manuscript, we will present in brief recent insights centred on their experimentally proved biosynthetic routes. Moreover, all reported compounds were extensively investigated for their in-silico drug-likeness and pharmacokinetics properties which intriguingly highlighted a list of 20 anthraquinone-containing compounds that could be considered as potential drug lead scaffolds.

Comparative genome and transcriptome of Rhodococcus pyridinivorans GF3 for analyzing the detoxification mechanism of anthraquinone compounds

Anthraquinone compounds (ACs) could be efficiently degraded and detoxified by bacteria. However, the molecular mechanism of bacterial degradation and detoxification of ACs remains unclear. In this study, 1-aminoanthraquinone-2-sulfonate (ASA-2) was used as a model anthraquinone compound, the response mechanism of Rhodococcus pyridinivorans GF3 to ASA-2 using genomics and transcriptomics techniques was investigated. Comparative genome analysis showed that strain GF3 owned an especial gene region (Genes 1337-1399) containing the genes encoding cytochrome P450, monooxygenase, dehydrogenase and oxidoreductase, which did not commonly exist in Rhodococcus genus. The amino acid sequences of these genes were similar to those of the cleavage enzymes of anthraquinone ring in Aspergillus genus. Moreover, the transcriptions of Genes 1392-1394 (cytochrome 450 gene cluster) displayed 1.8-3.1-fold up-regulation under ASA-2 exposure. Meanwhile, as an intermediate product of ASA-2, catechol was degraded to acetyl-CoA, succinyl-CoA and pyruvate, resulting in the enhanced tricarboxylic acid cycle and ATP generation. This process also promoted the up-regulation of the genes encoding resistance, efflux, transporter and anti-oxidation pressure proteins, which were involved in resisting ASA-2 and maintaining the homeostasis of cells. These results provided us with a further understanding of the molecular mechanism of degradation and detoxification of ACs.

Branching and converging pathways in fungal natural product biosynthesis

In nature, organic molecules with great structural diversity and complexity are synthesized by utilizing a relatively small number of starting materials. A synthetic strategy adopted by nature is pathway branching, in which a common biosynthetic intermediate is transformed into different end products. A natural product can also be synthesized by the fusion of two or more precursors generated from separate metabolic pathways. This review article summarizes several representative branching and converging pathways in fungal natural product biosynthesis to illuminate how fungi are capable of synthesizing a diverse array of natural products.

Bienzyme-Catalytic and Dioxygenation-Mediated Anthraquinone Ring Opening

The C-10-C-4a bond cleavage of anthraquinone is believed to be a crucial step in fungal seco-anthraquinone biosynthesis and has long been proposed as a classic Baeyer-Villiger oxidation. Nonetheless, genetic, enzymatic, and chemical information on ring opening remains elusive. Here, a revised questin ring-opening mechanism was elucidated by in vivo gene disruption, in vitro enzymatic analysis, and 18O chasing experiments. It has been confirmed that the reductase GedF is responsible for the reduction of the keto group at C-10 in questin to a hydroxyl group with the aid of NADPH. The C-10-C-4a bond of the resultant questin hydroquinone is subsequently cleaved by the atypical cofactor-free dioxygenase GedK, giving rise to desmethylsulochrin. This proposed bienzyme-catalytic and dioxygenation-mediated anthraquinone ring-opening reaction shows universality.

Heterologous Biosynthesis of Tetrahydroxanthone Dimers: Determination of Key Factors for Selective or Divergent Synthesis

Tetrahydroxanthone dimers are fungal products, among which secalonic acid D (1) is one of the most studied compounds because of its potent biological activity. Because the biosynthetic gene cluster of 1 has been previously identified, we sought to heterologously produce 1 in Aspergillus oryzae by expressing the relevant biosynthetic genes. However, our initial attempt of the total biosynthesis of 1 failed; instead, it produced four isomers of 1 due to the activity of an endogenous enzyme of A. oryzae. Subsequent overexpression of the Baeyer-Villiger monooxygenase, AacuH, which competes with the endogenous enzyme, altered the product profile and successfully generated 1. Characterization of the key biosynthetic enzymes revealed the surprising substrate promiscuity of the dimerizing enzyme, AacuE, and indicated that efficient synthesis of 1 requires highly selective preparation of the tetrahydroxanthone monomer, which is apparently controlled by AacuH. This study facilitates engineered biosynthesis of tetrahydroxanthone dimers both in a selective and divergent manner.

Structure revision of cryptosporioptides and determination of the genetic basis for dimeric xanthone biosynthesis in fungi

Three novel dimeric xanthones, cryptosporioptides A-C were isolated from Cryptosporiopsis sp. 8999 and their structures elucidated. Methylation of cryptosporioptide A gave a methyl ester with identical NMR data to cryptosporioptide, a compound previously reported to have been isolated from the same fungus. However, HRMS analysis revealed that cryptosporioptide is a symmetrical dimer, not a monomer as previously proposed, and the revised structure was elucidated by extensive NMR analysis. The genome of Cryptosporiopsis sp. 8999 was sequenced and the dimeric xanthone (dmx) biosynthetic gene cluster responsible for the production of the cryptosporioptides was identified. Gene disruption experiments identified a gene (dmxR5) encoding a cytochrome P450 oxygenase as being responsible for the dimerisation step late in the biosynthetic pathway. Disruption of dmxR5 led to the isolation of novel monomeric xanthones. Cryptosporioptide B and C feature an unusual ethylmalonate subunit: a hrPKS and acyl CoA carboxylase are responsible for its formation. Bioinformatic analysis of the genomes of several fungi producing related xanthones, e.g. the widely occurring ergochromes, and related metabolites allows detailed annotation of the biosynthetic genes, and a rational overall biosynthetic scheme for the production of fungal dimeric xanthones to be proposed.

Characterisation of the biosynthetic pathway to agnestins A and B reveals the reductive route to chrysophanol in fungi

Two new dihydroxy-xanthone metabolites, agnestins A and B, were isolated from Paecilomyces variotii along with a number of related benzophenones and xanthones including monodictyphenone. The structures were elucidated by NMR analyses and X-ray crystallography. The agnestin (agn) biosynthetic gene cluster was identified and targeted gene disruptions of the PKS, Baeyer-Villiger monooxygenase, and other oxido-reductase genes revealed new details of fungal xanthone biosynthesis. In particular, identification of a reductase responsible for in vivo anthraquinone to anthrol conversion confirms a previously postulated essential step in aromatic deoxygenation of anthraquinones, e.g. emodin to chrysophanol.

Classic fungal natural products in the genomic age: the molecular legacy of Harold Raistrick

Covering: 1893 to 2017Harold Raistrick was involved in the discovery of many of the most important classes of fungal metabolites during the 20th century. This review focusses on how these discoveries led to developments in isotopic labelling, biomimetic chemistry and the discovery, analysis and exploitation of biosynthetic gene clusters for major classes of fungal metabolites including: alternariol; geodin and metabolites of the emodin pathway; maleidrides; citrinin and the azaphilones; dehydrocurvularin; mycophenolic acid; and the tropolones. Key recent advances in the molecular understanding of these important pathways, including the discovery of biosynthetic gene clusters, the investigation of the molecular and chemical aspects of key biosynthetic steps, and the reengineering of key components of the pathways are reviewed and compared. Finally, discussion of key relationships between metabolites and pathways and the most important recent advances and opportunities for future research directions are given.

Comparative genomics reveals high biological diversity and specific adaptations in the industrially and medically important fungal genus Aspergillus

BACKGROUND

The fungal genus Aspergillus is of critical importance to humankind. Species include those with industrial applications, important pathogens of humans, animals and crops, a source of potent carcinogenic contaminants of food, and an important genetic model. The genome sequences of eight aspergilli have already been explored to investigate aspects of fungal biology, raising questions about evolution and specialization within this genus.

RESULTS

We have generated genome sequences for ten novel, highly diverse Aspergillus species and compared these in detail to sister and more distant genera. Comparative studies of key aspects of fungal biology, including primary and secondary metabolism, stress response, biomass degradation, and signal transduction, revealed both conservation and diversity among the species. Observed genomic differences were validated with experimental studies. This revealed several highlights, such as the potential for sex in asexual species, organic acid production genes being a key feature of black aspergilli, alternative approaches for degrading plant biomass, and indications for the genetic basis of stress response. A genome-wide phylogenetic analysis demonstrated in detail the relationship of the newly genome sequenced species with other aspergilli.

CONCLUSIONS

Many aspects of biological differences between fungal species cannot be explained by current knowledge obtained from genome sequences. The comparative genomics and experimental study, presented here, allows for the first time a genus-wide view of the biological diversity of the aspergilli and in many, but not all, cases linked genome differences to phenotype. Insights gained could be exploited for biotechnological and medical applications of fungi.

Three Pairs of New Isopentenyl Dibenzo[b,e]oxepinone Enantiomers from Talaromyces flavus, a Wetland Soil-Derived Fungus

Three pairs of new isopentenyl dibenzo[b,e]oxepinone enantiomers, (+)-(5S)-arugosin K (1a), (−)-(5R)-arugosin K (1b), (+)-(5S)-arugosin L (2a), (−)-(5R)-arugosin L (2b), (+)-(5S)-arugosin M (3a), (−)-(5R)-arugosin M (3b), and a new isopentenyl dibenzo[b,e]oxepinone, arugosin N (4), were isolated from a wetland soil-derived fungus Talaromyces flavus, along with two known biosynthetically-related compounds 5 and 6. Among them, arugosin N (4) and 1,6,10-trihydroxy-8-methyl-2-(3-methyl-2-butenyl)-dibenz[b,e]oxepin-11(6H)-one (CAS: 160585-91-1, 5) were obtained as the tautomeric mixtures. The structures of isolated compounds were determined by detailed spectroscopic analysis. In addition, the absolute configurations of these three pairs of new enantiomers were determined by quantum chemical ECD calculations.

Elucidation of cladofulvin biosynthesis reveals a cytochrome P450 monooxygenase required for anthraquinone dimerization

Anthraquinones are a large family of secondary metabolites (SMs) that are extensively studied for their diverse biological activities. These activities are determined by functional group decorations and the formation of dimers from anthraquinone monomers. Despite their numerous medicinal qualities, very few anthraquinone biosynthetic pathways have been elucidated so far, including the enzymatic dimerization steps. In this study, we report the elucidation of the biosynthesis of cladofulvin, an asymmetrical homodimer of nataloe-emodin produced by the fungus Cladosporium fulvum A gene cluster of 10 genes controls cladofulvin biosynthesis, which begins with the production of atrochrysone carboxylic acid by the polyketide synthase ClaG and the β-lactamase ClaF. This compound is decarboxylated by ClaH to yield emodin, which is then converted to chrysophanol hydroquinone by the reductase ClaC and the dehydratase ClaB. We show that the predicted cytochrome P450 ClaM catalyzes the dimerization of nataloe-emodin to cladofulvin. Remarkably, such dimerization dramatically increases nataloe-emodin cytotoxicity against mammalian cell lines. These findings shed light on the enzymatic mechanisms involved in anthraquinone dimerization. Future characterization of the ClaM enzyme should facilitate engineering the biosynthesis of novel, potent, dimeric anthraquinones and structurally related compound families.

Transcriptomic and metabolomic profiling of ionic liquid stimuli unveils enhanced secondary metabolism in Aspergillus nidulans

BACKGROUND

The inherent potential of filamentous fungi, especially of Ascomycota, for producing diverse bioactive metabolites remains largely silent under standard laboratory culture conditions. Innumerable strategies have been described to trigger their production, one of the simplest being manipulation of the growth media composition. Supplementing media with ionic liquids surprisingly enhanced the diversity of extracellular metabolites generated by penicillia. This finding led us to evaluate the impact of ionic liquids' stimuli on the fungal metabolism in Aspergillus nidulans and how it reflects on the biosynthesis of secondary metabolites (SMs).

RESULTS

Whole transcriptional profiling showed that exposure to 0.7 M cholinium chloride or 1-ethyl-3-methylimidazolium chloride dramatically affected expression of genes encoding both primary and secondary metabolism. Both ionic liquids apparently induced stress responses and detoxification mechanisms but response profiles to each stimulus were unique. Primary metabolism was up-regulated by choline, but down-regulated by 1-ethyl-3-methylimidazolium chloride; both stimulated production of acetyl-CoA (key precursor to numerous SMs) and non proteinogenic amino acids (building blocks of bioactive classes of SMs). In total, twenty one of the sixty six described backbone genes underwent up-regulation. Accordingly, differential analysis of the fungal metabolome showed that supplementing growth media with ionic liquids resulted in ca. 40 differentially accumulated ion masses compared to control conditions. In particular, it stimulated production of monodictyphenone and orsellinic acid, otherwise cryptic. Expression levels of genes encoding corresponding polyketide biosynthetic enzymes (i.e. backbone genes) increased compared to control conditions. The corresponding metabolite extracts showed increased cell polarity modulation potential in an ex vivo whole tissue assay (The lial Live Targeted Epithelia; theLiTE™).

CONCLUSIONS

Ionic liquids, a diverse class of chemicals composed solely of ions, can provide an unexpected means to further resolve the diversity of natural compounds, guiding discovery of fungal metabolites with clinical potential.

Ionic Liquids as Unforeseen Assets to Fight Life-Threatening Mycotic Diseases

Ionic liquids discovery has celebrated 100 years. They consist solely of ions, one of which is typically organic and asymmetrical. Remarkable physical and chemical properties stirred their use as alternative solvents in many chemical processes. The recent demonstration of their occurrence in nature might boost their interest in biological sciences. In the search of mechanistic understandings of ionic liquids’ ecotoxicological impacts in fungi, we have analyzed the proteome, transcriptome, and metabolome responses to this chemical stress. Data illuminated new hypotheses that altered our research path – exploit ionic liquids as tools for the discovery of pathways and metabolites that may impact fungal development and pathogenicity. As we get closer to solve the primary effects of each ionic liquid family and their specific gene targets, the vision of developing antifungal ionic liquids and/or materials, by taking advantage of elegant progresses in this field, might become a reality. Task-designed formulations may improve the performance of conventional antifungal drugs, build functional coatings for reducing allergens production, or aid in the recovery of antifungal plant polymers. The frontier research in this cross-disciplinary field may provide us unforeseen means to address the global concern of mycotic diseases. Pathogenic and opportunistic fungi are responsible for numerous infections, killing annually nearly 1.5 million immunocompromised individuals worldwide, a similar rate to malaria or tuberculosis. This perspective will review our major findings and current hypotheses, contextualizing how they might bring us closer to efficient strategies to prevent and fight mycotic diseases.

Evolution of Chemical Diversity in a Group of Non-Reduced Polyketide Gene Clusters: Using Phylogenetics to Inform the Search for Novel Fungal Natural Products

Fungal polyketides are a diverse class of natural products, or secondary metabolites (SMs), with a wide range of bioactivities often associated with toxicity. Here, we focus on a group of non-reducing polyketide synthases (NR-PKSs) in the fungal phylum Ascomycota that lack a thioesterase domain for product release, group V. Although widespread in ascomycete taxa, this group of NR-PKSs is notably absent in the mycotoxigenic genus Fusarium and, surprisingly, found in genera not known for their secondary metabolite production (e.g., the mycorrhizal genus Oidiodendron, the powdery mildew genus Blumeria, and the causative agent of white-nose syndrome in bats, Pseudogymnoascus destructans). This group of NR-PKSs, in association with the other enzymes encoded by their gene clusters, produces a variety of different chemical classes including naphthacenediones, anthraquinones, benzophenones, grisandienes, and diphenyl ethers. We discuss the modification of and transitions between these chemical classes, the requisite enzymes, and the evolution of the SM gene clusters that encode them. Integrating this information, we predict the likely products of related but uncharacterized SM clusters, and we speculate upon the utility of these classes of SMs as virulence factors or chemical defenses to various plant, animal, and insect pathogens, as well as mutualistic fungi.

Redundant synthesis of a conidial polyketide by two distinct secondary metabolite clusters in Aspergillus fumigatus

Filamentous fungi are renowned for the production of bioactive secondary metabolites. Typically, one distinct metabolite is generated from a specific secondary metabolite cluster. Here, we characterize the newly described trypacidin (tpc) cluster in the opportunistic human pathogen Aspergillus fumigatus. We find that this cluster as well as the previously characterized endocrocin (enc) cluster both contribute to the production of the spore metabolite endocrocin. Whereas trypacidin is eliminated when only tpc cluster genes are deleted, endocrocin production is only eliminated when both the tpc and enc non-reducing polyketide synthase-encoding genes, tpcC and encA, respectively, are deleted. EncC, an anthrone oxidase, converts the product released from EncA to endocrocin as a final product. In contrast, endocrocin synthesis by the tpc cluster likely results from incomplete catalysis by TpcK (a putative decarboxylase), as its deletion results in a nearly 10-fold increase in endocrocin production. We suggest endocrocin is likely a shunt product in all related non-reducing polyketide synthase clusters containing homologues of TpcK and TpcL (a putative anthrone oxidase), e.g. geodin and monodictyphenone. This finding represents an unusual example of two physically discrete secondary metabolite clusters generating the same natural product in one fungal species by distinct routes.

New Insights into the Conversion of Versicolorin A in the Biosynthesis of Aflatoxin B1

A crucial and enigmatic step in the complex biosynthesis of aflatoxin B1 is the oxidative rearrangement of versicolorin A to demethylsterigmatocystin. This step is thought to proceed by an oxidation-reduction-oxidation sequence, in which the NADPH-dependent oxidoreductase AflM catalyzes the enclosed reduction step. AflM from Aspergillus parasiticus, after heterologous production in E. coli and purification, however, catalyzed the reduction of the hydroquinoid form of the starting compound versicolorin A (25% conversion) to a so far unknown product of aflatoxin biosynthesis. The asymmetric reduction of emodin hydroquinone to (R)-3,8,9,10-tetrahydroxy-6-methyl-3,4-dihydroanthracen-1(2H)-one (up to 82% for AflM) has also been observed in previous studies using MdpC from Aspergillus nidulans (monodictyphenone biosynthetic gene cluster). The first (nonenzymatic) reduction of emodin to emodin hydroquinone, for example with sodium dithionite, is obligatory for the enzymatic reduction by AflM or MdpC. These results imply an unprecedented role of AflM in the complex enzymatic network of aflatoxin biosynthesis.

Xanthone dimers: a compound family which is both common and privileged

This Review seeks to systematically describe, for the first time, the widely-occurring and highly biologically-active family of dimeric xanthones from nature, encompassing several aspects of their biosynthesis, occurrence, contrasting structural features and wide variety of bioactivities.

Heterologous reconstitution of the intact geodin gene cluster in Aspergillus nidulans through a simple and versatile PCR based approach

Fungal natural products are a rich resource for bioactive molecules. To fully exploit this potential it is necessary to link genes to metabolites. Genetic information for numerous putative biosynthetic pathways has become available in recent years through genome sequencing. However, the lack of solid methodology for genetic manipulation of most species severely hampers pathway characterization. Here we present a simple PCR based approach for heterologous reconstitution of intact gene clusters. Specifically, the putative gene cluster responsible for geodin production from Aspergillus terreus was transferred in a two step procedure to an expression platform in A. nidulans. The individual cluster fragments were generated by PCR and assembled via efficient USER fusion prior to transformation and integration via re-iterative gene targeting. A total of 13 open reading frames contained in 25 kb of DNA were successfully transferred between the two species enabling geodin synthesis in A. nidulans. Subsequently, functions of three genes in the cluster were validated by genetic and chemical analyses. Specifically, ATEG_08451 (gedC) encodes a polyketide synthase, ATEG_08453 (gedR) encodes a transcription factor responsible for activation of the geodin gene cluster and ATEG_08460 (gedL) encodes a halogenase that catalyzes conversion of sulochrin to dihydrogeodin. We expect that our approach for transferring intact biosynthetic pathways to a fungus with a well developed genetic toolbox will be instrumental in characterizing the many exciting pathways for secondary metabolite production that are currently being uncovered by the fungal genome sequencing projects.

Accurate prediction of secondary metabolite gene clusters in filamentous fungi

Biosynthetic pathways of secondary metabolites from fungi are currently subject to an intense effort to elucidate the genetic basis for these compounds due to their large potential within pharmaceutics and synthetic biochemistry. The preferred method is methodical gene deletions to identify supporting enzymes for key synthases one cluster at a time. In this study, we design and apply a DNA expression array for Aspergillus nidulans in combination with legacy data to form a comprehensive gene expression compendium. We apply a guilt-by-association-based analysis to predict the extent of the biosynthetic clusters for the 58 synthases active in our set of experimental conditions. A comparison with legacy data shows the method to be accurate in 13 of 16 known clusters and nearly accurate for the remaining 3 clusters. Furthermore, we apply a data clustering approach, which identifies cross-chemistry between physically separate gene clusters (superclusters), and validate this both with legacy data and experimentally by prediction and verification of a supercluster consisting of the synthase AN1242 and the prenyltransferase AN11080, as well as identification of the product compound nidulanin A. We have used A. nidulans for our method development and validation due to the wealth of available biochemical data, but the method can be applied to any fungus with a sequenced and assembled genome, thus supporting further secondary metabolite pathway elucidation in the fungal kingdom.