[Efficient production of polyketide products in Streptomyces hosts - A review]

Polyketides represent an important class of structurally and functionally diverse secondary metabolites with high economic value. Among bacteria, Streptomycetes are the main producers of polyketides. To enhance polyketide production in Streptomyces hosts, rational metabolic engineering approaches have been applied, such as overexpressing rate-limiting enzymes, or transcriptional activator, increasing the supply of precursor, removing feedback inhibition by end products and heterologous expression of polyketide biosynthetic gene clusters. In this review, we discuss examples of successful metabolic engineering strategies used to improve polyketide production in Streptomycetes. Meanwhile, we also address future prospective, emerging synthetic biology strategies to dynamically adjust the metabolic fluxes of pathways related to polyketide synthesis.

Recent advances in the biosynthesis of unusual polyketide synthase substrates

This highlight provides an overview of recent advances in understanding the diversity of polyketide synthase (PKS) substrate building blocks. Substrates functioning as starter units and extender units contribute significantly to the chemical complexity and structural diversity exhibited by this class of natural products. This article complements and extends upon the current comprehensive reviews that have been published on these two topics (Moore and Hertweck, Nat. Prod. Rep., 2002, 19, 70; Chan et al., Nat. Prod. Rep., 2009, 1, 90; Wilson and Moore, Nat. Prod. Rep., 2012, 29, 72).

The Assembly Line Enzymology of Polyketide Biosynthesis

Polyketides are a structurally and functionally diverse family of bioactive natural products that have found widespread application as pharmaceuticals, agrochemicals, and veterinary medicines. In bacteria complex polyketides are biosynthesized by giant multifunctional megaenzymes, termed modular polyketide synthases (PKSs), which construct their products in a highly coordinated assembly line-like fashion from a pool of simple precursor substrates. Not only is the multifaceted enzymology of PKSs a fascinating target for study, but it also presents considerable opportunities for the reengineering of these systems affording access to functionally optimized unnatural natural products. Here we provide an introductory primer to modular polyketide synthase structure and function, and highlight recent advances in the characterization and exploitation of these systems.

Identification of the Polyketide Biosynthetic Machinery for the Indolizidine Alkaloid Cyclizidine

The cyclizidine biosynthetic gene cluster was identified from Streptomyces NCIB 11649, which revealed the polyketide biosynthetic machinery for cyclizidine alkaloid biosynthesis. Both in vivo mutagenesis study and in vitro biochemical analysis provided insight into the timing and mechanism of the biosynthetic enzymes that produce cyclizidine-type indolizidine alkaloids.

Substrate Flexibility of a Mutated Acyltransferase Domain and Implications for Polyketide Biosynthesis

Polyketides are natural products frequently used for the treatment of various diseases, but their structural complexity hinders efficient derivatization. In this context, we recently introduced enzyme-directed mutasynthesis to incorporate non-native extender units into the biosynthesis of erythromycin. Modeling and mutagenesis studies led to the discovery of a variant of an acyltransferase domain in the erythromycin polyketide synthase capable of accepting a propargylated substrate. Here, we extend molecular rationalization of enzyme-substrate interactions through modeling, to investigate the incorporation of substrates with different degrees of saturation of the malonic acid side chain. This allowed the engineered biosynthesis of new erythromycin derivatives and the introduction of additional mutations into the AT domain for a further shift of the enzyme's substrate scope. Our approach yields non-native polyketide structures with functional groups that will simplify future derivatization approaches, and provides a blueprint for the engineering of AT domains to achieve efficient polyketide synthase diversification.

Production of the polyketide 6-deoxyerythronolide B in the heterologous host Bacillus subtilis

Polyketides, such as erythromycin, are complex natural products with diverse therapeutic applications. They are synthesized by multi-modular megaenzymes, so-called polyketide synthases (PKSs). The macrolide core of erythromycin, 6-deoxyerythronolide B (6dEB), is produced by the deoxyerythronolide B synthase (DEBS) that consists of three proteins each with a size of 330–370 kDa. We cloned and investigated the expression of the corresponding gene cluster from Saccharopolyspora erythraea, which comprises more than 30 kb, in Bacillus subtilis. It is shown that the DEBS genes are functionally expressed in B. subtilis when the native eryAI–III operon was separated into three individual expression cassettes with optimized ribosomal binding sites. A synthesis of 6dEB could be detected by using the acetoin-inducible acoA promoter and a fed-batch simulating EnBase-cultivation strategy. B. subtilis was capable of the secretion of 6dEB into the medium. In order to improve the 6dEB production, several genomic modifications of this production strain were tested. This included the knockout of the native secondary metabolite clusters of B. subtilis for the synthesis of surfactin (26 kb), bacillaene (76 kb), and plipastatin (38 kb). It is revealed that the deletion of the prpBD operon, responsible for propionyl-CoA utilization, resulted in a significant increase of the 6dEB product yield when exogenous propionate is provided. Although the presented B. subtilis 6dEB production strain is not competitive with established Escherichia coli 6dEB production strains, the results of this study indicate that B. subtilis is a suitable heterologous host for the secretory production of a complex polyketide.

A study on the biosynthesis of hygrophorone B(12) in the mushroom Hygrophorus abieticola reveals an unexpected labelling pattern in the cyclopentenone moiety

The hitherto unknown natural formation of hygrophorones, antibacterial and antifungal cyclopentenone derivatives from mushrooms, was investigated for hygrophorone B(12) in Hygrophorus abieticola Krieglst. ex Gröger & Bresinsky by feeding experiments in the field using (13)C labelled samples of D-glucose and sodium acetate. The incorporation of (13)C isotopes was extensively studied using 1D and 2D NMR spectroscopy as well as ESI-HRMS analyses. In the experiment with [U-(13)C6]-glucose, six different (13)C2 labelled isotopomers were observed in the 2D INADEQUATE spectrum due to incorporation of [1,2-(13)C2]-acetyl-CoA. This labelling pattern demonstrated that hygrophorone B(12) is derived from a fatty acid-polyketide route instead of a 1,4-α-D-glucan derived anhydrofructose pathway. The experiment with [2-(13)C]-acetate revealed an unexpected incorporation pattern in the cyclopentenone functionality of hygrophorone B(12). Four single-labelled isotopomers, in particular [1-(13)C]-, [2-(13)C]-, [3-(13)C]-, and [4-(13)C]-hygrophorone B(12), were detected that showed only half enrichment in comparison to the respective labelled alkyl side chain carbons. This labelling pattern indicates the formation of a symmetrical intermediate during hygrophorone B(12) biosynthesis. Based on these observations, a biogenetic route via a 4-oxo fatty acid and a chrysotrione B homologue is discussed.

Efficient Biosynthesis of Fungal Polyketides Containing the Dioxabicyclo-octane Ring System

Aurovertins are fungal polyketides that exhibit potent inhibition of adenosine triphosphate synthase. Aurovertins contain a 2,6-dioxabicyclo[3.2.1]octane ring that is proposed to be derived from a polyene precursor through regioselective oxidations and epoxide openings. In this study, we identified only four enzymes required to produce aurovertin E. The core polyketide synthase produces a polyene α-pyrone. Following pyrone O-methylation by a methyltransferase, a flavin-dependent mono-oxygenase and an epoxide hydrolase can iteratively transform the terminal triene portion of the precursor into the dioxabicyclo[3.2.1]octane scaffold. We demonstrate that a tetrahydrofuranyl polyene is the first stable intermediate in the transformation, which can undergo epoxidation and anti-Baldwin 6-endo-tet ring opening to yield the cyclic ether product. Our results further demonstrate the highly concise and efficient ways in which fungal biosynthetic pathways can generate complex natural product scaffolds.

Computational approaches to natural product discovery

Starting with the earliest Streptomyces genome sequences, the promise of natural product genome mining has been captivating: genomics and bioinformatics would transform compound discovery from an ad hoc pursuit to a high-throughput endeavor. Until recently, however, genome mining has advanced natural product discovery only modestly. Here, we argue that the development of algorithms to mine the continuously increasing amounts of (meta)genomic data will enable the promise of genome mining to be realized. We review computational strategies that have been developed to identify biosynthetic gene clusters in genome sequences and predict the chemical structures of their products. We then discuss networking strategies that can systematize large volumes of genetic and chemical data and connect genomic information to metabolomic and phenotypic data. Finally, we provide a vision of what natural product discovery might look like in the future, specifically considering longstanding questions in microbial ecology regarding the roles of metabolites in interspecies interactions.

Conformation-activity relationships of polyketide natural products

Polyketides represent an important class of secondary metabolites that interact with biological targets connected to a variety of disease-associated pathways. Remarkably, nature's assembly lines, polyketide synthases, manufacture these privileged structures through a combinatorial mixture of just a few structural units. This review highlights the role of these structural elements in shaping a polyketide's conformational preferences, the use of computer-based molecular modeling and solution NMR studies in the identification of low-energy conformers, and the importance of conformational analogues in probing the bound conformation. In particular, this review covers several examples wherein conformational analysis complements classic structure-activity relationships in the design of biologically active natural product analogues.

Lindgomycin, an Unusual Antibiotic Polyketide from a Marine Fungus of the Lindgomycetaceae

An unusual polyketide with a new carbon skeleton, lindgomycin (1), and the recently described ascosetin (2) were extracted from mycelia and culture broth of different Lindgomycetaceae strains, which were isolated from a sponge of the Kiel Fjord in the Baltic Sea (Germany) and from the Antarctic. Their structures were established by spectroscopic means. In the new polyketide, two distinct domains, a bicyclic hydrocarbon and a tetramic acid, are connected by a bridging carbonyl. The tetramic acid substructure of compound 1 was proved to possess a unique 5-benzylpyrrolidine-2,4-dione unit. The combination of 5-benzylpyrrolidine-2,4-dione of compound 1 in its tetramic acid half and 3-methylbut-3-enoic acid pendant in its decalin half allow the assignment of a new carbon skeleton. The new compound 1 and ascosetin showed antibiotic activities with IC50 value of 5.1 (±0.2) µM and 3.2 (±0.4) μM, respectively, against methicillin-resistant Staphylococcus aureus.

The Biogeography of Putative Microbial Antibiotic Production

Understanding patterns in the distribution and abundance of functional traits across a landscape is of fundamental importance to ecology. Mapping these distributions is particularly challenging for species-rich groups with sparse trait measurement coverage, such as flowering plants, insects, and microorganisms. Here, we use likelihood-based character reconstruction to infer and analyze the spatial distribution of unmeasured traits. We apply this framework to a microbial dataset comprised of 11,732 ketosynthase alpha gene sequences extracted from 144 soil samples from three continents to document the spatial distribution of putative microbial polyketide antibiotic production. Antibiotic production is a key competitive strategy for soil microbial survival and performance. Additionally, novel antibiotic discovery is highly relevant to human health, making natural antibiotic production by soil microorganisms a major target for bioprospecting. Our comparison of trait-based biogeographical patterns to patterns based on taxonomy and phylogeny is relevant to our basic understanding of microbial biogeography as well as the pressing need for new antibiotics.

The biosynthesis, composition and assembly of the outer pollen wall: A tough case to crack

The formation of the durable outer pollen wall, largely composed of sporopollenin, is essential for the protection of the male gametophyte and plant reproduction. Despite its apparent strict conservation amongst land plants, the composition of sporopollenin and the biosynthetic pathway(s) yielding this recalcitrant biopolymer remain elusive. Recent molecular genetic studies in Arabidopsis thaliana (Arabidopsis) and rice have, however, identified key genes involved in sporopollenin formation, allowing a better understanding of the biochemistry and cell biology underlying sporopollenin biosynthesis and pollen wall development. Herein, current knowledge of the biochemical composition of the outer pollen wall is reviewed, with an emphasis on enzymes with characterized biochemical activities in sporopollenin and pollen coat biosynthesis. The tapetum, which forms the innermost sporophytic cell layer of the anther and envelops developing pollen, plays an essential role in sporopollenin and pollen coat formation. Recent studies show that several tapetum-expressed genes encode enzymes that metabolize fatty acid derived compounds to form putative sporopollenin precursors, including tetraketides derived from fatty acyl-CoA starter molecules, but analysis of mutants defective in pollen wall development indicate that other components are also incorporated into sporopollenin. Also highlighted are the many uncertainties remaining in the development of a sporopollenin-fortified pollen wall, particularly in relation to the mechanisms of sporopollenin precursor transport and assembly into the patterned form of the pollen wall. A working model for sporopollenin biosynthesis is proposed based on the data obtained largely from studies of Arabidopsis, and future challenges to complete our understanding of pollen wall biology are outlined.

Expanding our understanding of sequence-function relationships of type II polyketide biosynthetic gene clusters: bioinformatics-guided identification of Frankiamicin A from Frankia sp. EAN1pec

A large and rapidly increasing number of unstudied “orphan” natural product biosynthetic gene clusters are being uncovered in sequenced microbial genomes. An important goal of modern natural products research is to be able to accurately predict natural product structures and biosynthetic pathways from these gene cluster sequences. This requires both development of bioinformatic methods for global analysis of these gene clusters and experimental characterization of select products produced by gene clusters with divergent sequence characteristics. Here, we conduct global bioinformatic analysis of all available type II polyketide gene cluster sequences and identify a conserved set of gene clusters with unique ketosynthase α/β sequence characteristics in the genomes of Frankia species, a group of Actinobacteria with underexploited natural product biosynthetic potential. Through LC-MS profiling of extracts from several Frankia species grown under various conditions, we identified Frankia sp. EAN1pec as producing a compound with spectral characteristics consistent with the type II polyketide produced by this gene cluster. We isolated the compound, a pentangular polyketide which we named frankiamicin A, and elucidated its structure by NMR and labeled precursor feeding. We also propose biosynthetic and regulatory pathways for frankiamicin A based on comparative genomic analysis and literature precedent, and conduct bioactivity assays of the compound. Our findings provide new information linking this set of Frankia gene clusters with the compound they produce, and our approach has implications for accurate functional prediction of the many other type II polyketide clusters present in bacterial genomes.

Substrate controlled divergence in polyketide synthase catalysis

Biochemical characterization of polyketide synthases (PKSs) has relied on synthetic substrates functionalized as electrophilic esters to acylate the enzyme and initiate the catalytic cycle. In these efforts, N-acetylcysteamine thioesters have typically been employed for in vitro studies of full PKS modules as well as excised domains. However, substrate engineering approaches to control the catalytic cycle of a full PKS module harboring multiple domains remain underexplored. This study examines a series of alternatively activated native hexaketide substrates on the catalytic outcome of PikAIV, the sixth and final module of the pikromycin (Pik) pathway. We demonstrate the ability to control product formation with greater than 10:1 selectivity for either full module catalysis, leading to a 14-membered macrolactone, or direct cyclization to a 12-membered ring. This outcome was achieved through modifying the type of hexaketide ester employed, demonstrating the utility of substrate engineering in PKS functional studies and biocatalysis.

Synthetic biology approaches to fluorinated polyketides

The catalytic diversity of living systems offers a broad range of opportunities for developing new methods to produce small molecule targets such as fuels, materials, and pharmaceuticals. In addition to providing cost-effective and renewable methods for large-scale commercial processes, the exploration of the unusual chemical phenotypes found in living organisms can also enable the expansion of chemical space for discovery of novel function by combining orthogonal attributes from both synthetic and biological chemistry. In this context, we have focused on the development of new fluorine chemistry using synthetic biology approaches. While fluorine has become an important feature in compounds of synthetic origin, the scope of biological fluorine chemistry in living systems is limited, with fewer than 20 organofluorine natural products identified to date. In order to expand the diversity of biosynthetically accessible organofluorines, we have begun to develop methods for the site-selective introduction of fluorine into complex natural products by engineering biosynthetic machinery to incorporate fluorinated building blocks. To gain insight into how both enzyme active sites and metabolic pathways can be evolved to manage and select for fluorinated compounds, we have studied one of the only characterized natural hosts for organofluorine biosynthesis, the soil microbe Streptomyces cattleya. This information provides a template for designing engineered organofluorine enzymes, pathways, and hosts and has allowed us to initiate construction of enzymatic and cellular pathways for the production of fluorinated polyketides.

Decoding and reprogramming complex polyketide assembly lines: prospects for synthetic biology

Bacterial modular type I polyketide synthases (PKSs) represent giant megasynthases that produce a vast number of complex polyketides, many of which are pharmaceutically relevant. This review highlights recent advances in elucidating the mechanism of bacterial type I PKSs and associated enzymes, and outlines the ramifications of this knowledge for synthetic biology approaches to expand structural diversity. New insights into biosynthetic codes and structures of thiotemplate systems pave the way to rational bioengineering strategies. Through advances in genome mining, DNA recombination technologies, and biochemical analyses, the toolbox of non-canonical polyketide-modifying enzymes has been greatly enlarged. In addition to various chain-branching and chain-fusing enzymes, an increasing set of scaffold modifying biocatalysts is now available for synthetically hard-to-emulate reactions.

Heterologous production of curcuminoids

SUMMARY

Curcuminoids, components of the rhizome of turmeric, show several beneficial biological activities, including anticarcinogenic, antioxidant, anti-inflammatory, and antitumor activities. Despite their numerous pharmaceutically important properties, the low natural abundance of curcuminoids represents a major drawback for their use as therapeutic agents. Therefore, they represent attractive targets for heterologous production and metabolic engineering. The understanding of biosynthesis of curcuminoids in turmeric made remarkable advances in the last decade, and as a result, several efforts to produce them in heterologous organisms have been reported. The artificial biosynthetic pathway (e.g., in Escherichia coli) can start with the supplementation of the amino acid tyrosine or phenylalanine or of carboxylic acids and lead to the production of several natural curcuminoids. Unnatural carboxylic acids can also be supplemented as precursors and lead to the production of unnatural compounds with possibly novel therapeutic properties. In this paper, we review the natural conversion of curcuminoids in turmeric and their production by E. coli using an artificial biosynthetic pathway. We also explore the potential of other enzymes discovered recently or already used in other similar biosynthetic pathways, such as flavonoids and stilbenoids, to increase curcuminoid yield and activity.

Structure and Function of Macroalgal Natural Products

Since the initial discovery of marine phyco-derived secondary metabolites in the 1950s there has been a rapid increase in the description of new algal natural products. These metabolites have multiple ecological roles as well as commercial value as potential drugs or lead compounds. With the emergence of resistance to our current arsenal of drugs as well as the development of new chemotherapies for currently untreatable diseases, new compounds must be sourced. As outlined in this chapter algae produce a diverse range of chemicals many of which have potential for the treatment of human afflictions.In this chapter we outline the classes of metabolites produced by this chemically rich group of organisms as well as their respective ecological roles in the environment. Algae are found in nearly every environment on earth, with many of these organisms possessing the ability to shape the ecosystem they inhabit. With current challenges to climate stability, understanding how these important organisms interact with their environment as well as one another might afford better insight into how they respond to a changing climate.

Polyketide family of novel antibacterial 7-O-methyl-5'-hydroxy-3'-heptenoate-macrolactin from seaweed-associated Bacillus subtilis MTCC 10403

Seaweed-associated heterotrophic bacterial communities were screened to isolate potentially useful antimicrobial strains, which were characterized by phylogenetic analysis. The bacteria were screened for the presence of metabolite genes involved in natural product biosynthetic pathway, and the structural properties of secondary metabolites were correlated with the genes. Bioactivity-guided isolation of polyene antibiotic 7-O-methyl-5'-hydroxy-3'-heptenoate-macrolactin from Bacillus subtilis MTCC10403 associated with seaweed Anthophycus longifolius using mass spectrometry and extensive 2D-NMR studies was carried out. The newly isolated macrolactin compound is a bactericidal antibiotic with broad spectrum activity against human opportunistic clinical pathogens. The biosynthetic pathway of 7-O-methyl-5'-hydroxy-3'-heptenoate-macrolactin by means of a stepwise, decarboxylative condensation pathway established the PKS-assisted biosynthesis of the parent macrolactin and the side-chain 5-hydroxyhept-3-enoate moiety attached to the macrolactin ring system at C-7. Antimicrobial activity analysis combined with the results of amplifying genes encoding for polyketide synthetase and nonribosomal peptide synthetase showed that seaweed-associated bacteria had broad-spectrum antimicrobial activity. The present work may have an impact on the exploitation of macrolactins for pharmaceutical and biotechnological applications.

Evaluating nonpolar surface area and liquid chromatography/mass spectrometry response: an application for site occupancy measurements for enzyme intermediates in polyketide biosynthesis

RATIONALE

Site occupancy measurements using liquid chromatography/mass spectrometry (LC/MS) are reported throughout the literature. However, site occupancy quantification suffers from ionization bias between modified and unmodified peptides containing the active site. In this study, we explore the MS signal as a function of nonpolar surface area (NPSA) in order to better understand this bias in electrospray response. The correlation between hydrophobicity and LC/MS response was evaluated and applied to study enzyme intermediates in polyketide synthases.

METHODS

Site occupancy methods were developed to study acyltransferase activity. To further evaluate these methods, several standard peptides containing one cysteine residue were modified with alkylation reagents of increasing hydrophobicity to study the MS signal as a function of NPSA.

RESULTS

A consistent trend in MS response was observed which is dependent on the NPSA of the analyte. An optimal NPSA zone was observed for the peptides studied.

CONCLUSIONS

Nonpolar surface area can be used as metric to determine relative LC/MS response for peptides and evaluate site occupancy measurements.

One strain-many compounds (OSMAC) method for production of polyketides, azaphilones, and an isochromanone using the endophytic fungus Dothideomycete sp

Polyketides 1-6 were produced by a one strain-many compounds (OSMAC) approach using the endophytic fungus Dothideomycete sp. CRI7 as a producer. Metabolite production of the fungus Dothideomycete sp. CRI7 was sensitive to sources of potato and malt extract used for the preparation of PDB and Czapek malt media, respectively. Three hitherto unknown metabolites were obtained from the fungus CRI7 grown in PDB medium prepared from a commercial potato powder instead of fresh tubers of potato, while three others were obtained from the fungus cultivated in Czapek malt medium. Moreover, a source of malt extract used in the Czapek malt medium was found to influence metabolite production by the fungus CRI7. Structure elucidation of these compounds was achieved by analysis of spectroscopic data, as well as by single crystal X-ray analysis. Two of the compounds showed weak cytotoxic activity, while the remainders were inactive toward the cell lines tested. One compound exhibited radical scavenging activity with an IC50 value of 21.7 μM, and inhibited aromatase with an IC50 value of 12.3 μM.

ABCG26-mediated polyketide trafficking and hydroxycinnamoyl spermidines contribute to pollen wall exine formation in Arabidopsis

Pollen grains are encased by a multilayered, multifunctional wall. The sporopollenin and pollen coat constituents of the outer pollen wall (exine) are contributed by surrounding sporophytic tapetal cells. Because the biosynthesis and development of the exine occurs in the innermost cell layers of the anther, direct observations of this process are difficult. The objective of this study was to investigate the transport and assembly of exine components from tapetal cells to microspores in the intact anthers of Arabidopsis thaliana. Intrinsically fluorescent components of developing tapetum and microspores were imaged in intact, live anthers using two-photon microscopy. Mutants of ABCG26, which encodes an ATP binding cassette transporter required for exine formation, accumulated large fluorescent vacuoles in tapetal cells, with corresponding loss of fluorescence on microspores. These vacuolar inclusions were not observed in tapetal cells of double mutants of abcg26 and genes encoding the proposed sporopollenin polyketide biosynthetic metabolon (ACYL COENZYME A SYNTHETASE5, POLYKETIDE SYNTHASE A [PKSA], PKSB, and TETRAKETIDE α-PYRONE REDUCTASE1), providing a genetic link between transport by ABCG26 and polyketide biosynthesis. Genetic analysis also showed that hydroxycinnamoyl spermidines, known components of the pollen coat, were exported from tapeta prior to programmed cell death in the absence of polyketides, raising the possibility that they are incorporated into the exine prior to pollen coat deposition. We propose a model where ABCG26-exported polyketides traffic from tapetal cells to form the sporopollenin backbone, in coordination with the trafficking of additional constituents, prior to tapetum programmed cell death.

Aflatoxin and deconstruction of type I, iterative polyketide synthase function

In this viewpoint highlights are drawn from a deep analysis of the multifaceted problem of aflatoxin biosynthesis, one of the most highly rearranged polyketide natural products known. Fundamental chemical insights have emerged into how cytochrome P450-mediated skeletal rearrangements occur through probable cationic intermediates and oxidative dearomatizations, which are applicable more widely in natural product catabolism. So to where current experimental methods have failed in our hands, bioinformatic tools and fresh experimental strategies have been developed to identify linker regions in large, polydomain proteins and guide the dissection and reassembly of their component parts. It has been possible to deduce individual catalytic roles, how overall synthesis is coordinated and how these enzymes can be re-engineered in a rational manner to prepare non-natural products. These insights and innovations were often not planned or anticipated, but sprung from the inability to answer fundamental questions. Advances in science can take place by chance favoring the prepared mind, other times by refusing to give up and devising new solutions to address hard questions. Both ways forward played important roles in the investigation of aflatoxin biosynthesis. For these contributions I am pleased to share this special issue of NPR with John Vederas and Tom Simpson, who have been leaders in this field for the last third of a century.