An Unprecedented 1,2-Shift in the Biosynthesis of the 3-Aminosalicylate Moiety of Antimycins

A Standalone β-Ketoreductase Acts Concomitantly with Biosynthesis of the Antimycin Scaffold

Antimycins are anticancer compounds produced by a hybrid nonribosomal peptide synthetase/polyketide synthase (NRPS/PKS) pathway. The biosynthesis of these compounds is well characterized, with the exception of the standalone β-ketoreductase enzyme AntM that is proposed to catalyze the reduction of the C8 carbonyl of the antimycin scaffold. Inactivation of antM and structural characterization suggested that rather than functioning as a post-PKS tailoring enzyme, AntM acts upon the terminal biosynthetic intermediate while it is tethered to the PKS acyl carrier protein. Mutational analysis identified two amino acid residues (Tyr185 and Phe223) that are proposed to serve as checkpoints controlling substrate access to the AntM active site. Aromatic checkpoint residues are conserved in uncharacterized standalone β-ketoreductases, indicating that they may also act concomitantly with synthesis of the scaffold. These data provide novel mechanistic insights into the functionality of standalone β-ketoreductases and will enable their reprogramming for combinatorial biosynthesis.

A Flavoprotein Dioxygenase Steers Bacterial Tropone Biosynthesis via Coenzyme A-Ester Oxygenolysis and Ring Epoxidation

Bacterial tropone natural products such as tropolone, tropodithietic acid, or the roseobacticides play crucial roles in various terrestrial and marine symbiotic interactions as virulence factors, antibiotics, algaecides, or quorum sensing signals. We now show that their poorly understood biosynthesis depends on a shunt product from aerobic CoA-dependent phenylacetic acid catabolism that is salvaged by the dedicated acyl-CoA dehydrogenase-like flavoenzyme TdaE. Further characterization of TdaE revealed an unanticipated complex catalysis, comprising substrate dehydrogenation, noncanonical CoA-ester oxygenolysis, and final ring epoxidation. The enzyme thereby functions as an archetypal flavoprotein dioxygenase that incorporates both oxygen atoms from O₂ into the substrate, most likely involving flavin-N5-peroxide and flavin-N5-oxide species for consecutive CoA-ester cleavage and epoxidation, respectively. The subsequent spontaneous decarboxylation of the reactive enzyme product yields tropolone, which serves as a key virulence factor in rice panicle blight caused by pathogenic edaphic Burkholderia plantarii. Alternatively, the TdaE product is most likely converted to more complex sulfur-containing secondary metabolites such as tropodithietic acid from predominant marine Rhodobacteraceae (e.g., Phaeobacter inhibens).

Targeted accumulation of selective anticancer depsipeptides by reconstructing the precursor supply in the neoantimycin biosynthetic pathway

BACKGROUND

Neoantimycins are a group of 15-membered ring depsipeptides isolated from Streptomycetes with a broad-spectrum of anticancer activities. Neoantimycin biosynthesis is directed by the hybrid multimodular megaenzymes of non-ribosomal peptide synthetase and polyketide synthase. We previously discovered a new neoantimycin analogue unantimycin B, which was demonstrated to have selective anticancer activities and was produced from the neoantimycin biosynthetic pathway with a starter unit of 3-hydroxybenzoate, instead of the 3-formamidosalicylate unit that is common for neoantimycins. However, the low fermentation titre and tough isolation procedure have hindered in-depth pharmacological investigation of unantimycin B as an anticancer agent.

RESULTS

In this work, we genetically constructed two unantimycin B producer strains and inhibited neoantimycins production by removing natO and natJ-L genes essential for 3-formamidosalicylate biosynthesis, therefore facilitating chromatographic separation of unantimycin B from the complex fermentation extract. Based on the ΔnatO mutant, we improved unantimycin B production twofold, reaching approximately 12.8 mg/L, by feeding 3-hydroxybenzoate during fermentation. Furthermore, the production was improved more than sixfold, reaching approximately 40.0 mg/L, in the ΔnatO strain introduced with a chorismatase gene highly expressed under a strong promoter for endogenously over-producing 3-hydroxybenzoate.

CONCLUSION

This work provides a case of targeting accumulation and significant production improvement of medicinally interesting natural products via genetic manipulation of precursor biosynthesis in Streptomycetes, the talented producers of pharmaceutical molecules.

Identification and heterologous expression of an isoquinolinequinone biosynthetic gene cluster from Streptomyces albus J1074

Nitrogen heterocycle small molecules display various pharmaceutically important bioactivities and have great potential in drug development and application. Microbes are an important source for discovering nitrogen heterocycle natural products, and the elucidation of their biosynthetic pathways in microbes facilitates genetic manipulation of new nitrogen heterocycle products. In this study, we isolated three isoquinolinequinones from a Streptomyces albus J1074 conjugant and identified their biosynthetic gene cluster in the S. albus J1074 genome. The function of the biosynthetic gene cluster was confirmed by heterologous expression of the gene cluster in S. coelicolor M1146. This study uncovered a new biosynthetic machinery to produce nitrogen heterocycle natural products in microbes.

Biosynthesis of depsipeptides with a 3-hydroxybenzoate moiety and selective anticancer activities involves a chorismatase

Neoantimycins are anticancer compounds of 15-membered ring antimycin-type depsipeptides. They are biosynthesized by a hybrid multimodular protein complex of nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS), typically from the starting precursor 3-formamidosalicylate. Examining fermentation extracts of Streptomyces conglobatus, here we discovered four new neoantimycin analogs, unantimycins B-E, in which 3-formamidosalicylates are replaced by an unusual 3-hydroxybenzoate (3-HBA) moiety. Unantimycins B-E exhibited levels of anticancer activities similar to those of the chemotherapeutic drug cisplatin in human lung cancer, colorectal cancer, and melanoma cells. Notably, they mostly displayed no significant toxicity toward noncancerous cells, unlike the serious toxicities generally reported for antimycin-type natural products. Using site-directed mutagenesis and heterologous expression, we found that unantimycin productions are correlated with the activity of a chorismatase homolog, the nat-hyg5 gene, from a type I PKS gene cluster. Biochemical analysis confirmed that the catalytic activity of Nat-hyg5 generates 3-HBA from chorismate. Finally, we achieved selective production of unantimycins B and C by engineering a chassis host. On the basis of these findings, we propose that unantimycin biosynthesis is directed by the neoantimycin-producing NRPS-PKS complex and initiated with the starter unit of 3-HBA. The elucidation of the biosynthetic unantimycin pathway reported here paves the way to improve the yield of these compounds for evaluation in oncotherapeutic applications.

Regulation of Antimycin Biosynthesis Is Controlled by the ClpXP Protease

Natural products produced by Streptomyces species underpin many industrially and medically important compounds. However, the majority of the ∼30 biosynthetic pathways harbored by an average species are not expressed in the laboratory. This unrevealed biochemical diversity is believed to comprise an untapped resource for natural product drug discovery. Major roadblocks preventing the exploitation of unexpressed biosynthetic pathways are a lack of insight into their regulation and limited technology for activating their expression. Our findings reveal that the abundance of σ^AntA, which is the cluster-situated regulator of antimycin biosynthesis, is controlled by the ClpXP protease. These data link proteolysis to the regulation of natural product biosynthesis for the first time to our knowledge, and we anticipate that this will emerge as a major strategy by which actinobacteria regulate production of their natural products. Further study of this process will advance understanding of how expression of secondary metabolism is controlled and will aid pursuit of activating unexpressed biosynthetic pathways.

The survival of any microbe relies on its ability to respond to environmental change. Use of extracytoplasmic function (ECF) RNA polymerase sigma (σ) factors is a major strategy enabling dynamic responses to extracellular signals. Streptomyces species harbor a large number of ECF σ factors, nearly all of which are uncharacterized, but those that have been characterized generally regulate genes required for morphological differentiation and/or response to environmental stress, except for σ^AntA, which regulates starter-unit biosynthesis in the production of antimycin, an anticancer compound. Unlike a canonical ECF σ factor, whose activity is regulated by a cognate anti-σ factor, σ^AntA is an orphan, raising intriguing questions about how its activity may be controlled. Here, we reconstituted in vitro ClpXP proteolysis of σ^AntA but not of a variant lacking a C-terminal di-alanine motif. Furthermore, we show that the abundance of σ^AntAin vivo was enhanced by removal of the ClpXP recognition sequence and that levels of the protein rose when cellular ClpXP protease activity was abolished. These data establish direct proteolysis as an alternative and, thus far, unique control strategy for an ECF RNA polymerase σ factor and expands the paradigmatic understanding of microbial signal transduction regulation.

IMPORTANCE Natural products produced by Streptomyces species underpin many industrially and medically important compounds. However, the majority of the ∼30 biosynthetic pathways harbored by an average species are not expressed in the laboratory. This unrevealed biochemical diversity is believed to comprise an untapped resource for natural product drug discovery. Major roadblocks preventing the exploitation of unexpressed biosynthetic pathways are a lack of insight into their regulation and limited technology for activating their expression. Our findings reveal that the abundance of σ^AntA, which is the cluster-situated regulator of antimycin biosynthesis, is controlled by the ClpXP protease. These data link proteolysis to the regulation of natural product biosynthesis for the first time to our knowledge, and we anticipate that this will emerge as a major strategy by which actinobacteria regulate production of their natural products. Further study of this process will advance understanding of how expression of secondary metabolism is controlled and will aid pursuit of activating unexpressed biosynthetic pathways.

Efficient editing DNA regions with high sequence identity in actinomycetal genomes by a CRISPR-Cas9 system

Actinobacteria able to produce varieties of bioactive natural products have been long appreciated by the field of drug discovery and development. Recently, a few of CRISPR/Cas9 systems bearing different types of replicons (pSG5 and pIJ101) were developed to efficiently edit their genomes. Despite wide application in gene editing, their utility in editing challenging DNA regions e.g. high sequence identity has not been compared. In this study, we confirmed that the widely used temperature-sensitive pSG5 replicon is indeed not suitable for editing modular polyketide synthase (PKS) genes due to causing unpredicted gene recombination. This problem can be addressed by replacing the pSG5 with the segregationally unstable pIJ101 replicon. By introducing a counter-selection marker CodA, convenient cloning sites in the single guide RNAs (sgRNAs) and homologous template scaffolds, we developed a new CRISPR-Cas9 system pMWCas9. This system was successfully used to delete/replace erythromycin PKS and other biosynthetic genes in Saccharopolyspora erythraea and Streptomyces sp. AL2110. By swapping the promoters of antB and antC with ermE and kasOp, we achieved a deacyl-antimycin hyper producer which produces a 9-fold higher yield than the original Streptomyces sp. AL2110 strain. Our results provide a robust and useful Cas9 tool for genetic studies in Actinobacteria.

Directed Accumulation of Anticancer Depsipeptides by Characterization of Neoantimycins Biosynthetic Pathway and an NADPH-Dependent Reductase

Neoantimycins (NATs) are members of antimycin-types of depsipeptides with outstanding anticancer activities. We isolated NAT-A (1) and -F (2) from the fermentation extract of Streptomyces conglobatus. The NAT biosynthetic gene cluster ( nat BGC) was identified by genome sequencing and bioinformatics analysis. nat BGC includes two nonribosomal peptide synthetase (NRPS) and one polyketide synthase (PKS) gene, and a gene cassette (10 genes), of which the encoded enzymes share high homology to the ones responsible for 3-formamidosalicylate (3-FAS) biosynthesis in the antimycin biosynthetic pathway. Heterologous expression of the partial nat BGC without the 3-FAS gene cassette in the antimycin producer, Streptomyces albus J1074, results in the production of 1 and 2, suggesting that the nat BGC indeed directs NATs biosynthesis. Targeted in-frame deletion of the reductase gene ( natE) abolished the production of 1 and 2 but accumulated two NAT derivatives, the known NAT-H (3) and a new NAT-I (4). Biochemical verification demonstrated that the recombinant NatE indeed catalyzes an NADPH-dependent reaction of 3 or 4 to 1 or 2, respectively. Compound 3 presented significantly stronger activities against eight cancer cell lines than the ones using cisplatin, the clinical chemotherapy medicine. In particular, 3 displayed 559- and 57-fold higher activity toward human melanoma and cervix epidermoid carcinoma cells, respectively, compared with cisplatin. The new derivative, 4, was 1.5- to 10.9-fold more active than cisplatin toward five cancer cell lines. The evaluation of NATs biosynthesis depicted here will pave the way to generate new NAT derivatives through rational pathway engineering.

Biosynthesis of the 15-Membered Ring Depsipeptide Neoantimycin

Antimycins are a family of natural products possessing outstanding biological activities and unique structures, which have intrigued chemists for over a half century. Of particular interest are the ring-expanded antimycins that show promising anticancer potential and whose biosynthesis remains uncharacterized. Specifically, neoantimycin and its analogs have been shown to be effective regulators of the oncogenic proteins GRP78/BiP and K-Ras. The neoantimycin structural skeleton is built on a 15-membered tetralactone ring containing one methyl, one hydroxy, one benzyl, and three alkyl moieties, as well as an amide linkage to a conserved 3-formamidosalicylic acid moiety. Although the biosynthetic gene cluster for neoantimycins was recently identified, the enzymatic logic that governs the synthesis of neoantimycins has not yet been revealed. In this work, the neoantimycin gene cluster is identified, and an updated sequence and annotation is provided delineating a nonribosomal peptide synthetase/polyketide synthase (NRPS/PKS) hybrid scaffold. Using cosmid expression and CRISPR/Cas-based genome editing, several heterologous expression strains for neoantimycin production are constructed in two separate Streptomyces species. A combination of in vivo and in vitro analysis is further used to completely characterize the biosynthesis of neoantimycins including the megasynthases and trans-acting domains. This work establishes a set of highly tractable hosts for producing and engineering neoantimycins and their C11 oxidized analogs, paving the way for neoantimycin-based drug discovery and development.

A phylogenetic and evolutionary analysis of antimycin biosynthesis

Streptomyces species and other Actinobacteria are ubiquitous in diverse environments worldwide and are the source of, or inspiration for, the majority of antibiotics. The genomic era has enhanced biosynthetic understanding of these valuable chemical entities and has also provided a window into the diversity and distribution of natural product biosynthetic gene clusters. Antimycin is an inhibitor of mitochondrial cytochrome c reductase and more recently was shown to inhibit Bcl-2/Bcl-X_L-related anti-apoptotic proteins commonly overproduced by cancerous cells. Here we identify 73 putative antimycin biosynthetic gene clusters (BGCs) in publicly available genome sequences of Actinobacteria and classify them based on the presence or absence of cluster-situated genes antP and antQ, which encode a kynureninase and a phosphopantetheinyl transferase (PPTase), respectively. The majority of BGCs possess either both antP and antQ (L-form) or neither (S-form), while a minority of them lack either antP or antQ (I_Q- or I_P-form, respectively). We also evaluate the biogeographical distribution and phylogenetic relationships of antimycin producers and BGCs. We show that antimycin BGCs occur on five of the seven continents and are frequently isolated from plants and other higher organisms. We also provide evidence for two distinct phylogenetic clades of antimycin producers and gene clusters, which delineate S-form from L- and I-form BGCs. Finally, our findings suggest that the ancestral antimycin producer harboured an L-form gene cluster which was primarily propagated by vertical transmission and subsequently diversified into S-, I_Q- and I_P-form biosynthetic pathways.

Coordinate Regulation of Antimycin and Candicidin Biosynthesis

Natural products produced by members of the phylum Actinobacteria underpin many industrially and medically important compounds; however, the majority of the ~30 biosynthetic pathways harbored by an average species are not expressed in the laboratory. Understanding the diversity of regulatory strategies controlling the expression of these pathways is therefore critical if their biosynthetic potential is to be explored for new drug leads. Our findings reveal that the candicidin cluster-situated regulator FscRI coordinately controls the biosynthesis of both candicidin and antimycin, which is the first observation of cross-regulation of disparate biosynthetic gene clusters specifying unrelated natural products. We anticipate that this will emerge as a major strategy by which members of the phylum Actinobacteria coordinately produce natural products, which will advance our understanding of how the expression of secondary metabolism is controlled and will aid the pursuit of “silent” biosynthetic pathway activation.

Streptomyces species produce an incredible array of high-value specialty chemicals and medicinal therapeutics. A single species typically harbors ~30 biosynthetic pathways, but only a few them are expressed in the laboratory; thus, poor understanding of how natural-product biosynthesis is regulated is a major bottleneck in drug discovery. Antimycins are a large family of anticancer compounds widely produced by Streptomyces species, and their regulation is atypical compared to that of most other natural products. Here we demonstrate that antimycin production by Streptomyces albus S4 is regulated by FscRI, a PAS-LuxR family cluster-situated regulator of the polyene antifungal agent candicidin. We report that heterologous production of antimycins by Streptomyces coelicolor is dependent on FscRI and show that FscRI activates the transcription of key biosynthetic genes. We also demonstrate through chromatin immunoprecipitation sequencing that FscRI regulation is direct, and we provide evidence that this regulation strategy is conserved and unique to short-form antimycin gene clusters. Our study provides direct in vivo evidence of the cross-regulation of disparate biosynthetic gene clusters specifying unrelated natural products and expands the paradigmatic understanding of the regulation of secondary metabolism.

IMPORTANCE Natural products produced by members of the phylum Actinobacteria underpin many industrially and medically important compounds; however, the majority of the ~30 biosynthetic pathways harbored by an average species are not expressed in the laboratory. Understanding the diversity of regulatory strategies controlling the expression of these pathways is therefore critical if their biosynthetic potential is to be explored for new drug leads. Our findings reveal that the candicidin cluster-situated regulator FscRI coordinately controls the biosynthesis of both candicidin and antimycin, which is the first observation of cross-regulation of disparate biosynthetic gene clusters specifying unrelated natural products. We anticipate that this will emerge as a major strategy by which members of the phylum Actinobacteria coordinately produce natural products, which will advance our understanding of how the expression of secondary metabolism is controlled and will aid the pursuit of “silent” biosynthetic pathway activation.

Biosynthesis and molecular engineering of templated natural products

Bioactive small molecules that are produced by living organisms, often referred to as natural products (NPs), historically play a critical role in the context of both medicinal chemistry and chemical biology. How nature creates these chemical entities with stunning structural complexity and diversity using a limited range of simple substrates has not been fully understood. Focusing on two types of NPs that share a highly evolvable ‘template’-biosynthetic logic, we here provide specific examples to highlight the conceptual and technological leaps in NP biosynthesis and witness the area of progress since the beginning of the twenty-first century. The biosynthesis of polyketides, non-ribosomal peptides and their hybrids that share an assembly-line enzymology of modular multifunctional proteins exemplifies an extended ‘central dogma’ that correlates the genotype of catalysts with the chemotype of products; in parallel, post-translational modifications of ribosomally synthesized peptides involve a number of unusual biochemical mechanisms for molecular maturation. Understanding the biosynthetic processes of these templated NPs would largely facilitate the design, development and utilization of compatible biosynthetic machineries to address the challenge that often arises from structural complexity to the accessibility and efficiency of current chemical synthesis.

Antimycin-type depsipeptides: discovery, biosynthesis, chemical synthesis, and bioactivities

This review discusses the isolation, structural variation, biosynthesis, chemical synthesis, and biological activities of antimycin-type depsipeptides.

Recent highlights in biosynthesis research using stable isotopes

The long and successful history of isotopic labeling experiments within natural products research has both changed and deepened our understanding of biosynthesis. As demonstrated in this article, the usage of isotopes is not at all old-fashioned, but continues to give important insights into biosynthetic pathways of secondary metabolites. This review with 85 cited references is structured by separate discussions of compounds from different classes including polyketides, non-ribosomal peptides, their hybrids, terpenoids, and aromatic compounds formed via the shikimate pathway. The text does not aim at a comprehensive overview, but instead a selection of recent important examples of isotope usage within biosynthetic studies is presented, with a special emphasis on mechanistic surprises.

WITHDRAWN: Identification of an antimycin gene cluster and characterization of the tryptophan 2,3-dioxygenase from the deep sea-derived Streptomyces somaliensis HND1201

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Biosynthesis of antimycins with a reconstituted 3-formamidosalicylate pharmacophore in Escherichia coli

Antimycins are a family of natural products generated from a hybrid nonribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) assembly line. Although they possess an array of useful biological activities, their structural complexity makes chemical synthesis challenging, and their biosynthesis has thus far been dependent on slow-growing source organisms. Here, we reconstituted the biosynthesis of antimycins in Escherichia coli, a versatile host that is robust and easy to manipulate genetically. Along with Streptomyces genetic studies, the heterologous expression of different combinations of ant genes enabled us to systematically confirm the functions of the modification enzymes, AntHIJKL and AntO, in the biosynthesis of the 3-formamidosalicylate pharmacophore of antimycins. Our E. coli-based antimycin production system can not only be used to engineer the increased production of these bioactive compounds, but it also paves the way for the facile generation of novel and diverse antimycin analogues through combinatorial biosynthesis.

Regulation of antimycin biosynthesis by the orphan ECF RNA polymerase sigma factor σ (AntA.)

Antimycins are an extended family of depsipeptides that are made by filamentous actinomycete bacteria and were first isolated more than 60 years ago. Recently, antimycins have attracted renewed interest because of their activities against the anti-apoptotic machineries inside human cells which could make them promising anti-cancer compounds. The biosynthetic pathway for antimycins was recently characterised but very little is known about the organisation and regulation of the antimycin (ant) gene cluster. Here we report that the ant gene cluster in Streptomyces albus is organized into four transcriptional units; the antBA, antCDE, antGF and antHIJKLMNO operons. Unusually for secondary metabolite clusters, the antG and antH promoters are regulated by an extracytoplasmic function (ECF) RNA polymerase sigma factor named σ^AntA which represents a new sub-family of ECF σ factors that is only found in antimycin producing strains. We show that σ^AntA controls production of the unusual precursor 3-aminosalicylate which is absolutely required for the production of antimycins. σ^AntA is highly conserved in antimycin producing strains and the −10 and −35 elements at the σ^AntA regulated antG and antH promoters are also highly conserved suggesting a common mechanism of regulation. We also demonstrate that altering the C-terminal Ala-Ala residues found in all σ^AntA proteins to Asp-Asp increases expression of the antFG and antGHIJKLMNO operons and we speculate that this Ala-Ala motif may be a signal for the protease ClpXP.

A single Sfp-type phosphopantetheinyl transferase plays a major role in the biosynthesis of PKS and NRPS derived metabolites in Streptomyces ambofaciens ATCC23877

The phosphopantetheinyl transferases (PPTases) are responsible for the activation of the carrier protein domains of the polyketide synthases (PKS), non ribosomal peptide synthases (NRPS) and fatty acid synthases (FAS). The analysis of the Streptomyces ambofaciens ATCC23877 genome has revealed the presence of four putative PPTase encoding genes. One of these genes appears to be essential and is likely involved in fatty acid biosynthesis. Two other PPTase genes, samT0172 (alpN) and samL0372, are located within a type II PKS gene cluster responsible for the kinamycin production and an hybrid NRPS-PKS cluster involved in antimycin production, respectively, and their products were shown to be specifically involved in the biosynthesis of these secondary metabolites. Surprisingly, the fourth PPTase gene, which is not located within a secondary metabolite gene cluster, appears to play a pleiotropic role. Its product is likely involved in the activation of the acyl- and peptidyl-carrier protein domains within all the other PKS and NRPS complexes encoded by S. ambofaciens. Indeed, the deletion of this gene affects the production of the spiramycin and stambomycin macrolide antibiotics and of the grey spore pigment, all three being PKS-derived metabolites, as well as the production of the nonribosomally produced compounds, the hydroxamate siderophore coelichelin and the pyrrolamide antibiotic congocidine. In addition, this PPTase seems to act in concert with the product of samL0372 to activate the ACP and/or PCP domains of the antimycin biosynthesis cluster which is also responsible for the production of volatile lactones.

Genome mining of Streptomyces ambofaciens

Since the discovery of the streptomycin produced by Streptomyces griseus in the middle of the last century, members of this bacterial genus have been largely exploited for the production of secondary metabolites with wide uses in medicine and in agriculture. They have even been recognized as one of the most prolific producers of natural products among microorganisms. With the onset of the genomic era, it became evident that these microorganisms still represent a major source for the discovery of novel secondary metabolites. This was highlighted with the complete genome sequencing of Streptomyces coelicolor A3(2) which revealed an unexpected potential of this organism to synthesize natural products undetected until then by classical screening methods. Since then, analysis of sequenced genomes from numerous Streptomyces species has shown that a single species can carry more than 30 secondary metabolite gene clusters, reinforcing the idea that the biosynthetic potential of this bacterial genus is far from being fully exploited. This review highlights our knowledge on the potential of Streptomyces ambofaciens ATCC 23877 to synthesize natural products. This industrial strain was known for decades to only produce the drug spiramycin and another antibacterial compound, congocidine. Mining of its genome allowed the identification of 23 clusters potentially involved in the production of other secondary metabolites. Studies of some of these clusters resulted in the characterization of novel compounds and of previously known compounds but never characterized in this Streptomyces species. In addition, genome mining revealed that secondary metabolite gene clusters of phylogenetically closely related Streptomyces are mainly species-specific.

The regulation and biosynthesis of antimycins

Antimycins (>40 members) were discovered nearly 65 years ago but the discovery of the gene cluster encoding antimycin biosynthesis in 2011 has facilitated rapid progress in understanding the unusual biosynthetic pathway. Antimycin A is widely used as a piscicide in the catfish farming industry and also has potent killing activity against insects, nematodes and fungi. The mode of action of antimycins is to inhibit cytochrome c reductase in the electron transport chain and halt respiration. However, more recently, antimycin A has attracted attention as a potent and selective inhibitor of the mitochondrial anti-apoptotic proteins Bcl-2 and Bcl-x_L. Remarkably, this inhibition is independent of the main mode of action of antimycins such that an artificial derivative named 2-methoxyantimycin A inhibits Bcl-x_L but does not inhibit respiration. The Bcl-2/Bcl-x_L family of proteins are over-produced in cancer cells that are resistant to apoptosis-inducing chemotherapy agents, so antimycins have great potential as anticancer drugs used in combination with existing chemotherapeutics. Here we review what is known about antimycins, the regulation of the ant gene cluster and the unusual biosynthetic pathway.

Enzymatic synthesis of dilactone scaffold of antimycins

Antimycins are a family of natural products possessing outstanding biological activities and unique structures, which have intrigued chemists for over a half century. The antimycin structural skeleton is built on a nine-membered dilactone ring containing one alkyl, one acyloxy, two methyl moieties, and an amide linkage connecting to a 3-formamidosalicylic acid. Although a biosynthetic gene cluster for antimycins was recently identified, the enzymatic logic that governs the synthesis of antimycins has not yet been revealed. In this work, the biosynthetic pathway for antimycins was dissected by both genetic and enzymatic studies for the first time. A minimum set of enzymes needed for generation of the antimycin dilactone scaffold were identified, featuring a hybrid nonribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) assembly line containing both cis- and trans-acting components. Several antimycin analogues were further produced using in vitro enzymatic total synthesis based on the substrate promiscuity of this NRPS-PKS machinery.