Uncovering the formation and selection of benzylmalonyl-CoA from the biosynthesis of splenocin and enterocin reveals a versatile way to introduce amino acids into polyketide carbon scaffolds

Characterization of three succinyl-CoA acyltransferases involved in polyketide chain assembly

Polyketides are a class of natural products with astonishing structural diversities, fascinating biological activities, and a versatile of applications. In polyketides biosynthesis, acyltransferases (ATs) are the 'gatekeeping' enzymes selecting the specific CoA-activated acyl groups as building blocks and transferring them onto the phosphopantetheine arm of acyl carrier proteins (ACPs) to enable the following condensation reactions to assemble the polyketide chain. Herein, the Art2 protein from aurantinins, a group of the antibacterial polyketides, is characterized in vitro as an AT that can load a CoA-activated succinyl unit onto the first ACP domain of Art17 (ACP_Art17-1). In addition, another two proteins, GbnB and EtnB, involved in the biosynthesis of gladiolin and etnangien respectively, were traced by literature mining, homologous searching, and product structure analysis and then identified as functional succinyl-CoA ATs by the ACP_Art17-1 assays. Taken together, by the assay method employing ACP_Art17-1 as an acyl acceptor, we identified three ATs that can introduce a succinyl unit into the polyketide assembly line, which enriches the toolbox of polyketide biosynthetic enzymes and sets a stage for incorporating a succinyl unit into polyketide backbones in synthetic biological manners. KEY POINTS: • Three acyltransferases that are able to load ACP with a succinyl unit were characterized in vitro. • ACP_Art17-1 can be used as an acceptor to assay succinyl-CoA AT from different polyketides. • The succinyl unit can be incorporated into polyketides assembly process.

Biosynthesis of Tetronates by a Nonribosomal Peptide Synthetase-Polyketide Synthase System

A cryptic tetronate biosynthetic pathway was identified in Kitasatospora niigatensis DSM 44781 via heterologous expression. Distinct from the currently known biosynthetic pathways, this system utilizes a partially functional nonribosomal peptide synthetase and a broadly selective polyketide synthase to direct the assembly and lactonization of the tetronate scaffold. By employing a permissive crotonyl-CoA reductase/carboxylase to provide different extender units, seven new tetronates (kitaniitetronins A-G) were obtained via precursor-directed biosynthesis.

Investigation of Streptomyces sp. Strain EMB24 Secondary Metabolite Profile Has Unraveled Its Extraordinary Antibacterial Potency Against Drug-Resistant Bacteria

With the overuse and misuse of antibiotics amid COVID-19 pandemic, the antimicrobial resistance, which is already a global challenge, has accelerated its pace significantly. Finding novel and potential antibiotics seems one of the probable solutions. In this work, a novel Streptomyces sp. strain EMB24 was isolated and found to be an excellent source of antimicrobials as confirmed by agar-plug assay. It showed antibacterial activity against infection-causing bacteria, namely Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. In addition, Streptomyces sp. strain EMB24 inhibited the growth methicillin-resistant Staphylococcus aureus (MRSA), tetracycline-resistant Neisseria gonorrhoeae, and ampicillin-resistant Neisseria gonorrhoeae. Furthermore, to get deep insights about the genome and biosynthetic gene clusters producing antibiotics, whole genome sequencing was done. The strain EMB24 is closely related to the Streptomyces longispororuber as revealed by phylogenetic analysis which is a potential source of antibiotics and pigments as undecylprodigiosin and metacycloprodigiosin belonging to the class prodigiosin. Naphthyridinomycin, alkylresorcinols, desferrioxamine B and E, venezuelin, aborycin, MS-271, and siamycin are potent therapeutics that shared 100% similarity with the reference strain as revealed by the online antiSMASH tool.

Genetic origin of homopyrones, a rare type of hybrid phenylpropanoid- and polyketide-derived yellow pigments from Aspergillus homomorphus

In recent years, there has been an increasing demand for the replacement of synthetic food colorants with naturally derived alternatives. Filamentous fungi are prolific producers of secondary metabolites including polyketide-derived pigments, many of which have not been fully characterized yet. During our ongoing investigations of black aspergilli, we noticed that Aspergillus homomorphus turned yellow when cultivated on malt extract agar plates. Chemical discovery guided by UV and MS led to the isolation of two novel yellow natural products, and their structures were elucidated as aromatic α-pyrones homopyrones A (1) and B (2) by HRMS and NMR. Combined investigations including retro-biosynthesis, genome mining, and gene deletions successfully linked both compounds to their related biosynthetic gene clusters. This demonstrated that homopyrones are biosynthesized by using cinnamoyl-CoA as the starter unit, followed by extension with three malonyl-CoA units, and lactonization to give the core hybrid backbone structure. The polyketide synthase AhpA includes a C-methylation domain, which however seems to be promiscuous since only 2 is C-methylated. Altogether, the homopyrones represent a rare case of hybrid phenylpropanoid- and polyketide-derived natural products in filamentous fungi. KEY POINTS: • Homopyrones represent a rare type of fungal polyketides synthesized from cinnamic-CoA. • CRISPR/Cas9 technology has been firstly applied in Aspergillus homomorphus.

Benzylmalonyl-CoA dehydrogenase, an enzyme involved in bacterial auxin degradation

A novel acyl-CoA dehydrogenase involved in degradation of the auxin indoleacetate by Aromatoleum aromaticum was identified as a decarboxylating benzylmalonyl-CoA dehydrogenase (IaaF). It is encoded within the iaa operon coding for enzymes of indoleacetate catabolism. Using enzymatically produced benzylmalonyl-CoA, the reaction was characterized as simultaneous oxidation and decarboxylation of benzylmalonyl-CoA to cinnamoyl-CoA and CO₂. Oxygen served as electron acceptor and was reduced to H₂O₂, whereas electron transfer flavoprotein or artificial dyes serving as electron acceptors for other acyl-CoA dehydrogenases were not used. The enzyme is homotetrameric, contains an FAD cofactor and is enantiospecific in benzylmalonyl-CoA turnover. It shows high catalytic efficiency and strong substrate inhibition with benzylmalonyl-CoA, but otherwise accepts only a few medium-chain alkylmalonyl-CoA compounds as alternative substrates with low activities. Its reactivity of oxidizing 2-carboxyacyl-CoA with simultaneous decarboxylation is unprecedented and indicates a modified reaction mechanism for acyl-CoA dehydrogenases, where elimination of the 2-carboxy group replaces proton abstraction from C2.

Unravelling the Biosynthetic Flexibility of UK-2A Enables Enzymatic Synthesis of Its Structural Variants

Emerging antimicrobial resistant fungal pathogens are a growing threat, and fungicides with novel modes of action are urgently needed to prevent critical failures in global food security. Fenpicoxamid, the prodrug of UK-2A, is a member of a new class of antifungal agents that displays no cross-resistance to other fungicides. Rational engineering of its structure using a biosynthetic approach is a promising avenue for developing more potent fungicides. Herein, through in vitro enzymatic reconstitution, we elucidate the biosynthetic pathway of UK-2A. Its biosynthesis involves a flexible AMP-binding protein and dilactone formation assembly enzymes that are able to select and incorporate highly diverse substituted salicylic acids into the dilactone scaffold. By introducing diverse salicylic acids into the in vitro biosynthetic pathway, we successfully generate 14 novel deacyl UK-2A analogues. This study reveals the flexibility of the biosynthetic pathway of UK-2A and provides an effective solution to rationally engineer its crucial C3 moiety.

ClusterCAD: a computational platform for type I modular polyketide synthase design

ClusterCAD is a web-based toolkit designed to leverage the collinear structure and deterministic logic of type I modular polyketide synthases (PKSs) for synthetic biology applications. The unique organization of these megasynthases, combined with the diversity of their catalytic domain building blocks, has fueled an interest in harnessing the biosynthetic potential of PKSs for the microbial production of both novel natural product analogs and industrially relevant small molecules. However, a limited theoretical understanding of the determinants of PKS fold and function poses a substantial barrier to the design of active variants, and identifying strategies to reliably construct functional PKS chimeras remains an active area of research. In this work, we formalize a paradigm for the design of PKS chimeras and introduce ClusterCAD as a computational platform to streamline and simplify the process of designing experiments to test strategies for engineering PKS variants. ClusterCAD provides chemical structures with stereochemistry for the intermediates generated by each PKS module, as well as sequence- and structure-based search tools that allow users to identify modules based either on amino acid sequence or on the chemical structure of the cognate polyketide intermediate. ClusterCAD can be accessed at https://clustercad.jbei.org and at http://clustercad.igb.uci.edu.

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.

Engineering the Substrate Specificity of a Modular Polyketide Synthase for Installation of Consecutive Non-Natural Extender Units

There is significant interest in diversifying the structures of polyketides to create new analogues of these bioactive molecules. This has traditionally been done by focusing on engineering the acyltransferase (AT) domains of polyketide synthases (PKSs) responsible for the incorporation of malonyl-CoA extender units. Non-natural extender units have been utilized by engineered PKSs previously; however, most of the work to date has been accomplished with ATs that are either naturally promiscuous and/or located in terminal modules lacking downstream bottlenecks. These limitations have prevented the engineering of ATs with low native promiscuity and the study of any potential gatekeeping effects by domains downstream of an engineered AT. In an effort to address this gap in PKS engineering knowledge, the substrate preferences of the final two modules of the pikromycin PKS were compared for several non-natural extender units and through active site mutagenesis. This led to engineering of the methylmalonyl-CoA specificity of both modules and inversion of their selectivity to prefer consecutive non-natural derivatives. Analysis of the product distributions of these bimodular reactions revealed unexpected metabolites resulting from gatekeeping by the downstream ketoreductase and ketosynthase domains. Despite these new bottlenecks, AT engineering provided the first full-length polyketide products incorporating two non-natural extender units. Together, this combination of tandem AT engineering and the identification of previously poorly characterized bottlenecks provides a platform for future advancements in the field.

Evaluation of Inhibitory Activities of UK-2A, an Antimycin-Type Antibiotic, and Its Synthetic Analogues against the Production of Anti-inflammatory Cytokine IL-4

The inhibitory activities of the antimycin-class antibiotics UK-2A, antimycin A, and splenocin B against the production of anti-inflammatory cytokine IL-4, which is related to IgE-mediated allergic responses in rat basophilic leukemia (RBL-2H3) cells, were evaluated. Although antimycin A and splenocin B showed cytotoxicity at concentrations at which IL-4 release from the cells was restricted, UK-2A was found to restrict IL-4 release without cytotoxicity. Three UK-2A analogues (4-6) were then synthesized and assessed. Compound 5 restricted IL-4 release dose-dependently without cytotoxicity, and its effect was more potent than that of UK-2A.

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.

Structural Basis of a Broadly Selective Acyltransferase from the Polyketide Synthase of Splenocin

Polyketides are a large family of pharmaceutically important natural products, and the structural modification of their scaffolds is significant for drug development. Herein, we report high-resolution X-ray crystal structures of the broadly selective acyltransferase (AT) from the splenocin polyketide synthase (SpnD-AT) in the apo form and in complex with benzylmalonyl and pentynylmalonyl extender unit mimics. These structures revealed the molecular basis for the stereoselectivity and substrate specificity of SpnD-AT, and enabled the engineering of the industrially important Ery-AT6 to broaden its substrate scope to include three new types of extender units.

New WS9326A Derivatives and One New Annimycin Derivative with Antimalarial Activity are Produced by Streptomyces asterosporus DSM 41452 and Its Mutant

In this study, we report that Streptomyces asterosporus DSM 41452 is a producer of new molecules related to the nonribosomal cyclodepsipeptide WS9326A and the polyketide annimycin. S. asterosporus DSM 41452 is shown to produce six cyclodepsipeptides and peptides, WS9326A to G. Notably, the compounds WS9326F and WS9326G have not been described before. The genome of S. asterosporus DSM 41452 was sequenced, and a putative WS9326A gene cluster was identified. Gene-deletion experiments confirmed that this cluster was responsible for the biosynthesis of WS9326A to G. Additionally, a gene-deletion experiment demonstrated that sas16 encoding a cytochrome P450 monooxygenase was involved in the synthesis of the novel (E)-2,3-dehydrotyrosine residue found in WS9326A and its derivatives. An insertion mutation within the putative annimycin gene cluster led to the production of a new annimycin derivative, annimycin B, which exhibited modest inhibitory activity against Plasmodium falciparum.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Neoantimycins A and B, Two Unusual Benzamido Nine-Membered Dilactones from Marine-Derived Streptomyces antibioticus H12-15

An actinomycete strain (H12-15) isolated from a sea sediment in a mangrove district was identified as Streptomycesantibioticus on the basis of 16S rDNA gene sequence analysis as well as the investigation of its morphological, physiological, and biochemical characteristics. Two novel benzamido nonacyclic dilactones, namely neoantimycins A (1) and B (2), together with the known antimycins A_1ab (3a,b), A_2a (4), and A₉ (5), were isolated from the culture broth of this strain. Compounds 1 and 2 are the first natural modified ATNs with an unusual benzamide unit. The structures of these new compounds, including their absolute configuration, were established on the basis of HRMS, NMR spectroscopic data, and quantum chemical ECD calculations. Their cytotoxicities against human breast adenocarcinoma cell line MCF-7, the human glioblastoma cell line SF-268, and the human lung cancer cell line NCI-H460 were also tested. All compounds exhibited mild cytotoxic activity. However, Compounds 1 and 2 showed no activity against C. albicans at the test concentration of 1 mg/mL via paper disc diffusion, while the known antimycins showed obvious antifungal activity.

Comprehensive in Vitro Analysis of Acyltransferase Domain Exchanges in Modular Polyketide Synthases and Its Application for Short-Chain Ketone Production

Type I modular polyketide synthases (PKSs) are polymerases that utilize acyl-CoAs as substrates. Each polyketide elongation reaction is catalyzed by a set of protein domains called a module. Each module usually contains an acyltransferase (AT) domain, which determines the specific acyl-CoA incorporated into each condensation reaction. Although a successful exchange of individual AT domains can lead to the biosynthesis of a large variety of novel compounds, hybrid PKS modules often show significantly decreased activities. Using monomodular PKSs as models, we have systematically analyzed the segments of AT domains and associated linkers in AT exchanges in vitro and have identified the boundaries within a module that can be used to exchange AT domains while maintaining protein stability and enzyme activity. Importantly, the optimized domain boundary is highly conserved, which facilitates AT domain replacements in most type I PKS modules. To further demonstrate the utility of the optimized AT domain boundary, we have constructed hybrid PKSs to produce industrially important short-chain ketones. Our in vitro and in vivo analysis demonstrated production of predicted ketones without significant loss of activities of the hybrid enzymes. These results greatly enhance the mechanistic understanding of PKS modules and prove the benefit of using engineered PKSs as a synthetic biology tool for chemical production.

A crotonyl-CoA reductase-carboxylase independent pathway for assembly of unusual alkylmalonyl-CoA polyketide synthase extender units

Type I modular polyketide synthases assemble diverse bioactive natural products. Such multienzymes typically use malonyl and methylmalonyl-CoA building blocks for polyketide chain assembly. However, in several cases more exotic alkylmalonyl-CoA extender units are also known to be incorporated. In all examples studied to date, such unusual extender units are biosynthesized via reductive carboxylation of α, β-unsaturated thioesters catalysed by crotonyl-CoA reductase/carboxylase (CCRC) homologues. Here we show using a chemically-synthesized deuterium-labelled mechanistic probe, and heterologous gene expression experiments that the unusual alkylmalonyl-CoA extender units incorporated into the stambomycin family of polyketide antibiotics are assembled by direct carboxylation of medium chain acyl-CoA thioesters. X-ray crystal structures of the unusual β-subunit of the acyl-CoA carboxylase (YCC) responsible for this reaction, alone and in complex with hexanoyl-CoA, reveal the molecular basis for substrate recognition, inspiring the development of methodology for polyketide bio-orthogonal tagging via incorporation of 6-azidohexanoic acid and 8-nonynoic acid into novel stambomycin analogues.

Polyketides are typically assembled from a starter unit and malonyl- and/or methylmalonyl-CoA-derived extender units, but the macrolide antibiotics stambomycins incorporate non-standard alkylmalonyl-CoA extender units. Here, the authors describe the biosynthetic pathway responsible for this unusual synthesis.

Bio-based production of fuels and industrial chemicals by repurposing antibiotic-producing type I modular polyketide synthases: opportunities and challenges

Complex polyketides comprise a large number of natural products that have broad application in medicine and agriculture. They are produced in bacteria and fungi from large enzyme complexes named type I modular polyketide synthases (PKSs) that are composed of multifunctional polypeptides containing discrete enzymatic domains organized into modules. The modular nature of PKSs has enabled a multitude of efforts to engineer the PKS genes to produce novel polyketides of predicted structure. We have repurposed PKSs to produce a number of short-chain mono- and di-carboxylic acids and ketones that could have applications as fuels or industrial chemicals.

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.

Recent Advances in Understanding and Engineering Polyketide Synthesis

Polyketides are a diverse group of natural products that form the basis of many important drugs. The engineering of the polyketide synthase (PKS) enzymes responsible for the formation of these compounds has long been considered to have great potential for producing new bioactive molecules. Recent advances in this field have contributed to the understanding of this powerful and complex enzymatic machinery, particularly with regard to domain activity and engineering, unique building block formation and incorporation, and programming rules and limitations. New developments in tools for in vitro biochemical analysis, full-length megasynthase structural studies, and in vivo heterologous expression will continue to improve our fundamental understanding of polyketide synthesis as well as our ability to engineer the production of polyketides.

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).

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.

Harnessing natural product assembly lines: structure, promiscuity, and engineering

Many therapeutically relevant natural products are biosynthesized by the action of giant mega-enzyme assembly lines. By leveraging the specificity, promiscuity, and modularity of assembly lines, a variety of strategies has been developed that enables the biosynthesis of modified natural products. This review briefly summarizes recent structural advances related to natural product assembly lines, discusses chemical approaches to probing assembly line structures in the absence of traditional biophysical data, and surveys efforts that harness the inherent or engineered promiscuity of assembly lines for the synthesis of non-natural polyketides and non-ribosomal peptide analogues.

Identification of Middle Chain Fatty Acyl-CoA Ligase Responsible for the Biosynthesis of 2-Alkylmalonyl-CoAs for Polyketide Extender Unit

Understanding the biosynthetic mechanism of the atypical polyketide extender unit is important for the development of bioactive natural products. Reveromycin (RM) derivatives produced by Streptomyces sp. SN-593 possess several aliphatic extender units. Here, we studied the molecular basis of 2-alkylmalonyl-CoA formation by analyzing the revR and revS genes, which form a transcriptional unit with the revT gene, a crotonyl-CoA carboxylase/reductase homolog. We mainly focused on the uncharacterized adenylate-forming enzyme (RevS). revS gene disruption resulted in the reduction of all RM derivatives, whereas reintroduction of the gene restored the yield of RMs. Although RevS was classified in the fatty acyl-AMP ligase clade based on phylogenetic analysis, biochemical characterization revealed that the enzyme catalyzed the middle chain fatty acyl-CoA ligase (FACL) but not the fatty acyl-AMP ligase activity, suggesting the molecular evolution for acyl-CoA biosynthesis. Moreover, we examined the in vitro conversion of fatty acid into 2-alkylmalonyl-CoA using purified RevS and RevT. The coupling reaction showed efficient conversion of hexenoic acid into butylmalonyl-CoA. RevS efficiently catalyzed C8-C10 middle chain FACL activity; therefore, we speculated that the acyl-CoA precursor was truncated via β-oxidation and converted into (E)-2-enoyl-CoA, a RevT substrate. To determine whether the β-oxidation process is involved between the RevS and RevT reaction, we performed the feeding experiment using [1,2,3,4-(13)C]octanoic acid. (13)C NMR analysis clearly demonstrated incorporation of the [3,4-(13)C]octanoic acid moiety into the structure of RM-A. Our results provide insight into the role of uncharacterized RevS homologs that may catalyze middle chain FACL to produce a unique polyketide extender unit.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as coprisamide A from a bacterium isolated from Copris tripartitus.