With growing understanding of complex biosynthetic pathways to natural products, mutasynthesis, which combines metabolic engineering with chemical synthesis, is becoming an increasingly important tool to produce novel compounds. Mupirocin, isolated from Pseudomonas fluorescens, is a mixture of pseudomonic acids (PAs) that exhibit antibiotic activity against Gram-positive bacteria including methicillin-resistant Staphylococcus aureus. We have developed a flexible approach, based on mutasynthesis, for the preparation of a library of novel PA analogues. The antimicrobial activities of the natural products and synthetic analogues were evaluated against Bacillus subtilis and four Staphylococcus aureus strains. Interestingly, one of the analogues retained antimicrobial activity in all the assays but lacked structural features that render PA-A unstable, that is, the 10,11-epoxide (replaced by an alkene) and ester linkage (replaced by a ketone). In addition, mutasynthesis allowed the preparation of an analogue of a key biosynthetic intermediate (desepoxy-PA-B) in which the C9 hydroxy fatty acid is replaced by a C7 analogue. Feeding studies with mutant strains of Pseudomonas fluorescens revealed that C7-desepoxy-PA-B was converted to the novel metabolite C7-PA-C with loss of the 8-hydroxyl group but with no extension to the C9 side chain.

The deep sea remains a major reservoir of underexplored microbial diversity and biosynthetic novelty. Here, we describe three bacterial species isolated from Atlantic sponges, including Stappia quadratibracata sp. nov., Bacillus crepusculi sp. nov., and Psychrobacter noctis sp. nov. Genome sequencing, phylogenomics, and phenotypic characterization confirmed their novelty and biosynthetic potential. A targeted 'One Strain Many Active Compounds' (OSMAC) screen revealed previously silent antibacterial activity from Stappia quadratibracata sp. nov., only when grown with the carbon source succinate. Metabolomics and molecular networking analysis indicated that this activity was attributable to an unstable thiazole alkaloid with spectral data closely related to, but distinct from, the epimeric siderophores agrochelin and massiliachelin. Spectroscopic studies, in tandem with comparative analysis of the biosynthetic gene cluster for this metabolite, are consistent with agrochelin II, a previously unreported thiazole alkaloid diastereoisomer. Agrochelin II exhibits iron-enhanced antibacterial activity against Staphylococcus aureus, underscoring the ecological role of iron acquisition in microbial competition. Our findings highlight the value of OSMAC-guided bioprospecting in uncovering antimicrobial metabolites from sponge-associated bacteria.IMPORTANCEBioactive microbial natural products remain the preeminent source of new lead compounds for drug development. Due to the increasingly high levels of strain and compound rediscovery from terrestrial environments, the deep-ocean is increasingly considered an attractive starting point for bioprospecting programs, which seek to isolate and characterize novel chemical scaffolds. Here, we use a combination of genomics, metabolomics, and chemical analysis, to establish the biosynthetic potential of three bacterial species isolated from deep-ocean Atlantic sponges and report the discovery and characterization of a new antimicrobial thiazole alkaloid, agrochelin II. Our findings demonstrate the usefulness of integrated cultivation-screening-metabolomics-genomics pipelines for microbial metabolite discovery and identify the genus Stappia as a hitherto neglected source of bioactive natural products.

ABSTRACT Peroxy natural products, including endoperoxides, acyclic peroxides, and hydroperoxides, are widely distributed across all domains of life, with many, such as artemisinin and prostaglandins, serving as clinically important agents. However, the enzymatic mechanisms by which nature installs O─O bonds have largely remained elusive. To date, only a limited number of endoperoxide‐forming enzymes have been identified, while the enzymatic basis for acyclic peroxide assembly remains unknown. Here, we identify two homologous flavin‐dependent enzymes, OxaJ and OtnJ, that catalyze enantioselective acyclic peroxide formation in the biosynthesis of oxanthromicin natural products. A conserved structural motif acts as a redox gate by blocking NADPH access to the active site, thereby promoting peroxide installation. Removal of this motif permits NADPH binding and redirects the enzyme's activity toward hydroxylation. This work establishes the first example of peroxide formation by a flavin‐dependent enzyme and introduces redox gating as a previously unrecognized strategy for controlling oxidative divergence in enzymatic catalysis.

Sorangium cellulosum is a cellulolytic myxobacterium that produces a vast array of complex natural products with diverse chemical scaffolds and biological activities. However, biosynthetic investigations of these metabolites have been hindered by the scarcity of genetic manipulation tools available for their producing microorganisms. Here, we develop an efficient electroporation method for transforming foreign DNA into various Sorangium strains, enabling effective genetic engineering via homologous recombination. This facilitates delineation of the biosynthetic pathway to ambruticin, unveiling several previously undisclosed steps. Notably, AmbK is identified as the elusive epoxide hydrolase responsible for the formation of the tetrahydropyran ring during post-polyketide synthase (PKS) modification, while the terminal PKS module AmbH is shown to catalyse dual rounds of chain elongation during polyketide assembly. Our findings provide significant insights into the intricate molecular machinery governing myxobacterial natural product biosynthesis and greatly enhance our ability to further engineer Sorangium strains to unlock their biosynthetic potentials.

A novel ionized heavy-atom-free two-dimensional organic nanosheet was prepared and exhibited highly selective generation of singlet oxygen under both light and ultrasound excitation. These ionized nanosheets displayed excellent dispersibility in water and enhanced singlet oxygen production efficiency compared to their non-assembled monomers. Antimicrobial experiments have revealed their potent bactericidal effects on drug-resistant E. coli and S. aureus under both visible light and ultrasound irradiation.

Less steric ketones exhibited low stereoselectivity toward M5 due to their difficulty in restricting the free rotation of the imine intermediate. An engineered enantio-complementary imine reductase from M5 was obtained with catalytic activity. We identified four key residues that play essential roles in controlling stereoselectivity. Two mutants, I149Y-W234L (up to 99% S ee) and L200M-F260M (up to 99% R ee), were achieved, showing excellent stereoselectivity toward the tested substrates, offering valuable biocatalysts for synthesizing alkylated amphetamines.

The biomedical field is increasingly utilizing organic semiconducting materials, driving interest in the green synthesis of conjugated polymers from biomass-derived monomers. This study introduces an efficient C-H arylation method to synthesize furan-based conjugated polymers using oligofurans as building blocks. The resulting polymers exhibit excellent solubility and photostability, and as photosensitizers, they can generate singlet oxygen under both light and ultrasound excitation, effectively eradicating bacteria. Notably, long oligofurans demonstrate greater reactivity than individual furan monomers, which is crucial for producing high-molecular-weight furan-based conjugated polymers via direct C-H arylation. Moreover, adjusting the length of the oligofuran building blocks enables the tuning of the polymers' bandgaps across the visible to near-infrared regions. This work presents a promising eco-friendly synthesis strategy for developing furan-based conjugated polymers with tailored properties for biomedical applications.

The ambruticins are a family of potent antifungal polyketide derived natural products isolated from the myxobacterium Sorangium cellulosum. Their unusual structures include a trisubstituted cyclopropyl group and two oxygen heterocycles, a tetrahydropyran (THP) and dihydropyran (DHP). Herein we report a flexible modular approach for the total synthesis of ambruticins which is used to prepare ambruticins F and S as well as in the first total synthesis of 20,21-dihydroambruticin F. The flexible strategy unites 3 fragments via Julia-Kocienski olefinations and provides important standards for investigation of dihydropyran formation in ambruticin biosynthesis. Cultures of wild-type S. cellulosum So ce10 produce mainly ambruticin S and the VS series of metabolites. An efficient electroporation method enabled gene knockout experiments which revealed that the ΔambP-S mutant of S. cellulosum accumulated the bisTHP polyketide 20,21-dihydroambruticin F. In contrast, the ΔambN-S mutant gave ambruticin F with the 20,21-alkene as the major metabolite confirming that AmbP and AmbO (a Rieske enzyme and flavin-dependent monooxygenase respectively) are implicated in 20,21-alkene formation. The results of feeding studies to a Sorangium strain containing only ambP and ambO are in accord with formation of the 20,21-alkene occurring prior to generation of the C3 to C7 dihydroxylated tetrahydropyran in ambruticin biosynthesis.

The delineation of the complex biosynthesis of the potent antibiotic mupirocin, which consists of a mixture of pseudomonic acids (PAs) isolated from Pseudomonas fluorescens NCIMB 10586, presents significant challenges, and the timing and mechanisms of several key transformations remain elusive. Particularly intriguing are the steps that process the linear backbone from the initial polyketide assembly phase to generate the first cyclic intermediate PA-B. These include epoxidation as well as incorporation of the tetrahydropyran (THP) ring and fatty acid side chain required for biological activity. Herein, we show that the mini-module MmpE performs a rare online (ACP-substrate) epoxidation and is integrated ('in-cis') into the polyketide synthase via a docking domain. A linear polyketide fragment with six asymmetric centres was synthesised using a convergent approach and used to demonstrate substrate flux via an atypical KS0 and a previously unannotated ACP (MmpE_ACP). MmpE_ACP-bound synthetic substrates were critical in demonstrating successful epoxidation in vitro by the purified MmpE oxidoreductase domain. Alongside feeding studies, these results confirm the timing as well as chain length dependence of this selective epoxidation. These mechanistic studies pinpoint the location and nature of the polyketide substrate prior to the key formation of the THP ring and esterification that generate PA-B.

After publication of this article [...].

AbstractMupirocin is a clinically important antibiotic produced by a trans‐AT Type I polyketide synthase (PKS) in Pseudomonas fluorescens. The major bioactive metabolite, pseudomonic acid A (PA−A), is assembled on a tetrasubstituted tetrahydropyran (THP) core incorporating a 6‐hydroxy group proposed to be introduced by α‐hydroxylation of the thioester of the acyl carrier protein (ACP) bound polyketide chain. Herein, we describe an in vitro approach combining purified enzyme components, chemical synthesis, isotopic labelling, mass spectrometry and NMR in conjunction with in vivo studies leading to the first characterisation of the α‐hydroxylation bimodule of the mupirocin biosynthetic pathway. These studies reveal the precise timing of hydroxylation by MupA, substrate specificity and the ACP dependency of the enzyme components that comprise this α‐hydroxylation bimodule. Furthermore, using purified enzyme, it is shown that the MmpA KS0 shows relaxed substrate specificity, suggesting precise spatiotemporal control of in trans MupA recruitment in the context of the PKS. Finally, the detection of multiple intermodular MupA/ACP interactions suggests these bimodules may integrate MupA into their assembly.

Objective To determine the secondary metabolites of Pseudomonas promysalinigenes RW10S1. Methods MCI column chromatography and semi-preparative HPLC were used for the isolation and purification of fermentated products. The structures of isolated compounds were elucidated by HR-ESI-MS, NMR and other related spectroscopic analyses. Results Three compounds were identified as promysalin（1）, promynicotin（2） and dihydrodeoxypromynicotin（3）. Conclusion Promynicotin（2） and dihydrodeoxypromynicotin（3） have been identified for the first time.

Tetrahydropyrans (THPs) are common structural motifs found in natural products and synthetic therapeutic molecules. In Nature these 6-membered oxygen heterocycles are often assembled via intramolecular reactions involving either oxy-Michael additions or ring opening of epoxy-alcohols. Indeed, the polyether natural products have been particularly widely studied due to their fascinating structures and important biological properties; these are commonly formed via endo-selective epoxide-opening cascades. In this review we outline synthetic approaches for endo-selective intramolecular epoxide ring opening (IERO) of 4,5-epoxy-alcohols and their applications in natural product synthesis. In addition, the biosynthesis of THP-containing natural products which utilise IERO reactions are reviewed.

Synthetic biology-based methods (Sbio) and chemical synthesis (Csyn) are two independent approaches that are both widely used for synthesizing biomolecules. In the current study, two systems were combined for the overproduction of chanoclavine (CC), a structurally complex ergot alkaloid. The whole synthetic pathway for CC was split into three sections: enzymatic synthesis of 4-Br-Trp (4-Bromo-trptophan) using cell-lysate catalysis (CLC), chemical synthesis of prechanoclavine (PCC) from 4-Br-Trp, and overproduction CC from PCC using a whole-cell catalysis (WCC) platform. The final titer of the CC is over 3 g/L in this Sbio-Csyn hybrid system, the highest yield reported so far, to the best of our knowledge. The development of such a combined route could potentially avoid the limitations of both Sbio and Csyn systems and boost the overproduction of complex natural products.

The ambruticins are a family of polyketide natural products which exhibit potent antifungal activity. Gene knockout experiments are in accord with the proposal that the tetrahydropyran ring of the ambruticins is formed via the AmbJ catalysed epoxidation of the unsaturated 3,5-dihydroxy acid, ambruticin J, followed by regioselective cyclisation to ambruticin F. Herein, a convergent approach to the total synthesis of ambruticin J is described as well as model studies involving epoxidation and cyclisations of unsaturated hydroxy esters to give tetrahydropyrans and tetrahydrofurans. The total synthesis involves preparation of three key fragments which were united via a Suzuki-Miyaura cross-coupling and Julia-Kocienski olefination to generate the required carbon framework. Global deprotection to a triol and selective oxidation of the primary alcohol gave, after hydrolysis of the lactone, ambruticin J.

To tackle the growing problem of antibiotic resistance, it is essential to identify new bioactive compounds that are effective against resistant microbes and safe to use. Natural products and their derivatives are, and will continue to be, an important source of these molecules. Sea sponges harbour a diverse microbiome that co-exists with the sponge, and these bacterial communities produce a rich array of bioactive metabolites for protection and resource competition. For these reasons, the sponge microbiota constitutes a potential source of clinically relevant natural products. To date, efforts in bioprospecting for these compounds have focused predominantly on sponge specimens isolated from shallow water, with much still to be learned about samples from the deep sea. Here we report the isolation of a new Micromonospora strain, designated 28ISP2-46T, recovered from the microbiome of a mid-Atlantic deep-sea sponge. Whole-genome sequencing reveals the capacity of this bacterium to produce a diverse array of natural products, including kosinostatin and isoquinocycline B, which exhibit both antibiotic and antitumour properties. Both compounds were isolated from 28ISP2-46T fermentation broths and were found to be effective against a plethora of multidrug-resistant clinical isolates. This study suggests that the marine production of isoquinocyclines may be more widespread than previously supposed and demonstrates the value of targeting the deep-sea sponge microbiome as a source of novel microbial life with exploitable biosynthetic potential.

Mupirocin, a commercially available antibiotic produced by Pseudomonas fluorescens NCIMB 10586, and thiomarinol, isolated from the marine bacterium Pseudoalteromonas sp. SANK 73390, both consist of a polyketide-derived monic acid homologue esterified with either 9-hydroxynonanoic acid (mupirocin, 9HN) or 8-hydroxyoctanoic acid (thiomarinol, 8HO). The mechanisms of formation of these deceptively simple 9HN and 8HO fatty acid moieties in mup and tml, respectively, remain unresolved. To define starter unit generation, the purified mupirocin proteins Mupq. MupS, and MacpD and their thiomarinol equivalents (TmIQ, TmIS and TacpD) have been expressed and shown to convert malonyl coenzyme A (CoA) and succinyl CoA to 3-hydroxypropionoyl (3-HP) or 4-hydroxybutyryl (4-HB) fatty acid starter units, respectively, via the MupQ/TmlQ catalyzed generation of an unusual bis-CoA/acyl carrier protein (ACP) thioester, followed by MupS/TmlS catalyzed reduction. Mix and match experiments show MupQ/TmlQ to be highly selective for the correct CoA. MacpD/TacpD were interchangeable but alternate trans-acting ACPs from the mupirocin pathway (MacpA/TacpA) or a heterologous ACP (BatA) were nonfunctional. MupS and TmIS selectivity was more varied, and these reductases differed in their substrate and ACP selectivity. The solution structure of MacpD determined by NMR revealed a C-terminal extension with partial helical character that has been shown to be important for maintaining high titers of mupirocin. We generated a truncated MacpD construct, MacpD_T, which lacks this C-terminal extension but retains an ability to generate 3-HP with MupS and MupQ suggesting further downstream roles in protein-protein interactions for this region of the ACP.

With growing understanding of the underlying pathways of polyketide biosynthesis, along with the continual expansion of the synthetic biology toolkit, it is becoming possible to rationally engineer and fine-tune the polyketide biosynthetic machinery for production of new compounds with improved properties such as stability and/or bioactivity. However, engineering the pathway to the thiomarinol antibiotics has proved challenging. Here we report that genes from a marine Pseudoalternomonas sp. producing thiomarinol can be expressed in functional form in the biosynthesis of the clinically important antibiotic mupirocin from the soil bacterium Pseudomonas fluorescens. It is revealed that both pathways employ the same unusual mechanism of tetrahydropyran (THP) ring formation and the enzymes are cross compatible. Furthermore, the efficiency of downstream processing of 10,11-epoxy versus 10,11-alkenic metabolites are comparable. Optimisation of the fermentation conditions in an engineered strain in which production of pseudomonic acid A (with the 10,11-epoxide) is replaced by substantial titres of the more stable pseudomonic acid C (with a 10,11-alkene) pave the way for its development as a more stable antibiotic with wider applications than mupirocin.

AbstractThe presence of β‐branches in the structure of polyketides that possess potent biological activity underpins the widespread importance of this structural feature. Kalimantacin is a polyketide antibiotic with selective activity against staphylococci, and its biosynthesis involves the unprecedented incorporation of three different and sequential β‐branching modifications. We use purified single and multi‐domain enzyme components of the kalimantacin biosynthetic machinery to address in vitro how the pattern of β‐branching in kalimantacin is controlled. Robust discrimination of enzyme products required the development of a generalisable assay that takes advantage of 13C NMR of a single 13C label incorporated into key biosynthetic mimics combined with favourable dynamic properties of an acyl carrier protein. We report a previously unassigned modular enoyl‐CoA hydratase (mECH) domain and the assembly of enzyme constructs and cascades that are able to generate each specific β‐branch.