Discovery of a Cyclic Depsipeptide from Chaetomium mollipilium by the Genome Mining Approach

BGC heteroexpression strategy for production of novel microbial secondary metabolites

Biosynthetic gene clusters (BGCs) in microbial genomes play a crucial role in the biosynthesis of diverse secondary metabolites (SMs) with pharmaceutical potential. However, most BGCs remain silent under conventional conditions, resulting in the frequently repeated discovery of known SMs. Fortunately, in the past two decades, the heterologous expression of BGCs in genetically tractable hosts has emerged as a powerful strategy to awaken microbial metabolic pathways for making novel microbial SMs. In this review, we comprehensively delineated the development and application of this strategy, highlighting various BGC cloning and assembly techniques and their technical characteristics. We also summarized 519 novel SMs from BGC hetero-expression-derived strains and described their occurrence, bioactivity, mode of action, and biosynthetic logic. Lastly, current challenges and future perspectives for developing more efficient BGC hetero-expression strategies were discussed in this review.

Enzymatic ester bond formation strategies in fungal macrolide skeletons

Covering: up to August 2024Macrolides, the core skeletons of numerous marketed drugs and bioactive natural products, have garnered considerable scientific interest owing to their structural diversity and broad spectrum of pharmaceutical activities. The formation of intramolecular ester bonds is a critical biocatalytic step in constructing macrolide skeletons. Here, we summarised enzymatic ester bond formation strategies in fungal polyketide (PK)-type, nonribosomal peptide (NRP)-type, and PK-NRP hybrid-type macrolides. In PK-type macrolides, ester bond formation is commonly catalysed by a trans-acting thioesterase (TE) or a cis-acting TE domain during the product release process. In NRP-type and PK-NRP hybrid-type macrolides, the ester bond is usually introduced through condensation (C) domain-catalysed esterification during the elongation or product release step. Although the TE and C domains share similarities in their catalytic mechanism, using hydroxyl groups as nucleophiles in an intramolecular nucleophilic attack, they differ in terms of the hydroxyl origin, the timing of ester bond formation, and domain location. Furthermore, some TE domains are utilized as chemoenzymatic catalysts to construct macrolides with different ring sizes. A comparison of ester bond formation between fungi and bacteria is also discussed. Exploring the biosynthetic pathways of fungal macrolides, elucidating the diverse strategies employed in the formation of ester bonds, and understanding the application of enzymes/domains in chemoenzymatic synthesis hold promise for the discovery of new bioactive macrolides in the future.

Genome mining of labdane-related diterpenoids: Discovery of the two-enzyme pathway leading to (-)-sandaracopimaradiene in the fungus Arthrinium sacchari

Labdane-related diterpenoids (LRDs) in fungi are a pharmaceutically important, but underexplored family of natural products. In the biosynthesis of fungal LRDs, bifunctional terpene cyclases (TCs) consisting of αβγ domains are generally used to synthesize the polycyclic skeletones of LRDs. Herein, we conducted genome mining of LRDs in our fungal genome database and identified a unique pair of TCs, AsPS and AsCPS, in the fungus Arthrinium sacchari. AsPS consists of catalytically active α and inactive β domains, whereas AsCPS contains βγ domains and a truncated α domain. Heterologous expression in Aspergillus oryzae and biochemical characterization of recombinant proteins demonstrated that AsCPS synthesized copalyl diphosphate and that AsPS then converted it to (-)-sandaracopimaradiene. Since AsPS and AsCPS have distinct domain organizations from those of known fungal TCs and are likely generated through fusion or loss of catalytic domains, our findings provide insight into the evolution of TCs in fungi.

Identification and Functional Characterization of Fungal Chalcone Synthase and Chalcone Isomerase

By mining fungal genomic information, a noncanonical iterative type I PKS fused with an N-terminal adenylation-thiolation didomain, which catalyzes the formation of naringenin chalcone, was found. Structural prediction and molecular docking analysis indicated that a C-terminal thioesterase domain was involved in the Claisen-type cyclization. An enzyme responsible for formation of (2S)-flavanone in the biosynthesis of fungal flavonoids was also identified. Collectively, these findings demonstrate unprecedented fungal biosynthetic machinery leading to plant-like metabolites.

Reactive Azlactone Intermediate Drives Fungal Secondary Metabolite Cross-Pathway Generation

Pathogenic fungi of Aspergillus section Fumigati are known to produce various secondary metabolites. A reported isolation of a compound with an atypical carbon skeleton called fumimycin from A. fumisynnematus prompted us to examine a related fungus, A. lentulus, for production of similar products. Here we report the isolation of fumimycin and a related new racemic compound we named lentofuranine. Detailed analyses revealed that both compounds were assembled by a nonenzymatic condensation of a polyketide intermediate from the terrein biosynthetic pathway and a highly reactive azlactone intermediate produced by an unrelated nonribosomal peptide synthetase carrying a terminal condensation-like domain. While highly reactive azlactone is commonly used in chemical synthesis, its production by a conventional non-metalloenzyme and employment as a biosynthetic pathway intermediate is unprecedented. The observed unusual carbon skeleton formation is likely due to the reactivity of azlactone. Our finding provides another example of a chemical principle being aptly exploited by a biological system.

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A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as hyperispirone A from Hypericum beanii.