Synthesis of Gamma-Butyrolactone Hormones Enables Understanding of Natural Product Induction

Identification of γ-butyrolactone signalling molecules in diverse actinomycetes using resin-assisted isolation and chemoenzymatic synthesis

Actinomycetes are prolific producers of secondary metabolites with diverse bioactivities. Secondary metabolism in actinomycetes is regulated by signalling molecules, often termed "bacterial hormones." In Streptomyces griseus, the γ-butyrolactone (GBL) A-factor (1) plays a key role in regulating secondary metabolism, including streptomycin production. The widespread presence of afsA, the gene encoding A-factor synthase, suggests that GBLs are a major class of signalling molecules in actinomycetes. However, their identification is hindered by the requirement for large-scale cultures. This study presents two methodologies for identifying natural GBLs. First, a resin-assisted culture method combined with MS-guided screening enabled the isolation and structural determination of GBLs (2-5) from smaller-scale cultures. Second, a chemoenzymatic synthesis method involving one-pot three enzymatic reactions was developed, allowing the production of GBL standards (10a-10l). Using these standards, HR-LCMS analysis of 31 strains across 10 actinomycetes genera identified GBLs in nearly half of the tested strains, including genera where GBLs were detected for the first time. Chiral HPLC analysis further revealed the presence of the (3S)-isomer of GBL (11), an enantiomer of known GBLs. This study uncovers the widespread distribution and structural diversity of GBLs among actinomycetes, providing insights into their regulatory roles and potential for activating secondary metabolism, which may facilitate the discovery of new natural products.

Strain-release enables access to carbonyl conjugated allylic diborons and alkenyl boronates having multiple contiguous stereocenters in a one-pot process

Allylic diboronates are highly valuable reagents in organic synthesis. Existing methods predominantly yield alkyl-substituted allylic diboronates, while the incorporation of electrophilic carbonyl groups conjugated to these allylic systems remains unknown. We present a strain-release promoted cycloaddition-based strategy that enabled access to unprecedented carbonyl conjugated secondary allylic diborons. This mild base-free method also facilitated a one-pot multicomponent cycloaddition-allylboration sequence for a highly diastereoselective installation of contiguous quaternary, tertiary and secondary carbon centers on a scaffold featuring valuable β-hydroxy ester, β-vinyl ester and vinyl boronate motifs. The synthetic utility of these densely functionalized products was demonstrated through their transformation into other rare and sterically congested alkenylborons such as borylated spiro-fused β-lactones and bicyclic γ-butyrolactones. Detailed 11B NMR, deuterium labeling and mass spectrometry studies provided insights on an unexpected base-free deboronative allylic shift reaction of conjugated allylic diboronates.

A Diels-Alder probe for discovery of natural products containing furan moieties

Natural products (NPs) are fantastic sources of inspiration for novel pharmaceuticals, oftentimes showing unique bioactivity against interesting targets. Specifically, NPs containing furan moieties show activity against a variety of diseases including fungal infections, and cancers. However, it is challenging to discover and isolate these small molecules from cell supernatant. The work described herein showcases the development of a molecular probe that can covalently modify furan moieties via a [4 + 2] Diels-Alder cycloaddition, making them easily identifiable on liquid chromatography-mass spectrometry (LC-MS). The molecular probe, which undergoes this reaction with a variety of furans, was designed with both a UV-tag and a mass tag to enable easy identification. The probe has been tested with a variety of purified furans, including natural products, methylenomycin furan (MMF) hormones, and MMF derivatives. Moreover, the molecular probe has been tested in crude supernatants of various Streptomyces strains and enables identification of MMFs.

Methodology for awakening the potential secondary metabolic capacity in actinomycetes

Secondary metabolites produced by actinomycete strains undoubtedly have great potential for use in applied research areas such as drug discovery. However, it is becoming difficult to obtain novel compounds because of repeated isolation around the world. Therefore, a new strategy for discovering novel secondary metabolites is needed. Many researchers believe that actinomycetes have as yet unanalyzed secondary metabolic activities, and the associated undiscovered secondary metabolite biosynthesis genes are called "silent" genes. This review outlines several approaches to further activate the metabolic potential of actinomycetes.

Small molecule inducers of actinobacteria natural product biosynthesis

Actinobacteria are a large and diverse group of bacteria that are known to produce a wide range of secondary metabolites, many of which have important biological activities, including antibiotics, anti-cancer agents, and immunosuppressants. The biosynthesis of these compounds is often highly regulated with many natural products (NPs) being produced at very low levels in laboratory settings. Environmental factors, such as small molecule elicitors, can induce the production of secondary metabolites. Specifically, they can increase titers of known NPs as well as enabling discovery of novel NPs typically produced at undetectable levels. These elicitors can be NPs, including antibiotics or hormones, or synthetic compounds. In recent years, there has been a growing interest in the use of small molecule elicitors to induce the production of secondary metabolites from actinobacteria, especially for the discovery of NPs from "silent" biosynthetic gene clusters. This review aims to highlight classes of molecules that induce secondary metabolite production in actinobacteria and to describe the potential mechanisms of induction.

ONE-SENTENCE SUMMARY

This review describes chemical elicitors of actinobacteria natural products described to date and the proposed mechanisms of induction.