Insect-Associated Bacteria Assemble the Antifungal Butenolide Gladiofungin by Non-Canonical Polyketide Chain Termination

Microbial associates of the elm leaf beetle: uncovering the absence of resident bacteria and the influence of fungi on insect performance

Microbial symbionts play crucial roles in the biology of many insects. While bacteria have been the primary focus of research on insect-microbe symbiosis, recent studies suggest that fungal symbionts may be just as important. The elm leaf beetle (ELB, Xanthogaleruca luteola) is a serious pest species of field elm (Ulmus minor). Using culture-dependent and independent methods, we investigated the abundance and species richness of bacteria and fungi throughout various ELB life stages and generations, while concurrently analyzing microbial communities on elm leaves. No persistent bacterial community was found to be associated with the ELB or elm leaves. By contrast, fungi were persistently present in the beetle's feeding life stages and on elm leaves. Fungal community sequencing revealed a predominance of the genera Penicillium and Aspergillus in insects and on leaves. Culture-dependent surveys showed a high prevalence of two fungal colony morphotypes closely related to Penicillium lanosocoeruleum and Aspergillus flavus. Among these, the Penicillium morphotype was significantly more abundant on feeding-damaged compared with intact leaves, suggesting that the fungus thrives in the presence of the ELB. We assessed whether the detected prevalent fungal morphotypes influenced ELB's performance by rearing insects on (i) surface-sterilized leaves, (ii) leaves inoculated with Penicillium spores, and (iii) leaves inoculated with Aspergillus spores. Insects feeding on Penicillium-inoculated leaves gained more biomass and tended to lay larger egg clutches than those consuming surface-sterilized leaves or Aspergillus-inoculated leaves. Our results demonstrate that the ELB does not harbor resident bacteria and that it might benefit from associating with Penicillium fungi.IMPORTANCEOur study provides insights into the still understudied role of microbial symbionts in the biology of the elm leaf beetle (ELB), a major pest of elms. Contrary to expectations, we found no persistent bacterial symbionts associated with the ELB or elm leaves. Our research thus contributes to the growing body of knowledge that not all insects rely on bacterial symbionts. While no persistent bacterial symbionts were detectable in the ELB and elm leaf samples, our analyses revealed the persistent presence of fungi, particularly Penicillium and Aspergillus on both elm leaves and in the feeding ELB stages. Moreover, when ELB were fed with fungus-treated elm leaves, we detected a potentially beneficial effect of Penicillium on the ELB's development and fecundity. Our results highlight the significance of fungal symbionts in the biology of this insect.

Insights into group-specific pattern of secondary metabolite gene cluster in Burkholderia genus

Burkholderia is a versatile strain that has expanded into several genera. It has been steadily reported that the genome features of Burkholderia exhibit activities ranging from plant growth promotion to pathogenicity across various isolation areas. The objective of this study was to investigate the secondary metabolite patterns of 366 Burkholderia species through comparative genomics. Samples were selected based on assembly quality assessment and similarity below 80% in average nucleotide identity. Duplicate samples were excluded. Samples were divided into two groups using FastANI analysis. Group A included B. pseudomallei complex. Group B included B. cepacia complex. The limitations of MLST were proposed. The detection of genes was performed, including environmental and virulence-related genes. In the pan-genome analysis, each complex possessed a similar pattern of cluster for orthologous groups. Group A (n = 185) had 14,066 cloud genes, 2,465 shell genes, 682 soft-core genes, and 2,553 strict-core genes. Group B (n = 181) had 39,867 cloud genes, 4,986 shell genes, 324 soft-core genes, 222 core genes, and 2,949 strict-core genes. AntiSMASH was employed to analyze the biosynthetic gene cluster (BGC). The results were then utilized for network analysis using BiG-SCAPE and CORASON. Principal component analysis was conducted and a table was constructed using the results obtained from antiSMASH. The results were divided into Group A and Group B. We expected the various species to show similar patterns of secondary metabolite gene clusters. For in-depth analysis, a network analysis of secondary metabolite gene clusters was conducted, exemplified by BiG-SCAPE analysis. Depending on the species and complex, Burkholderia possessed several kinds of siderophore. Among them, ornibactin was possessed in most Burkholderia and was clustered into 4,062 clans. There was a similar pattern of gene clusters depending on the species. NRPS_04014 belonged to siderophore BGCs including ornibactin and indigoidine. However, it was observed that each family included a similar species. This suggests that, besides siderophores being species-specific, the ornibactin gene cluster itself might also be species-specific. The results suggest that siderophores are associated with environmental adaptation, possessing a similar pattern of siderophore gene clusters among species, which could provide another perspective on species-specific environmental adaptation mechanisms.

Deazaflavin metabolite produced by endosymbiotic bacteria controls fungal host reproduction

The endosymbiosis between the pathogenic fungus Rhizopus microsporus and the toxin-producing bacterium Mycetohabitans rhizoxinica represents a unique example of host control by an endosymbiont. Fungal sporulation strictly depends on the presence of endosymbionts as well as bacterially produced secondary metabolites. However, an influence of primary metabolites on host control remained unexplored. Recently, we discovered that M. rhizoxinica produces FO and 3PG-F420, a derivative of the specialized redox cofactor F420. Whether FO/3PG-F420 plays a role in the symbiosis has yet to be investigated. Here, we report that FO, the precursor of 3PG-F420, is essential to the establishment of a stable symbiosis. Bioinformatic analysis revealed that the genetic inventory to produce cofactor 3PG-F420 is conserved in the genomes of eight endofungal Mycetohabitans strains. By developing a CRISPR/Cas-assisted base editing strategy for M. rhizoxinica, we generated mutant strains deficient in 3PG-F420 (M. rhizoxinica ΔcofC) and in both FO and 3PG-F420 (M. rhizoxinica ΔfbiC). Co-culture experiments demonstrated that the sporulating phenotype of apo-symbiotic R. microsporus is maintained upon reinfection with wild-type M. rhizoxinica or M. rhizoxinica ΔcofC. In contrast, R. microsporus is unable to sporulate when co-cultivated with M. rhizoxinica ΔfbiC, even though the fungus was observed by super-resolution fluorescence microscopy to be successfully colonized. Genetic and chemical complementation of the FO deficiency of M. rhizoxinica ΔfbiC led to restoration of fungal sporulation, signifying that FO is indispensable for establishing a functional symbiosis. Even though FO is known for its light-harvesting properties, our data illustrate an important role of FO in inter-kingdom communication.

The Genomic-Driven Discovery of Glutarimide-Containing Derivatives from Burkholderia gladioli

Glutarimide-containing polyketides exhibiting potent antitumor and antimicrobial activities were encoded via conserved module blocks in various strains that favor the genomic mining of these family compounds. The bioinformatic analysis of the genome of Burkholderia gladioli ATCC 10248 showed a silent trans-AT PKS biosynthetic gene cluster (BGC) on chromosome 2 (Chr2C8), which was predicted to produce new glutarimide-containing derivatives. Then, the silent polyketide synthase gene cluster was successfully activated via in situ promoter insertion and heterologous expression. As a result, seven glutarimide-containing analogs, including five new ones, gladiofungins D-H (3-7), and two known gladiofungin A/gladiostatin (1) and 2 (named gladiofungin C), were isolated from the fermentation of the activated mutant. Their structures were elucidated through the analysis of HR-ESI-MS and NMR spectroscopy. The structural diversities of gladiofungins may be due to the degradation of the butenolide group in gladiofungin A (1) during the fermentation and extraction process. Bioactivity screening showed that 2 and 4 had moderate anti-inflammatory activities. Thus, genome mining combined with promoter engineering and heterologous expression were proved to be effective strategies for the pathway-specific activation of the silent BGCs for the directional discovery of new natural products.

Discovery and Biosynthesis of Anthrochelin, a Growth-Promoting Metallophore of the Human Pathogen Luteibacter anthropi

Various human pathogens have emerged from environmental strains by adapting to higher growth temperatures and the ability to produce virulence factors. A remarkable example of a pathoadapted bacterium is found in the genus Luteibacter, which typically comprises harmless soil microbes, yet Luteibacter anthropi was isolated from the blood of a diseased child. Up until now, nothing has been known about the specialized metabolism of this pathogen. By comparative genome analyses we found that L. anthropi has a markedly higher biosynthetic potential than other bacteria of this genus and uniquely bears an NRPS gene locus tentatively coding for the biosynthesis of a metallophore. By metabolic profiling, stable isotope labeling, and NMR investigation of a gallium complex, we identified a new family of salicylate-derived nonribosomal peptides named anthrochelins A-D. Surprisingly, anthrochelins feature a C-terminal homocysteine tag, which might be introduced during peptide termination. Mutational analyses provided insight into the anthrochelin assembly and revealed the unexpected involvement of a cytochrome P450 monooxygenase in oxazole formation. Notably, this heterocycle plays a key role in the binding of metals, especially copper(II). Bioassays showed that anthrochelin significantly promotes the growth of L. anthropi in the presence of low and high copper concentrations, which occur during infections.

Genomic and phenotypic characterization of a red-pigmented strain of Massilia frigida isolated from an Antarctic microbial mat

The McMurdo Dry Valleys of Antarctica experience a range of selective pressures, including extreme seasonal variation in temperature, water and nutrient availability, and UV radiation. Microbial mats in this ecosystem harbor dense concentrations of biomass in an otherwise desolate environment. Microbial inhabitants must mitigate these selective pressures via specialized enzymes, changes to the cellular envelope, and the production of secondary metabolites, such as pigments and osmoprotectants. Here, we describe the isolation and characterization of a Gram-negative, rod-shaped, motile, red-pigmented bacterium, strain DJPM01, from a microbial mat within the Don Juan Pond Basin of Wright Valley. Analysis of strain DJMP01's genome indicates it can be classified as a member of the Massilia frigida species. The genome contains several genes associated with cold and salt tolerance, including multiple RNA helicases, protein chaperones, and cation/proton antiporters. In addition, we identified 17 putative secondary metabolite gene clusters, including a number of nonribosomal peptides and ribosomally synthesized and post-translationally modified peptides (RiPPs), among others, and the biosynthesis pathway for the antimicrobial pigment prodigiosin. When cultivated on complex agar, multiple prodiginines, including the antibiotic prodigiosin, 2-methyl-3-propyl-prodiginine, 2-methyl-3-butyl-prodiginine, 2-methyl-3-heptyl-prodiginine, and cycloprodigiosin, were detected by LC-MS. Genome analyses of sequenced members of the Massilia genus indicates prodigiosin production is unique to Antarctic strains. UV-A radiation, an ecological stressor in the Antarctic, was found to significantly decrease the abundance of prodiginines produced by strain DJPM01. Genomic and phenotypic evidence indicates strain DJPM01 can respond to the ecological conditions of the DJP microbial mat, with prodiginines produced under a range of conditions, including extreme UV radiation.

Labelling studies in the biosynthesis of polyketides and non-ribosomal peptides

Covering: 2015 to 2022In this review, we discuss the recent advances in the use of isotopically labelled compounds to investigate the biosynthesis of polyketides, non-ribosomally synthesised peptides, and their hybrids. Also, we highlight the use of isotopes in the elucidation of their structures and investigation of enzyme mechanisms. The biosynthetic pathways of selected examples are presented in detail to reveal the principles of the discussed labelling experiments. The presented examples demonstrate that the application of isotopically labelled compounds is still the state of the art and can provide valuable information for the biosynthesis of natural products.

Compilation of the Antimicrobial Compounds Produced by Burkholderia Sensu Stricto

Due to the increase in multidrug-resistant microorganisms, the investigation of novel or more efficient antimicrobial compounds is essential. The World Health Organization issued a list of priority multidrug-resistant bacteria whose eradication will require new antibiotics. Among them, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae are in the "critical" (most urgent) category. As a result, major investigations are ongoing worldwide to discover new antimicrobial compounds. Burkholderia, specifically Burkholderia sensu stricto, is recognized as an antimicrobial-producing group of species. Highly dissimilar compounds are among the molecules produced by this genus, such as those that are unique to a particular strain (like compound CF66I produced by Burkholderia cepacia CF-66) or antimicrobials found in a number of species, e.g., phenazines or ornibactins. The compounds produced by Burkholderia include N-containing heterocycles, volatile organic compounds, polyenes, polyynes, siderophores, macrolides, bacteriocins, quinolones, and other not classified antimicrobials. Some of them might be candidates not only for antimicrobials for both bacteria and fungi, but also as anticancer or antitumor agents. Therefore, in this review, the wide range of antimicrobial compounds produced by Burkholderia is explored, focusing especially on those compounds that were tested in vitro for antimicrobial activity. In addition, information was gathered regarding novel compounds discovered by genome-guided approaches.

Adaptive Plasticity of Insect Eggs in Response to Environmental Challenges

Insect eggs are exposed to a plethora of abiotic and biotic threats. Their survival depends on both an innate developmental program and genetically determined protective traits provided by the parents. In addition, there is increasing evidence that (a) parents adjust the egg phenotype to the actual needs, (b) eggs themselves respond to environmental challenges, and (c) egg-associated microbes actively shape the egg phenotype. This review focuses on the phenotypic plasticity of insect eggs and their capability to adjust themselves to their environment. We outline the ways in which the interaction between egg and environment is two-way, with the environment shaping the egg phenotype but also with insect eggs affecting their environment. Specifically, insect eggs affect plant defenses, host biology (in the case of parasitoid eggs), and insect oviposition behavior. We aim to emphasize that the insect egg, although it is a sessile life stage, actively responds to and interacts with its environment.

Synthesis and Antifungal Activity of New butenolide Containing Methoxyacrylate Scaffold

In order to improve the antifungal activity of new butenolides containing oxime ether moiety, a series of new butenolide compounds containing methoxyacrylate scaffold were designed and synthesized, based on the previous reports. Their structures were characterized by ¹H NMR, ¹³C NMR, HR-MS spectra, and X-ray diffraction analysis. The in vitro antifungal activities were evaluated by the mycelium growth rate method. The results showed that the inhibitory activities of these new compounds against Sclerotinia sclerotiorum were significantly improved, in comparison with that of the lead compound 3-8; the EC₅₀ values of V-6 and VI-7 against S. sclerotiorum were 1.51 and 1.81 mg/L, nearly seven times that of 3-8 (EC₅₀ 10.62 mg/L). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation indicated that compound VI-3 had a significant impact on the structure and function of the hyphal cell of S. sclerotiorum mycelium and the positive control trifloxystrobin. Molecular simulation docking results indicated that the introduction of methoxyacrylate scaffold is beneficial to improving the antifungal activity of these compounds against S. sclerotiorum, which can be used as the lead for further structure optimization.

Biotechnology approaches for natural product discovery, engineering, and production based on Burkholderia bacteria

Bacterial natural products (NPs) retain high value in discovery efforts for applications in medicine and agriculture. Burkholderia β-Proteobacteria are a promising source of NPs. In this review, we summarize the recently developed genetic manipulation techniques used to access silent/cryptic biosynthetic gene clusters from Burkholderia native producers. We also discuss the development of Burkholderia bacteria as heterologous hosts and the application of Burkholderia in industrial-scale production of NPs. Genetic engineering and fermentation media optimization have enabled the industrial-scale production of at least two Burkholderia NPs. The biotechnology approaches discussed here will continue to facilitate the discovery and development of NPs from Burkholderia.

Deciphering Chemical Mediators Regulating Specialized Metabolism in a Symbiotic Cyanobacterium

Genomes of cyanobacteria feature a variety of cryptic biosynthetic pathways for complex natural products, but the peculiarities limiting the discovery and exploitation of the metabolic dark matter are not well understood. Here we describe the discovery of two cell density‐dependent chemical mediators, nostoclide and nostovalerolactone, in the symbiotic model strain Nostoc punctiforme, and demonstrate their pronounced impact on the regulation of specialized metabolism. Through transcriptional, bioinformatic and labeling studies we assigned two adjacent biosynthetic gene clusters to the biosynthesis of the two polyketide mediators. Our findings provide insight into the orchestration of specialized metabolite production and give lessons for the genomic mining and high‐titer production of cyanobacterial bioactive compounds.

Beetle-Bacterial Symbioses: Endless Forms Most Functional

Beetles are hosts to a remarkable diversity of bacterial symbionts. In this article, we review the role of these partnerships in promoting beetle fitness following a surge of recent studies characterizing symbiont localization and function across the Coleoptera. Symbiont contributions range from the supplementation of essential nutrients and digestive or detoxifying enzymes to the production of bioactive compounds providing defense against natural enemies. Insights on this functional diversity highlight how symbiosis can expand the host's ecological niche, but also constrain its evolutionary potential by promoting specialization. As bacterial localization can differ within and between beetle clades, we discuss how it corresponds to the microbe's beneficial role and outline the molecular and behavioral mechanisms underlying symbiont translocation and transmission by its holometabolous host. In reviewing this literature, we emphasize how the study of symbiosis can inform our understanding of the phenotypic innovations behind the evolutionary success of beetles.

Needles in haystacks: reevaluating old paradigms for the discovery of bacterial secondary metabolites

Covering: up to 2021Natural products research is in the midst of a renaissance ushered in by a modern understanding of microbiology and the technological explosions of genomics and metabolomics. As the exploration of uncharted chemical space expands into high-throughput discovery campaigns, it has become increasingly clear how design elements influence success: (bio)geography, habitat, community dynamics, culturing/induction methods, screening methods, dereplication, and more. We explore critical considerations and assumptions in natural products discovery. We revisit previous estimates of chemical rediscovery and discuss their relatedness to study design and producer taxonomy. Through frequency analyses of biosynthetic gene clusters in publicly available genomic data, we highlight phylogenetic biases that influence rediscovery rates. Through selected examples of how study design at each level determines discovery outcomes, we discuss the challenges and opportunities for the future of high-throughput natural product discovery.

Chain release mechanisms in polyketide and non-ribosomal peptide biosynthesis

Review covering up to mid-2021The structure of polyketide and non-ribosomal peptide natural products is strongly influenced by how they are released from their biosynthetic enzymes. As such, Nature has evolved a diverse range of release mechanisms, leading to the formation of bioactive chemical scaffolds such as lactones, lactams, diketopiperazines, and tetronates. Here, we review the enzymes and mechanisms used for chain release in polyketide and non-ribosomal peptide biosynthesis, how these mechanisms affect natural product structure, and how they could be utilised to introduce structural diversity into the products of engineered biosynthetic pathways.

Discovery of the Pseudomonas Polyyne Protegencin by a Phylogeny-Guided Study of Polyyne Biosynthetic Gene Cluster Diversity

Natural products that possess alkyne or polyyne moieties have been isolated from a variety of biological sources and possess a broad a range of bioactivities. In bacteria, the basic biosynthesis of polyynes is known, but their biosynthetic gene cluster (BGC) distribution and evolutionary relationship to alkyne biosynthesis have not been addressed. Through comprehensive genomic and phylogenetic analyses, the distribution of alkyne biosynthesis gene cassettes throughout bacteria was explored, revealing evidence of multiple horizontal gene transfer events. After investigation of the evolutionary connection between alkyne and polyyne biosynthesis, a monophyletic clade was identified that possessed a conserved seven-gene cassette for polyyne biosynthesis that built upon the conserved three-gene cassette for alkyne biosynthesis. Further diversity mapping of the conserved polyyne gene cassette revealed a phylogenetic subclade for an uncharacterized polyyne BGC present in several Pseudomonas species, designated pgn. Pathway mutagenesis and high-resolution analytical chemistry showed the Pseudomonas protegenspgn BGC directed the biosynthesis of a novel polyyne, protegencin. Exploration of the biosynthetic logic behind polyyne production, through BGC mutagenesis and analytical chemistry, highlighted the essentiality of a triad of desaturase proteins and a thioesterase in both the P. protegenspgn and Trinickia caryophylli (formerly Burkholderia caryophylli) caryoynencin pathways. We have unified and expanded knowledge of polyyne diversity and uniquely demonstrated that alkyne and polyyne biosynthetic gene clusters are evolutionarily related and widely distributed within bacteria. The systematic mapping of conserved biosynthetic genes across the available bacterial genomic diversity proved to be a fruitful method for discovering new natural products and better understanding polyyne biosynthesis.

Sesbanimide R, a Novel Cytotoxic Polyketide Produced by Magnetotactic Bacteria

Genomic information from various magnetotactic bacteria suggested that besides their common ability to form magnetosomes, they potentially also represent a source of bioactive natural products. By using targeted deletion and transcriptional activation, we connected a large biosynthetic gene cluster (BGC) of the trans-acyltransferase polyketide synthase (trans-AT PKS) type to the biosynthesis of a novel polyketide in the alphaproteobacterium Magnetospirillum gryphiswaldense Structure elucidation by mass spectrometry and nuclear magnetic resonance spectroscopy (NMR) revealed that this secondary metabolite resembles sesbanimides, which were very recently reported from other taxa. However, sesbanimide R exhibits an additional arginine moiety the presence of which reconciles inconsistencies in the previously proposed sesbanimide biosynthesis pathway observed when comparing the chemical structure and the potential biochemistry encoded in the BGC. In contrast to the case with sesbanimides D, E, and F, we were able to assign the stereocenter of the arginine moiety experimentally and two of the remaining three stereocenters by predictive biosynthetic tools. Sesbanimide R displayed strong cytotoxic activity against several carcinoma cell lines.IMPORTANCE The findings of this study contribute a new secondary metabolite member to the glutarimide-containing polyketides. The determined structure of sesbanimide R correlates with its cytotoxic bioactivity, characteristic for members of this family. Sesbanimide R represents the first natural product isolated from magnetotactic bacteria and identifies this highly diverse group as a so-far-untapped source for the future discovery of novel secondary metabolites.

Phylogenetic analysis of the salinipostin γ-butyrolactone gene cluster uncovers new potential for bacterial signalling-molecule diversity

Bacteria communicate by small-molecule chemicals that facilitate intra- and inter-species interactions. These extracellular signalling molecules mediate diverse processes including virulence, bioluminescence, biofilm formation, motility and specialized metabolism. The signalling molecules produced by members of the phylum Actinobacteria generally comprise γ-butyrolactones, γ-butenolides and furans. The best-known actinomycete γ-butyrolactone is A-factor, which triggers specialized metabolism and morphological differentiation in the genus Streptomyces . Salinipostins A–K are unique γ-butyrolactone molecules with rare phosphotriester moieties that were recently characterized from the marine actinomycete genus Salinispora . The production of these compounds has been linked to the nine-gene biosynthetic gene cluster (BGC) spt . Critical to salinipostin assembly is the γ-butyrolactone synthase encoded by spt9 . Here, we report the surprising distribution of spt9 homologues across 12 bacterial phyla, the majority of which are not known to produce γ-butyrolactones. Further analyses uncovered a large group of spt -like gene clusters outside of the genus Salinispora , suggesting the production of new salinipostin-like diversity. These gene clusters show evidence of horizontal transfer and location-specific recombination among Salinispora strains. The results suggest that γ-butyrolactone production may be more widespread than previously recognized. The identification of new γ-butyrolactone BGCs is the first step towards understanding the regulatory roles of the encoded small molecules in Actinobacteria.

Identification of trans-AT polyketide clusters in two marine bacteria reveals cryptic similarities between distinct symbiosis factors

Glutarimide-containing polyketides are known as potent antitumoral and antimetastatic agents. The associated gene clusters have only been identified in a few Streptomyces producers and Burkholderia gladioli symbiont. The new glutarimide-family polyketides, denominated sesbanimides D, E and F along with the previously known sesbanimide A and C, were isolated from two marine alphaproteobacteria Stappia indica PHM037 and Labrenzia aggregata PHM038. Structures of the isolated compounds were elucidated based on 1D and 2D homo and heteronuclear NMR analyses and ESI-MS spectrometry. All compounds exhibited strong antitumor activity in lung, breast and colorectal cancer cell lines. Subsequent whole genome sequencing and genome mining revealed the presence of the trans-AT PKS gene cluster responsible for the sesbanimide biosynthesis, described as sbn cluster. Strikingly, the modular architecture of downstream mixed type PKS/NRPS, SbnQ, revealed high similarity to PedH in pederin and Lab13 in labrenzin gene clusters, although those clusters are responsible for the production of structurally completely different molecules. The unexpected presence of SbnQ homologues in unrelated polyketide gene clusters across phylogenetically distant bacteria, raises intriguing questions about the evolutionary relationship between glutarimide-like and pederin-like pathways, as well as the functionality of their synthetic products.

Genomics-Driven Activation of Silent Biosynthetic Gene Clusters in Burkholderia gladioli by Screening Recombineering System

The Burkholderia genus possesses ecological and metabolic diversities. A large number of silent biosynthetic gene clusters (BGCs) in the Burkholderia genome remain uncharacterized and represent a promising resource for new natural product discovery. However, exploitation of the metabolomic potential of Burkholderia is limited by the absence of efficient genetic manipulation tools. Here, we screened a bacteriophage recombinase system Redγ-BAS, which was functional for genome modification in the plant pathogen Burkholderia gladioli ATCC 10248. By using this recombineering tool, the constitutive promoters were precisely inserted in the genome, leading to activation of two silent nonribosomal peptide synthetase gene clusters (bgdd and hgdd) and production of corresponding new classes of lipopeptides, burriogladiodins A–H (1–8) and haereogladiodins A–B (9–10). Structure elucidation revealed an unnatural amino acid Z- dehydrobutyrine (Dhb) in 1–8 and an E-Dhb in 9–10. Notably, compounds 2–4 and 9 feature an unusual threonine tag that is longer than the predicted collinearity assembly lines. The structural diversity of burriogladiodins was derived from the relaxed substrate specificity of the fifth adenylation domain as well as chain termination conducted by water or threonine. The recombinase-mediating genome editing system is not only applicable in B. gladioli, but also possesses great potential for mining meaningful silent gene clusters from other Burkholderia species.

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 pedrolide from Euphorbia pedroi.

Insect-Associated Bacteria Assemble the Antifungal Butenolide Gladiofungin by Non-Canonical Polyketide Chain Termination

Genome mining of one of the protective symbionts (Burkholderia gladioli) of the invasive beetle Lagria villosa revealed a cryptic gene cluster that codes for the biosynthesis of a novel antifungal polyketide with a glutarimide pharmacophore. Targeted gene inactivation, metabolic profiling, and bioassays led to the discovery of the gladiofungins as previously‐overlooked components of the antimicrobial armory of the beetle symbiont, which are highly active against the entomopathogenic fungus Purpureocillium lilacinum. By mutational analyses, isotope labeling, and computational analyses of the modular polyketide synthase, we found that the rare butenolide moiety of gladiofungins derives from an unprecedented polyketide chain termination reaction involving a glycerol‐derived C3 building block. The key role of an A‐factor synthase (AfsA)‐like offloading domain was corroborated by CRISPR‐Cas‐mediated gene editing, which facilitated precise excision within a PKS domain.

Functional Analysis of P450 Monooxygenase SrrO in the Biosynthesis of Butenolide-Type Signaling Molecules in Streptomyces rochei

Streptomyces rochei 7434AN4 produces two structurally unrelated polyketide antibiotics lankacidin and lankamycin, and their biosynthesis is tightly controlled by butenolide-type signaling molecules SRB1 and SRB2. SRBs are synthesized by SRB synthase SrrX, and induce lankacidin and lankamycin production at 40 nM concentration. We here investigated the role of a P450 monooxygenase gene srrO (orf84), which is located adjacent to srrX (orf85), in SRB biosynthesis. An srrO mutant KA54 accumulated lankacidin and lankamycin at a normal level when compared with the parent strain. To elucidate the chemical structures of the signaling molecules accumulated in KA54 (termed as KA54-SRBs), this mutant was cultured (30 L) and the active components were purified. Two active components (KA54-SRB1 and KA54-SRB2) were detected in ESI-MS and chiral HPLC analysis. The molecular formulae for KA54-SRB1 and KA54-SRB2 are C₁₅H₂₆O₄ and C₁₆H₂₈O₄, whose values are one oxygen smaller and two hydrogen larger when compared with those for SRB1 and SRB2, respectively. Based on extensive NMR analysis, the signaling molecules in KA54 were determined to be 6'-deoxo-SRB1 and 6'-deoxo-SRB2. Gel shift analysis indicated that a ligand affinity of 6'-deoxo-SRB1 to the specific receptor SrrA was 100-fold less than that of SRB1. We performed bioconversion of the synthetic 6'-deoxo-SRB1 in the Streptomyces lividans recombinant carrying SrrO-expression plasmid. Substrate 6'-deoxo-SRB1 was converted through 6'-deoxo-6'-hydroxy-SRB1 to SRB1 in a time-dependent manner. Thus, these results clearly indicated that SrrO catalyzes the C-6' oxidation at a final step in SRB biosynthesis.

MmfL catalyses formation of a phosphorylated butenolide intermediate in methylenomycin furan biosynthesis

MmfL forms phosphorylated butenolides that undergo dephosphorylation and rearrangement to yield methylenomycin furan (MMF) signalling molecules that induce antibiotic production in Streptomyces coelicolor.