The unusual mode of action of the polyketide glycoside antibiotic cervimycin C.

Cervimycins A-D are bis-glycosylated polyketide antibiotics produced by Streptomyces tendae HKI 0179 with bactericidal activity against Gram-positive bacteria. In this study, cervimycin C (CmC) treatment caused a spaghetti-like phenotype in Bacillus subtilis 168, with elongated curved cells, which stayed joined after cell division, and exhibited a chromosome segregation defect, resulting in ghost cells without DNA. Electron microscopy of CmC-treated Staphylococcus aureus (3 × MIC) revealed swollen cells, misshapen septa, cell wall thickening, and a rough cell wall surface. Incorporation tests in B. subtilis indicated an effect on DNA biosynthesis at high cervimycin concentrations. Indeed, artificial downregulation of the DNA gyrase subunit B gene (gyrB) increased the activity of cervimycin in agar diffusion tests, and, in high concentrations (starting at 62.5 × MIC), the antibiotic inhibited S. aureus DNA gyrase supercoiling activity in vitro. To obtain a more global view on the mode of action of CmC, transcriptomics and proteomics of cervimycin treated versus untreated S. aureus cells were performed. Interestingly, 3 × MIC of cervimycin did not induce characteristic responses, which would indicate disturbance of the DNA gyrase activity in vivo. Instead, cervimycin induced the expression of the CtsR/HrcA heat shock operon and the expression of autolysins, exhibiting similarity to the ribosome-targeting antibiotic gentamicin. In summary, we identified the DNA gyrase as a target, but at low concentrations, electron microscopy and omics data revealed a more complex mode of action of cervimycin, which comprised induction of the heat shock response, indicating protein stress in the cell.IMPORTANCEAntibiotic resistance of Gram-positive bacteria is an emerging problem in modern medicine, and new antibiotics with novel modes of action are urgently needed. Secondary metabolites from Streptomyces species are an important source of antibiotics, like the cervimycin complex produced by Streptomyces tendae HKI 0179. The phenotypic response of Bacillus subtilis and Staphylococcus aureus toward cervimycin C indicated a chromosome segregation and septum formation defect. This effect was at first attributed to an interaction between cervimycin C and the DNA gyrase. However, omics data of cervimycin treated versus untreated S. aureus cells indicated a different mode of action, because the stress response did not include the SOS response but resembled the response toward antibiotics that induce mistranslation or premature chain termination and cause protein stress. In summary, these results point toward a possibly novel mechanism that generates protein stress in the cells and subsequently leads to defects in cell and chromosome segregation.

A mutualistic bacterium rescues a green alga from an antagonist.

Photosynthetic protists, known as microalgae, are key contributors to primary production on Earth. Since early in evolution, they coexist with bacteria in nature, and their mode of interaction shapes ecosystems. We have recently shown that the bacterium Pseudomonas protegens acts algicidal on the microalga Chlamydomonas reinhardtii. It secretes a cyclic lipopeptide and a polyyne that deflagellate, blind, and lyse the algae [P. Aiyar et al., Nat. Commun. 8, 1756 (2017) and V. Hotter et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2107695118 (2021)]. Here, we report about the bacterium Mycetocola lacteus, which establishes a mutualistic relationship with C. reinhardtii and acts as a helper. While M. lacteus enhances algal growth, it receives methionine as needed organic sulfur and the vitamins B1, B3, and B5 from the algae. In tripartite cultures with the alga and the antagonistic bacterium P. protegens, M. lacteus aids the algae in surviving the bacterial attack. By combining synthetic natural product chemistry with high-resolution mass spectrometry and an algal Ca2+ reporter line, we found that M. lacteus rescues the alga from the antagonistic bacterium by cleaving the ester bond of the cyclic lipopeptide involved. The resulting linearized seco acid does not trigger a cytosolic Ca2+ homeostasis imbalance that leads to algal deflagellation. Thus, the algae remain motile, can swim away from the antagonistic bacteria and survive the attack. All three involved genera cooccur in nature. Remarkably, related species of Pseudomonas and Mycetocola also act antagonistically against C. reinhardtii or as helper bacteria in tripartite cultures.

Analysis of Rhizonin Biosynthesis Reveals Origin of Pharmacophoric Furylalanine Moieties in Diverse Cyclopeptides.

Rhizonin A and B are hepatotoxic cyclopeptides produced by bacterial endosymbionts (Mycetohabitans endofungorum) of the fungus Rhizopus microsporus. Their toxicity critically depends on the presence of 3-furylalanine (Fua) residues, which also occur in pharmaceutically relevant cyclopeptides of the endolide and bingchamide families. The biosynthesis and incorporation of Fua by non-ribosomal peptide synthetases (NRPS), however, has remained elusive. By genome sequencing and gene inactivation we elucidated the gene cluster responsible for rhizonin biosynthesis. A suite of isotope labeling experiments identified tyrosine and l-DOPA as Fua precursors and provided the first mechanistic insight. Bioinformatics, mutational analysis and heterologous reconstitution identified dioxygenase RhzB as necessary and sufficient for Fua formation. RhzB is a novel type of heme-dependent aromatic oxygenases (HDAO) that enabled the discovery of the bingchamide biosynthesis gene cluster through genome mining.

Streptomyces polyketides mediate bacteria-fungi interactions across soil environments.

Although the interaction between prokaryotic and eukaryotic microorganisms is crucial for the functioning of ecosystems, information about the processes driving microbial interactions within communities remains scarce. Here we show that arginine-derived polyketides (arginoketides) produced by Streptomyces species mediate cross-kingdom microbial interactions with fungi of the genera Aspergillus and Penicillium, and trigger the production of natural products. Arginoketides can be cyclic or linear, and a prominent example is azalomycin F produced by Streptomyces iranensis, which induces the cryptic orsellinic acid gene cluster in Aspergillus nidulans. Bacteria that synthesize arginoketides and fungi that decode and respond to this signal were co-isolated from the same soil sample. Genome analyses and a literature search indicate that arginoketide producers are found worldwide. Because, in addition to their direct impact, arginoketides induce a secondary wave of fungal natural products, they probably contribute to the wider structure and functioning of entire soil microbial communities.

Bacterial ectosymbionts in cuticular organs chemically protect a beetle during molting stages.

In invertebrates, the cuticle is the first and major protective barrier against predators and pathogen infections. While immune responses and behavioral defenses are also known to be important for insect protection, the potential of cuticle-associated microbial symbionts to aid in preventing pathogen entry during molting and throughout larval development remains unexplored. Here, we show that bacterial symbionts of the beetle Lagria villosa inhabit unusual dorsal invaginations of the insect cuticle, which remain open to the outer surface and persist throughout larval development. This specialized location enables the release of several symbiont cells and the associated protective compounds during molting. This facilitates ectosymbiont maintenance and extended defense during larval development against antagonistic fungi. One Burkholderia strain, which produces the antifungal compound lagriamide, dominates the community across all life stages, and removal of the community significantly impairs the survival probability of young larvae when exposed to different pathogenic fungi. We localize both the dominant bacterial strain and lagriamide on the surface of eggs, larvae, pupae, and on the inner surface of the molted cuticle (exuvia), supporting extended protection. These results highlight adaptations for effective defense of immature insects by cuticle-associated ectosymbionts, a potentially key advantage for a ground-dwelling insect when confronting pathogenic microbes.

Cervimycin-Resistant Staphylococcus aureus Strains Display Vancomycin-Intermediate Resistant Phenotypes.

Resistance to antibiotics is an increasing problem and necessitates novel antibacterial therapies. The polyketide antibiotics cervimycin A to D are natural products of Streptomyces tendae HKI 0179 with promising activity against multidrug-resistant staphylococci and vancomycin-resistant enterococci. To initiate mode of action studies, we selected cervimycin C- and D-resistant (CmR) Staphylococcus aureus strains. Genome sequencing of CmR mutants revealed amino acid exchanges in the essential histidine kinase WalK, the Clp protease proteolytic subunit ClpP or the Clp ATPase ClpC, and the heat shock protein DnaK. Interestingly, all characterized CmR mutants harbored a combination of mutations in walK and clpP or clpC. In vitro and in vivo analyses showed that the mutations in the Clp proteins abolished ClpP or ClpC activity, and the deletion of clpP rendered S. aureus but not all Bacillus subtilis strains cervimycin-resistant. The essential gene walK was the second mutational hotspot in the CmR S. aureus strains, which decreased WalK activity in vitro and generated a vancomycin-intermediate resistant phenotype, with a thickened cell wall, a lower growth rate, and reduced cell lysis. Transcriptomic and proteomic analyses revealed massive alterations in the CmR strains compared to the parent strain S. aureus SG511, with major shifts in the heat shock regulon, the metal ion homeostasis, and the carbohydrate metabolism. Taken together, mutations in the heat shock genes clpP, clpC, and dnaK, and the walK kinase gene in CmR mutants induced a vancomycin-intermediate resistant phenotype in S. aureus, suggesting cell wall metabolism or the Clp protease system as primary target of cervimycin. IMPORTANCE Staphylococcus aureus is a frequent cause of infections in both the community and hospital setting. Resistance development of S. aureus to various antibiotics is a severe problem for the treatment of this pathogen worldwide. New powerful antimicrobial agents against Gram-positives are needed, since antibiotics like vancomycin fail to cure vancomycin-intermediate resistant S. aureus (VISA) and vancomycin-resistant enterococci (VRE) infections. One candidate substance with promising activity against these organisms is cervimycin, which is an antibiotic complex with a yet unknown mode of action. In our study, we provide first insights into the mode of action of cervimycins. By characterizing cervimycin-resistant S. aureus strains, we revealed the Clp system and the essential kinase WalK as mutational hotspots for cervimycin resistance in S. aureus. It further emerged that cervimycin-resistant S. aureus strains show a VISA phenotype, indicating a role of cervimycin in perturbing the bacterial cell envelope.

Toxin-Producing Endosymbionts Shield Pathogenic Fungus against Micropredators.

The fungus Rhizopus microsporus harbors a bacterial endosymbiont (Mycetohabitans rhizoxinica) for the production of the antimitotic toxin rhizoxin. Although rhizoxin is the causative agent of rice seedling blight, the toxinogenic bacterial-fungal alliance is, not restricted to the plant disease. It has been detected in numerous environmental isolates from geographically distinct sites covering all five continents, thus raising questions regarding the ecological role of rhizoxin beyond rice seedling blight. Here, we show that rhizoxin serves the fungal host in fending off protozoan and metazoan predators. Fluorescence microscopy and coculture experiments with the fungivorous amoeba Protostelium aurantium revealed that ingestion of R. microsporus spores is toxic to P. aurantium. This amoebicidal effect is caused by the dominant bacterial rhizoxin congener rhizoxin S2, which is also lethal toward the model nematode Caenorhabditis elegans. By combining stereomicroscopy, automated image analysis, and quantification of nematode movement, we show that the fungivorous nematode Aphelenchus avenae actively feeds on R. microsporus that is lacking endosymbionts, whereas worms coincubated with symbiotic R. microsporus are significantly less lively. This study uncovers an unexpected ecological role of rhizoxin as shield against micropredators. This finding suggests that predators may function as an evolutionary driving force to maintain toxin-producing endosymbionts in nonpathogenic fungi. IMPORTANCE The soil community is a complex system characterized by predator-prey interactions. Fungi have developed effective strategies to defend themselves against predators. Understanding these strategies is of critical importance for ecology, medicine, and biotechnology. In this study, we shed light on the defense mechanisms of the phytopathogenic Rhizopus-Mycetohabitans symbiosis that has spread worldwide. We report an unexpected role of rhizoxin, a secondary metabolite produced by the bacterium M. rhizoxinica residing within the hyphae of R. microsporus. We show that this bacterial secondary metabolite is utilized by the fungal host to successfully fend off fungivorous protozoan and metazoan predators and thus identified a fundamentally new function of this infamous cytotoxic compound. This endosymbiont-dependent predator defense illustrates an unusual strategy employed by fungi that has broader implications, since it may serve as a model for understanding how animal predation acts as an evolutionary driving force to maintain endosymbionts in nonpathogenic fungi.

Alternative Benzoxazole Assembly Discovered in Anaerobic Bacteria Provides Access to Privileged Heterocyclic Scaffold.

Benzoxazole scaffolds feature prominently in diverse synthetic and natural product-derived pharmaceuticals. Our understanding of their bacterial biosynthesis is, however, limited to ortho-substituted heterocycles from actinomycetes. We report an overlooked biosynthetic pathway in anaerobic bacteria (typified in Clostridium cavendishii) that expands the benzoxazole chemical space to meta-substituted heterocycles and heralds a distribution beyond Actinobacteria. The first benzoxazoles from the anaerobic realm (closoxazole A and B) were elucidated by NMR and chemical synthesis. By genome editing in the native producer, heterologous expression in Escherichia coli, and systematic pathway dissection we show that closoxazole biosynthesis invokes an unprecedented precursor usage (3-amino-4-hydroxybenzoate) and manner of assembly. Synthetic utility was demonstrated by the precursor-directed biosynthesis of a tafamidis analogue. A bioinformatic survey reveals the pervasiveness of related gene clusters in diverse bacterial phyla.

Fusaric acid detoxification: a strategy of Gliocladium roseum involved in its antagonism against Fusarium verticillioides.

Fungal co-culture has several biotechnological applications including the discovery or the enhanced production of secondary metabolites. It is also a powerful tool aiding to elucidate the involvement of secondary metabolism in fungus-fungus interactions. The aim of this work was to investigate secondary metabolites produced when Fusarium verticillioides is co-cultured with Gliocladium roseum. Secreted metabolites were analyzed by HPLC-MS, and fusaric acid (FA) was quantified by HPLC-DAD. Four FA derivatives were identified only in the F. verticillioides-G. roseum co-culture. Mass spectrometry and one- and two-dimensional NMR spectra indicated that they were 5-butylpyridine-2-carboxylic acid methyl ester (5B2CAM), 4-(5-butylpicolinamido) butanoic acid (45BBA), methyl 4-(5-butylpicolinamido) butanoate (M45BBA), and bis(5-butyl-2-pyridinecarboxylate-N1,O2)-copper (B52P). 45BBA and M45BBA are reported for the first time and were FA biotransformation products generated by G. roseum. The antifungal activity of 5B2CAM, 45BBA, and M45BBA was evaluated in vitro against Botrytis cinerea and Aspergillus niger. They were less fungitoxic than FA, with 45BBA as the least toxic. Our results suggest that the effective antagonism exerted by G. roseum against F. verticillioides is due, at least in part, to its detoxifying ability against FA.

Endofungal bacteria boost anthelminthic host protection with the biosurfactant symbiosin.

Effective protection of soil fungi from predators is crucial for their survival in the niche. Thus, fungi have developed efficient defence strategies. We discovered that soil beneficial Mortierella fungi employ a potent cytotoxin (necroxime) against fungivorous nematodes. Interestingly, this anthelminthic agent is produced by bacterial endosymbionts (Candidatus Mycoavidus necroximicus) residing within the fungus. Analysis of the symbiont's genome indicated a rich biosynthetic potential, yet nothing has been known about additional metabolites and their potential synergistic functions. Here we report that two distinct Mortierella endosymbionts produce a novel cyclic lipodepsipeptide (symbiosin), that is clearly of bacterial origin, but has striking similarities to various fungal specialized metabolites. The structure and absolute configuration of symbiosin were fully elucidated. By comparative genomics of symbiosin-positive strains and in silico analyses of the deduced non-ribosomal synthetases, we assigned the (sym) biosynthetic gene cluster and proposed an assembly line model. Bioassays revealed that symbiosin is not only an antibiotic, in particular against mycobacteria, but also exhibits marked synergistic effects with necroxime in anti-nematode tests. By functional analyses and substitution experiments we found that symbiosin is a potent biosurfactant and that this particular property confers a boost in the anthelmintic action, similar to formulations of therapeutics in human medicine. Our findings illustrate that "combination therapies" against parasites already exist in ecological contexts, which may inspire the development of biocontrol agents and therapeutics.

A highly conserved gene locus in endofungal bacteria codes for the biosynthesis of symbiosis-specific cyclopeptides.

The tight association of the pathogenic fungus Rhizopus microsporus and its toxin-producing, bacterial endosymbionts (Mycetohabitans spp.) is distributed worldwide and has significance for agriculture, food production, and human health. Intriguingly, the endofungal bacteria are essential for the propagation of the fungal host. Yet, little is known about chemical mediators fostering the symbiosis, and universal metabolites that support the mutualistic relationship have remained elusive. Here, we describe the discovery of a complex of specialized metabolites produced by endofungal bacteria under symbiotic conditions. Through full genome sequencing and comparative genomics of eight endofungal symbiont strains from geographically distant regions, we discovered a conserved gene locus (hab) for a nonribosomal peptide synthetase as a unifying trait. Bioinformatics analyses, targeted gene deletions, and chemical profiling uncovered unprecedented depsipeptides (habitasporins) whose structures were fully elucidated. Computational network analysis and labeling experiments granted insight into the biosynthesis of their nonproteinogenic building blocks (pipecolic acid and ß-phenylalanine). Deletion of the hab gene locus was shown to impair the ability of the bacteria to enter their fungal host. Our study unveils a common principle of the endosymbiotic lifestyle of Mycetohabitans species and expands the repertoire of characterized chemical mediators of a globally occurring mutualistic association.

The bacterium Pseudomonas protegens antagonizes the microalga Chlamydomonas reinhardtii using a blend of toxins.

The unicellular alga Chlamydomonas reinhardtii and the bacterium Pseudomonas protegens serve as a model to study the interactions between photosynthetic and heterotrophic microorganisms. P. protegens secretes the cyclic lipopeptide orfamide A that interferes with cytosolic Ca2+ homeostasis in C. reinhardtii resulting in deflagellation of the algal cells. Here, we studied the roles of additional secondary metabolites secreted by P. protegens using individual compounds and co-cultivation of algae with bacterial mutants. Rhizoxin S2, pyrrolnitrin, pyoluteorin, 2,4-diacetylphloroglucinol (DAPG) and orfamide A all induce changes in cell morphology and inhibit the growth of C. reinhardtii. Rhizoxin S2 exerts the strongest growth inhibition, and its action depends on the spatial structure of the environment (agar versus liquid culture). Algal motility is unaffected by rhizoxin S2 and is most potently inhibited by orfamide A (IC50  = 4.1 µM). Pyrrolnitrin and pyoluteorin both interfere with algal cytosolic Ca2+ homeostasis and motility whereas high concentrations of DAPG immobilize C. reinhardtii without deflagellation or disturbance of Ca2+ homeostasis. Co-cultivation with a regulatory mutant of bacterial secondary metabolism (ΔgacA) promotes algal growth under spatially structured conditions. Our results reveal how a single soil bacterium uses an arsenal of secreted antialgal compounds with complementary and partially overlapping activities.

Multimodal Molecular Imaging and Identification of Bacterial Toxins Causing Mushroom Soft Rot and Cavity Disease.

Soft rot disease of edible mushrooms leads to rapid degeneration of fungal tissue and thus severely affects farming productivity worldwide. The bacterial mushroom pathogen Burkholderia gladioli pv. agaricicola has been identified as the cause. Yet, little is known about the molecular basis of the infection, the spatial distribution and the biological role of antifungal agents and toxins involved in this infectious disease. We combine genome mining, metabolic profiling, MALDI-Imaging and UV Raman spectroscopy, to detect, identify and visualize a complex of chemical mediators and toxins produced by the pathogen during the infection process, including toxoflavin, caryoynencin, and sinapigladioside. Furthermore, targeted gene knockouts and in vitro assays link antifungal agents to prevalent symptoms of soft rot, mushroom browning, and impaired mycelium growth. Comparisons of related pathogenic, mutualistic and environmental Burkholderia spp. indicate that the arsenal of antifungal agents may have paved the way for ancestral bacteria to colonize niches where frequent, antagonistic interactions with fungi occur. Our findings not only demonstrate the power of label-free, in vivo detection of polyyne virulence factors by Raman imaging, but may also inspire new approaches to disease control.

Mining and unearthing hidden biosynthetic potential.

Genetically encoded small molecules (secondary metabolites) play eminent roles in ecological interactions, as pathogenicity factors and as drug leads. Yet, these chemical mediators often evade detection, and the discovery of novel entities is hampered by low production and high rediscovery rates. These limitations may be addressed by genome mining for biosynthetic gene clusters, thereby unveiling cryptic metabolic potential. The development of sophisticated data mining methods and genetic and analytical tools has enabled the discovery of an impressive array of previously overlooked natural products. This review shows the newest developments in the field, highlighting compound discovery from unconventional sources and microbiomes.

AoiQ Catalyzes Geminal Dichlorination of 1,3-Diketone Natural Products.

Enzymes that can perform halogenation of aliphatic carbons are of significant interest to the synthetic and biocatalysis communities. Here we describe the characterization of AoiQ, a single-component flavin-dependent halogenase (FDH) that catalyzes gem-dichlorination of 1,3-diketone substrates in the biosynthesis of dichlorodiaporthin. AoiQ represents the first biochemically reconstituted FDH that can halogenate an enolizable sp3-hybridized carbon atom.

Structural and Mechanistic Insights into C-S Bond Formation in Gliotoxin.

Glutathione-S-transferases (GSTs) usually detoxify xenobiotics. The human pathogenic fungus Aspergillus fumigatus however uses the exceptional GST GliG to incorporate two sulfur atoms into its virulence factor gliotoxin. Because these sulfurs are essential for biological activity, glutathionylation is a key step of gliotoxin biosynthesis. Yet, the mechanism of carbon-sulfur linkage formation from a bis-hydroxylated precursor is unresolved. Here, we report structures of GliG with glutathione (GSH) and its reaction product cyclo[-l-Phe-l-Ser]-bis-glutathione, which has been purified from a genetically modified A. fumigatus strain. The structures argue for stepwise processing of first the Phe and second the Ser moiety. Enzyme-mediated dehydration of the substrate activates GSH and a helix dipole stabilizes the resulting anion via a water molecule for the nucleophilic attack. Activity assays with mutants validate the interactions of GliG with the ligands and enrich our knowledge about enzymatic C-S bond formation in gliotoxin and epipolythiodioxopiperazine (ETP) natural compounds in general.

Highly parallelized droplet cultivation and prioritization of antibiotic producers from natural microbial communities.

Antibiotics from few culturable microorganisms have saved millions of lives since the 20th century. But with resistance formation, these compounds become increasingly ineffective, while the majority of microbial and with that chemical compound diversity remains inaccessible for cultivation and exploration. Culturing recalcitrant bacteria is a stochastic process. But conventional methods are limited to low throughput. By increasing (i) throughput and (ii) sensitivity by miniaturization, we innovate microbiological cultivation to comply with biological stochasticity. Here, we introduce a droplet-based microscale cultivation system, which is directly coupled to a high-throughput screening for antimicrobial activity prior to strain isolation. We demonstrate that highly parallelized in-droplet cultivation starting from single cells results in the cultivation of yet uncultured species and a significantly higher bacterial diversity than standard agar plate cultivation. Strains able to inhibit intact reporter strains were isolated from the system. A variety of antimicrobial compounds were detected for a selected potent antibiotic producer.

Antibiotics are chemicals derived from microorganisms that can kill the bacteria that harm human health. In the 20^th and 21^st centuries antibiotics saved millions of lives, but new strains of dangerous bacteria that cannot be killed by antibiotics, known as antibiotic resistant strains, are becoming more frequent. Most antibiotics are produced by only a small group of microorganisms, but many more microorganisms exist in nature. So it is possible that microorganisms outside this small group can produce different antibiotics that are effective against antibiotic resistant strains. Unfortunately, finding the microorganisms that produce these alternative antibiotics is challenging because researchers do not know which bacteria are producing these molecules and how to grow these microorganisms in the laboratory. To solve this problem, Mahler et al. developed a new method for growing a new subset of microorganisms in the laboratory. This would allow researchers to study the new microorganisms under controlled conditions, and determine whether any of the substances they produce have antibiotic properties. Mahler et al. generated tiny droplets that could only contain a single cell of a microorganism, so each microbe could grow alone in its own protected environment. Using this approach, it was possible to grow completely different types of microorganisms than with traditional techniques, and keep them isolated from each other. This allowed each different species of microbe to be screened for antimicrobial activity, allowing the identification of chemicals that could potentially be developed into new antibiotics. This new method is automated and miniaturized, paving the way for growing many more cells in few hours, with very low material and space requirements. These results showcase a way of growing new types of microorganisms in the laboratory, making it easier and faster to study them and determine what chemicals they produce. Understanding a greater variety of microorganisms in detail can help identify new chemicals for industrial applications, including new ways of combating infections.

Biosynthesis of Sinapigladioside, an Antifungal Isothiocyanate from Burkholderia Symbionts.

Sinapigladioside is a rare isothiocyanate-bearing natural product from beetle-associated bacteria (Burkholderia gladioli) that might protect beetle offspring against entomopathogenic fungi. The biosynthetic origin of sinapigladioside has been elusive, and little is known about bacterial isothiocyanate biosynthesis in general. On the basis of stable-isotope labeling, bioinformatics, and mutagenesis, we identified the sinapigladioside biosynthesis gene cluster in the symbiont and found that an isonitrile synthase plays a key role in the biosynthetic pathway. Genome mining and network analyses indicate that related gene clusters are distributed across various bacterial phyla including producers of both nitriles and isothiocyanates. Our findings support a model for bacterial isothiocyanate biosynthesis by sulfur transfer into isonitrile precursors.

N-Heterocyclization in Gliotoxin Biosynthesis is Catalyzed by a Distinct Cytochrome P450 Monooxygenase.

Gliotoxin and related epidithiodiketopiperazines (ETP) from diverse fungi feature highly functionalized hydroindole scaffolds with an array of medicinally and ecologically relevant activities. Mutation analysis, heterologous reconstitution, and biotransformation experiments revealed that a cytochrome P450 monooxygenase (GliF) from the human-pathogenic fungus Aspergillus fumigatus plays a key role in the formation of the complex heterocycle. In vitro assays using a biosynthetic precursor from a blocked mutant showed that GliF is specific to ETPs and catalyzes an unprecedented heterocyclization reaction that cannot be emulated with current synthetic methods. In silico analyses indicate that this rare biotransformation takes place in related ETP biosynthetic pathways.