Fungi of the genus Mortierella occur ubiquitously in soils where they play pivotal roles in carbon cycling, xenobiont degradation, and promoting plant growth. These important fungi are, however, threatened by micropredators such as fungivorous nematodes, and yet little is known about their protective tactics. We report that Mortierella verticillata NRRL 6337 harbors a bacterial endosymbiont that efficiently shields its host from nematode attacks with anthelmintic metabolites. Microscopic investigation and 16S ribosomal DNA analysis revealed that a previously overlooked bacterial symbiont belonging to the genus Mycoavidus dwells in M. verticillata hyphae. Metabolic profiling of the wild-type fungus and a symbiont-free strain obtained by antibiotic treatment as well as genome analyses revealed that highly cytotoxic macrolactones (CJ-12,950 and CJ-13,357, syn. necroxime C and D), initially thought to be metabolites of the soil-inhabiting fungus, are actually biosynthesized by the endosymbiont. According to comparative genomics, the symbiont belongs to a new species (Candidatus Mycoavidus necroximicus) with 12% of its 2.2 Mb genome dedicated to natural product biosynthesis, including the modular polyketide-nonribosomal peptide synthetase for necroxime assembly. Using Caenorhabditis elegans and the fungivorous nematode Aphelenchus avenae as test strains, we show that necroximes exert highly potent anthelmintic activities. Effective host protection was demonstrated in cocultures of nematodes with symbiotic and chemically complemented aposymbiotic fungal strains. Image analysis and mathematical quantification of nematode movement enabled evaluation of the potency. Our work describes a relevant role for endofungal bacteria in protecting fungi against mycophagous nematodes.

Closthioamide (CTA) is a unique symmetric nonribosomal peptide with six thioamide moieties that is produced by the Gram-positive obligate anaerobe Ruminiclostridium cellulolyticum. CTA displays potent inhibitory activity against important clinical pathogens, making it a promising drug candidate. Yet, the biosynthesis of this DNA gyrase-targeting antibiotic has remained enigmatic. Using a combination of genome mining, genome editing (targeted group II intron, CRISPR/Cas9), and heterologous expression, we show that CTA biosynthesis involves specialized enzymes for starter unit biosynthesis, amide bond formation, thionation, and dimerization. Surprisingly, CTA biosynthesis involves a novel thiotemplated peptide assembly line that markedly differs from known nonribosomal peptide synthetases. These findings provide the first insights into the biosynthesis of thioamide-containing nonribosomal peptides and offer a starting point for the discovery of related natural products.

Closthioamide (CTA) is au nique symmetric nonribosomal peptide with six thioamide moieties that is produced by the Gram-positive obligate anaerobe Ruminiclostridium cellulolyticum. CTAdisplays potent inhibitory activity against important clinical pathogens,m aking it ap romising drug candidate.Y et, the biosynthesis of this DNAg yrase-targeting antibiotic has remained enigmatic. Using ac ombination of genome mining,g enome editing (targeted group II intron, CRISPR/Cas9), and heterologous expression, we show that CTAbiosynthesis involves specialized enzymes for starter unit biosynthesis,a mide bond formation, thionation, and dimerization. Surprisingly,C TA biosynthesis involves an ovel thiotemplated peptide assembly line that markedly differs from knownn onribosomal peptide synthetases.T hese findings providet he first insights into the biosynthesis of thioamidecontaining nonribosomal peptides and offer astarting point for the discovery of related natural products.

The soil community is a complex system characterized by predator-prey interactions. Fungi have developed effective strategies to defend themselves against predators.

Closthioamide (CTA) is a symmetric nonribosomal peptide (NRP) comprised of two diaminopropane‐linked polythioamidated monomers. CTA is biosynthesized by Ruminiclostridium cellulolyticum via an atypical NRP synthetase (NRPS)‐independent biosynthetic pathway. Although the logic for monomer assembly was recently elucidated, the strategy for the biosynthesis and incorporation of the diamine linker remained a mystery. By means of genome editing, synthesis, and in vitro biochemical assays, we demonstrate that the final steps in CTA maturation proceed through a surprising split‐merge pathway involving the dual use of a thiotemplated intermediate. This pathway includes the first examples of an aldo‐keto reductase catalyzing the reductive release of a thiotemplated product, and of a transthioamidating transglutaminase. In addition to clarifying the remaining steps in CTA assembly, our data shed light on largely unexplored pathways for NRPS‐independent peptide biosynthesis.