Mining Symbionts of a Spider‐Transmitted Fungus Illuminates Uncharted Biosynthetic Pathways to Cytotoxic Benzolactones

Abstract

A spider‐transmitted fungus (Rhizopus microsporus) that was isolated from necrotic human tissue was found to harbor endofungal bacteria (Burkholderia sp.). Metabolic profiling of the symbionts revealed a complex of cytotoxic agents (necroximes). Their structures were characterized as oxime‐substituted benzolactone enamides with a peptidic side chain. The potently cytotoxic necroximes are also formed in symbiosis with the fungal host and could have contributed to the necrosis. Genome sequencing and computational analyses revealed a novel modular PKS/NRPS assembly line equipped with several non‐canonical domains. Based on gene‐deletion mutants, we propose a biosynthetic model for bacterial benzolactones. We identified specific traits that serve as genetic handles to find related salicylate macrolide pathways (lobatamide, oximidine, apicularen) in various other bacterial genera. Knowledge of the biosynthetic pathway enables biosynthetic engineering and genome‐mining approaches.

Genome Mining of Oxidation Modules in trans‐Acyltransferase Polyketide Synthases Reveals a Culturable Source for Lobatamides

“…For an expansion of this family, see a study on necroximes that was conducted concurrently with this work. [23] Structure 5 features an internal ester that is reversed in comparison to oocydin, pederin, and toblerol biosynthesis. To investigate the role of the LmbC-Ox module in its formation, we cloned the region encoding the Ox domain into a variant of the pET28a expression vector (see the Supporting Information) for expression in E. coli as an N-terminally His 6tagged protein ( Figure S10).…”