Reversible Redox Cycling: A Mechanism for Toxification and Detoxification in Fungal Alkyl Salicylaldehyde...

Understanding how natural product producers tolerate the reactive and toxic compounds they synthesize contributes to discovering novel therapeutics and deciphering unusual biosynthetic mechanisms. Alkyl salicylaldehydes are secondary metabolites widely distributed in fungi, exhibiting great structural diversity and various biological activities. However, the mechanism by which fungi partition these reactive species to avoid self-toxicity remains poorly understood. In this study, by heterologous reconstruction and biochemical characterization of the alkyl salicylaldehyde biosynthetic gene cluster from Stachybotrys sp. g12, we uncovered a multienzyme self-resistance strategy driven by a reversible redox cycle. The biosynthetic pathway employs the intracellular reductase StrM to detoxify the aldehyde precursor into a stable alcohol prodrug, ensuring safe biosynthesis and cellular efflux. Once exported from the cell, the extracellular oxidase, StrJ, triggers toxification by regenerating the aldehyde species. Furthermore, the redundant reductases StrH and StrK provide a "double-insurance" detoxification defense by rapidly re-reducing any toxic aldehydes that reenter the cell. Collectively, our findings characterize a self-resistance mechanism common in fungal alkyl salicylaldehyde biosynthesis and provide a strategy for the safe microbial synthesis of highly reactive aldehyde natural products.