Trichothecenes are a widespread family of sesquiterpenoid toxins that can pose significant risks to food and feed safety as well as environmental health. A defining feature of all trichothecenes is their central tricyclic 12,13-epoxytrichothec-9-ene (EPT) motif. Although the formation of the EPT central skeleton has long been presumed to be a spontaneous process, the nonenzymatic cyclization reaction forming the tetrahydropyran ring in EPT requires acid catalysis
otherwise, it occurs too slowly to sustain efficient trichothecene biosynthesis under physiological conditions. Here, we resolved this decades-old problem by identifying the missing enzymes for EPT biosynthesis. We demonstrate that the C11 hydroxyl group of universal trichothecene precursors, isotrichodiol and isotrichotriol, must be acetylated by a strictly conserved