Biosynthesis of the Central Tricyclic Skeleton of Trichothecene Mycotoxins.

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 -acetyltransferase Tri3 to furnish a better leaving group. These acetylated intermediates preferentially undergo spontaneous allylic rearrangement with water to give shunt products, trichodiol and trichotriol. Therefore, a novel cyclase, Tri14, which was previously annotated as a hypothetical protein, is required to overcome the kinetically unfavored oxide bridge closure and meanwhile suppress the spontaneous formation of any shunt products.