A Role in 15-Deacetylcalonectrin Acetylation in the Non-Enzymatic Cyclization of an Earlier Bicyclic Intermediate in Fusarium Trichothecene Biosynthesis

The trichothecene biosynthesis in Fusarium begins with the cyclization of farnesyl pyrophosphate to trichodiene, followed by subsequent oxygenation to isotrichotriol. This initial bicyclic intermediate is further cyclized to isotrichodermol (ITDmol), a tricyclic precursor with a toxic trichothecene skeleton. Although the first cyclization and subsequent oxygenation are catalyzed by enzymes encoded by Tri5 and Tri4, the second cyclization occurs non-enzymatically. Following ITDmol formation, the enzymes encoded by Tri101, Tri11, Tri3, and Tri1 catalyze 3-O-acetylation, 15-hydroxylation, 15-O-acetylation, and A-ring oxygenation, respectively. In this study, we extensively analyzed the metabolites of the corresponding pathway-blocked mutants of Fusarium graminearum. The disruption of these Tri genes, except Tri3, led to the accumulation of tricyclic trichothecenes as the main products: ITDmol due to Tri101 disruption; a mixture of isotrichodermin (ITD), 7-hydroxyisotrichodermin (7-HIT), and 8-hydroxyisotrichodermin (8-HIT) due to Tri11 disruption; and a mixture of calonectrin and 3-deacetylcalonectrin due to Tri1 disruption. However, the ΔFgtri3 mutant accumulated substantial amounts of bicyclic metabolites, isotrichotriol and trichotriol, in addition to tricyclic 15-deacetylcalonectrin (15-deCAL). The ΔFgtri5ΔFgtri3 double gene disruptant transformed ITD into 7-HIT, 8-HIT, and 15-deCAL. The deletion of FgTri3 and overexpression of Tri6 and Tri10 trichothecene regulatory genes did not result in the accumulation of 15-deCAL in the transgenic strain. Thus, the absence of Tri3p and/or the presence of a small amount of 15-deCAL adversely affected the non-enzymatic second cyclization and C-15 hydroxylation steps.