Hydroxylations of trichothecene rings in the biosynthesis of Fusarium trichothecenes: evolution of alternative pathways in the nivalenol chemotype.

Fusarium sporotrichioides genes FsTri11, FsTri13, and FsTri1 encode cytochrome P450 monooxygenases (CYPs) responsible for hydroxylations at C-15, C-4, and C-8 of the trichothecene skeleton, respectively. However, the corresponding genes of nivalenol (NIV)-chemotype Fusarium graminearum remain to be functionally elucidated. In this study, we characterized the roles of these CYPs in NIV biosynthesis. Analyses of the metabolites of the F. graminearum Fgtri11- mutant, a disruptant of FgTri11 encoding isotrichodermin (ITD) C-15 hydroxylase, revealed a small amount of NIV-type trichothecenes suggesting that an alternative C-15 hydroxylase partially complemented FgTRI11p. In contrast, the C-7/C-8 hydroxylations depended solely on FgTRI1p, as suggested by the metabolite profiles of the Fgtri11- Fgtri1- double gene disruptant. Disruption of FgTri1 in both the wild-type and Fgtri13- mutant backgrounds revealed that FgTRI13p exhibits marginal activity toward calonectrin (CAL) and that it was the only C-4 hydroxylase. In addition, feeding experiments demonstrated that the C-4 hydroxylation of a 7-hydroxytrichothecene lacking C-8 ketone was extremely limited. The marginal activity of FgTRI13p toward CAL was advantageous for the C-7/C-8 hydroxylation steps in NIV biosynthesis, as transformation of a C-4 oxygenated trichothecene lacking C-7/C-8 modifications into NIV-type trichothecenes was quite inefficient. The significance of hydroxylation steps in the evolution of Fusarium trichothecenes is discussed.