Biosynthetic mechanism for mycotoxin fumonisins in the filamentous fungal pathogen Fusarium verticillioides
Filamentous fungi are rich in biologically active polyketides. However, the biosynthetic mechanism of these secondary metabolites is poorly understood. The goal of this Ph.D. study is to elucidate the biosynthetic mechanism for a group of fungal reduced polyketides (including the cholesterol-lowering drug lovastatin, the life-threatening mycotoxin fumonisins, and pathogenesis factor T-toxins) using both genetic and biochemical approaches. ^ In the first project, we established a genetic system that allows, for the first time, specific and functional manipulations of the catalytic domains of FUM1, which is a polyketide synthase (PKS) gene responsible for the 18-carbon polyketide chain of fumonisins produced by corn pathogen Fusarium verticillioides. Using this system, we have obtained fungal mutants containing a chimeric FUM1, in which the ketosynthase (KS) domain had been replaced by the corresponding domain from T-toxins PKS1 or lovastatin LDKS. The mutants containing T-toxin KS produced fumonisins, showing that the heterologous KS is able to functionally interact with six other domains of FUM1. On the other hand, the mutants containing lovastatin KS produced dihydroisocoumarins, which are known to have anti-malarial, antifungal, and anti-tubercular activities and had never been isolated from Fusaria. This work represents the first successful genetic manipulation of fungal PKS genes to produce new, biologically active metabolites in a filamentous fungus. The production of the aromatic metabolites by an engineered non-aromatic type PKS is unprecedented and shows that KS plays a key role in the control of fungal polyketides. ^ In the second project, we used the biochemical approach to determine the function of FUM10 in fumonisin tricarballylic esterifications, which are post-polyketide modifications crucial for the toxins' activity. We have expressed and purified two forms of Fum10p (His6-Fum10p and MBP-Fum10p) from E. coli. After attempts to characterize the obtained enzymes, preliminary data suggested that Fum10p most likely acts as a component of a nonribosomal peptide synthetase-like complex, consisting of Fum10p/Fum7p/Fum14p, together of which is responsible for fumonisin esterifications. ^
Zhu, Xiangcheng, "Biosynthetic mechanism for mycotoxin fumonisins in the filamentous fungal pathogen Fusarium verticillioides" (2007). ETD collection for University of Nebraska - Lincoln. AAI3274813.