Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2012
Publication Date: 9/19/2012
Citation: Uhlig, S., Busman, M., Shane, D.S., Ronning, H., Rise, F., Proctor, R. 2012. Identification of early fumonisin biosynthetic intermediates by inactivation of the FUM6 gene in Fusarium verticillioides. Journal of Agricultural and Food Chemistry. 60(41):10293-10301. Interpretive Summary: Mycotoxins are fungal metabolites that can contaminate crops and thereby pose health hazards to humans and livestock. Fumonisins are mycotoxins that are common contaminants of corn worldwide and are associated with multiple human and animal diseases. These mycotoxins are produced by the corn ear rot fungus Fusarium verticillioides. In the fungus, the biochemical pathway leading to the formation of fumonisins has been proposed based on a combination of genetic, enzymatic, and chemical studies. However, the metabolites that are formed early in this pathway have not yet been identified. In this study, we determined the chemical structures of compounds that accumulate in a mutant strain of F. verticillioides that is blocked in fumonisin production. We also demonstrated that one of these compounds is an early biochemical intermediate in synthesis of fumonisins. By doing this, we determined that the F. verticillioides enzyme encoded by the FUM6 gene catalyzes an early and critical step in fumonisin synthesis. The results of this study expand understanding of the genetic and enzymatic mechanisms that are responsible for fumonisin contamination of corn kernels. The results will be of interest to government, academic and industry scientists working to develop control strategies that limit mycotoxin contamination of cereals.
Technical Abstract: Fumonisins are polyketide mycotoxins produced by the maize pathogen Fusarium verticillioides and are associated with multiple human and animal diseases. A fumonisin biosynthetic pathway has been proposed, but structures of early pathway intermediates have not been demonstrated. The F. verticillioides FUM6 gene is required for an early pathway step. Here, metabolites produced by strains of the fungus with an inactivated FUM6 gene were purified and shown by mass spectrometry and NMR spectroscopy to have fumonisin-like structures but without substitutions at C-14 and C-15. The major metabolite was 2-amino-12,16-dimethylicosane-3,10-diol. Lesser amounts of 3-keto and triol analogues of the metabolite were also identified. In precursor feeding experiments, 2-amino-12,16-dimethylicosane-3,10-diol was transformed to fumonisins by an F. verticillioides strain with an inactive fumonisin polyketide synthase gene. The results support the hypothesis that the FUM6-encoded enzyme catalyzes fumonisin C-14 and C-15 hydroxylation and provide direct spectral and biochemical evidence for structures of early intermediates in fumonisin biosynthesis.