Determining the biosynthetic sequence in the early steps of the fumonisin pathway by use of three gene-disruption mutants of Fusarium verticillioides.

Fumonisins are polyketide-derived mycotoxins produced by Fusarium verticillioides, a fungal pathogen of corn plants. Although a gene cluster for the biosynthesis of fumonisins has been cloned, the biosynthetic pathway is still not clear. We have used three gene-disrupted mutants, designated DeltaFUM1, DeltaFUM6, and DeltaFUM8, to study the early steps of the pathway. Fumonisins were not produced in single-strain cultures of the DeltaFUM1, DeltaFUM6, and DeltaFUM8 mutants. However, fumonisins were produced by DeltaFUM1 or DeltaFUM8 mutants when they were cocultured with the DeltaFUM6 mutant. No fumonisins were produced when the DeltaFUM1 and DeltaFUM8 mutants were cocultured. These results suggest that the DeltaFUM6 mutant produces a fumonisin intermediate that can be further metabolized by fumonisin biosynthetic enzymes in the DeltaFUM1 and DeltaFUM8 mutants. To isolate the potential intermediates produced by DeltaFUM6, we followed a time course of cocultures of the DeltaFUM1 and DeltaFUM6 and the DeltaFUM8 and DeltaFUM6 mutants. Liquid chromatographic-mass spectrometric data suggested that metabolites having the general carbon skeleton of fumonisins with 1-4 hydroxyl groups were accumulated over a 7-day period. These results indicate that fumonisin biosynthesis starts with Fum1p-catalyzed carbon-chain assembly followed by the Fum8p-catalyzed alanine condensation. The resulting product then can be further oxidized by Fum6p and other enzymes.