Who Needs Neighbors? PKS8 Is a Stand-Alone Gene in Fusarium graminearum Responsible for Production of Gibepyrones and Prolipyrone B.

Genome sequencing of the genus Fusarium has revealed a great capacity for discovery of new natural products of potential economical and therapeutic importance. Several of these are unknown. In this study, we investigated the product of the PKS8 gene in Fusarium graminearum, which was recently linked to gibepyrones in F. fujikuroi. Genomic analyses showed that PKS8 constitutes a stand-alone gene in F. graminearum and related species. Overexpression of PKS8 resulted in production of gibepyrones A, B, D, G and prolipyrone B, which could not be detected in the wild type strain. Our results suggest that PKS8 produces the entry compound gibepyrone A, which is subsequently oxidized by one or several non-clustering cytochrome P450 monooxygenases ending with prolipyrone B.

Gibepyrone Biosynthesis in the Rice Pathogen Fusarium fujikuroi Is Facilitated by a Small Polyketide Synthase Gene Cluster.

The 2H-pyran-2-one gibepyrone A and its oxidized derivatives gibepyrones B-F have been isolated from the rice pathogenic fungus Fusarium fujikuroi already more than 20 years ago. However, these products have not been linked to the respective biosynthetic genes, and therefore, their biosynthesis has not yet been analyzed on a molecular level. Feeding experiments with isotopically labeled precursors clearly supported a polyketide origin for the formal monoterpenoid gibepyrone A, whereas the terpenoid pathway could be excluded. Targeted gene deletion verified that the F. fujikuroi polyketide synthase PKS13, designated Gpy1, is responsible for gibepyrone A biosynthesis. Next to Gpy1, the ATP-binding cassette transporter Gpy2 is encoded by the gibepyrone gene cluster. Gpy2 was shown to have only a minor impact on the actual efflux of gibepyrone A out of the cell. Instead, we obtained evidence that Gpy2 is involved in gene regulation as it represses GPY1 gene expression. Thus, GPY1 was up-regulated and gibepyrone A production was enhanced both extra- and intracellularly in Δgpy2 mutants. Furthermore, expression of GPY genes is strictly repressed by members of the fungus-specific velvet complex, Vel1, Vel2, and Lae1, whereas Sge1, a major regulator of secondary metabolism in F. fujikuroi, affects gibepyrone biosynthesis in a positive manner. The gibepyrone A derivatives gibepyrones B and D were shown to be produced by cluster-independent P450 monooxygenases, probably to protect the fungus from the toxic product. In contrast, the formation of gibepyrones E and F from gibepyrone A is a spontaneous process and independent of enzymatic activity.