Reconstruction of the biosynthetic pathway for the core fungal polyketide scaffold rubrofusarin in Saccharomyces cerevisiae

BACKGROUND Fungal polyketides include commercially important pharmaceuticals and food additives, e.g. the cholesterol-lowering statins and the red and orange monascus pigments. Presently, production relies on isolation of the compounds from the natural producers, and systems for heterologous production in easily fermentable and genetically engineerable organisms, such as Saccharomyces cerevisiae and Escherichia coli are desirable. Rubrofusarin is an orange polyketide pigment that is a common intermediate in many different fungal biosynthetic pathways. RESULTS In this study, we established a biosynthetic pathway for rubrofusarin in S. cerevisiae. First, the Fusarium graminearum gene encoding polyketide synthase 12 (PKS12) was heterologously co-expressed with the Aspergillus fumigatus gene encoding phosphopantetheinyl transferase (npgA) resulting in production of YWA1. This aromatic heptaketide intermediate was converted into nor-rubrofusarin upon expression of the dehydratase gene aurZ from the aurofusarin gene cluster of F. graminearum. Final conversion into rubrofusarin was achieved by expression of the O-methyltransferase encoding gene aurJ, also obtained from the aurofusarin gene cluster, resulting in a titer of 1.1 mg/L. Reduced levels of rubrofusarin were detected when expressing PKS12, npgA, and aurJ alone, presumably due to spontaneous conversion of YWA1 to nor-rubrofusarin. However, the co-expression of aurZ resulted in an approx. six-fold increase in rubrofusarin production. CONCLUSIONS The reconstructed pathway for rubrofusarin in S. cerevisiae allows the production of a core scaffold molecule with a branch-point role in several fungal polyketide pathways, thus paving the way for production of further natural pigments and bioactive molecules. Furthermore, the reconstruction verifies the suggested pathway, and as such, it is the first example of utilizing a synthetic biological "bottom up" approach for the validation of a complex fungal polyketide pathway.