Heterologous expression of intact biosynthetic gene clusters in Fusarium graminearum.

Filamentous fungi such as species from the genus Fusarium are capable of producing a wide palette of interesting metabolites relevant to health, agriculture and biotechnology. Secondary metabolites are formed from large synthase/synthetase enzymes often encoded in gene clusters containing additional enzymes cooperating in the metabolite's biosynthesis. The true potential of fungal metabolomes remain untapped as the majority of secondary metabolite gene clusters are silent under standard laboratory growth conditions. One way to achieve expression of biosynthetic pathways is to clone the responsible genes and express them in a well-suited heterologous host, which poses a challenge since Fusarium polyketide synthase and non-ribosomal peptide synthetase gene clusters can be large (e.g. as large as 80 kb) and comprise several genes necessary for product formation. The major challenge associated with heterologous expression of fungal biosynthesis pathways is thus handling and cloning large DNA sequences. In this paper we present the successful workflow for cloning, reconstruction and heterologous production of two previously characterized Fusarium pseudograminearum natural product pathways in Fusarium graminearum. In vivo yeast recombination enabled rapid assembly of the W493 (NRPS32-PKS40) and the Fusarium Cytokinin gene clusters. F. graminearum transformants were obtained through protoplast-mediated and Agrobacterium tumefaciens-mediated transformation. Whole genome sequencing revealed isolation of transformants carrying intact copies the gene clusters was possible. Known Fusarium cytokinin metabolites; fusatin, 8-oxo-fusatin, 8-oxo-isopentenyladenine, fusatinic acid together with cis- and trans-zeatin were detected by liquid chromatography and mass spectrometry, which confirmed gene functionality in F. graminearum. In addition the non-ribosomal lipopeptide products W493 A and B was heterologously produced in similar amounts to that observed in the F. pseudograminearum doner. The Fusarium pan-genome comprises more than 60 uncharacterized putative secondary metabolite gene clusters. We nominate the well-characterized F. graminearum as a heterologous expression platform for Fusarium secondary metabolite gene clusters, and present our experience cloning and introducing gene clusters into this species. We expect the presented methods will inspire future endevours in heterologous production of Fusarium metabolites and potentially aid the production and characterization of novel natural products.

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