Discovery and elucidation of the biosynthesis of aspernidgulenes, novel polyenes from Aspergillus nidulans, using serial promoter replacement.

Through serial promoter exchanges, we isolated several novel polyenes, the aspernidgulenes, from Aspergillus nidulans, and uncovered their succinct biosynthetic pathway involving only four enzymes. An enoyl reductase (ER)-less highly-reducing polyketide synthase (HR-PKS) putatively produces a 5,6-dihydro-α-pyrone polyene, which undergoes bisepoxidation, epoxide-ring opening, cyclization, and hydrolytic cleavage by three tailoring enzymes to generate aspernidgulene A1 and A2 (4 and 5). Our findings demonstrate the prowess of fungal tailoring enzymes to concisely and efficiently transform a polyketide scaffold into complex structures. Moreover, comparison with citreoviridin and aurovertin biosynthesis suggests that methylation of the α-pyrone hydroxyl group by methyltransferase (CtvB or AurB) is the branching point where the biosynthesis of these two classes of compounds diverge. Therefore, scanning for the presence or absence of the gatekeeping α-pyrone methyltransferase gene in homologous clusters might be a potential way to bioinformatically classify the product as belonging to methylated α-pyrone polyenes or polyenes containing rings derived from the cyclization of the unmethylated 5,6-dihydro-α-pyrone, such as 2,3-dimethyl-γ-lactone and oxabicyclo[2.2.1]heptane.