Elucidation of Pseurotin Biosynthetic Pathway Points to Trans-ActingC-Methyltransferase: Generation of Chemical Diversity

Pseurotins comprise a family of structurally related Aspergillal natural products having interesting bioactivity. However, little is known about the biosynthetic steps involved in the formation of their complex chemical features. Here, we systematically deleted the pseurotin biosynthetic genes in A. fumigatus and performed in vivo and in vitro characterization of the tailoring enzymes to determine the biosynthetic intermediates and the gene products responsible for the formation of each intermediate. This allowed us to elucidate the main biosynthetic steps leading to the formation of pseurotin A from the predominant precursor, azaspirene. The study revealed the combinatorial nature of the biosynthesis of the pseurotin family of compounds and the intermediates. Most interestingly, we report the first identification of an epoxidase–C-methyltransferase bifunctional fusion protein PsoF that appears to methylate the nascent polyketide backbone carbon atom in trans.