Functional expression of a highly-reducing polyketide synthase of Emericella variecolor IFM42010, an asteltoxin-producing strain, resulted in production of two polyenoic β-ketolactones with opposite stereochemistry.

Abstract

The asteltoxin-producing fungus Emericella variecolor IFM42010 possesses 22 highly-reducing polyketide synthase (HR-PKS) genes. Of these, an HR-PKS with a methyltransferase domain but lacking an enoylreductase domain could be involved in the biosynthesis of asteltoxin and related compounds. From six such candidate HR-PKS genes, Ev460pks was analyzed by gene disruption in E. variecolor and heterologous expression in Aspergillus oryzae. The Ev460pks-disrupted strain retained asteltoxin production ability, indicating that Ev460pks is not involved in asteltoxin biosynthesis. The A. oryzae transformant harboring the Ev460pks gene produced compounds 1 and 2, along with several unidentified products possibly decomposed from 2. Spectroscopic analyses revealed that 1 was a 4-methyl-β-ketolactone with a methylheptatriene side-chain at the C-5 position, and 2 was also a 4-methyl-β-ketolactone, bearing a dimethyltetradecahexaene side-chain at the same position. The relative configuration at C-4 in compounds 1 and 2 was opposite.