Genetic origin of homopyrones, a rare type of hybrid phenylpropanoid- and polyketide-derived yellow pigments from Aspergillus homomorphus

In recent years, there has been an increasing demand for the replacement of synthetic food colorants with naturally derived alternatives. Filamentous fungi are prolific producers of secondary metabolites including polyketide-derived pigments, many of which have not been fully characterized yet. During our ongoing investigations of black aspergilli, we noticed that Aspergillus homomorphus turned yellow when cultivated on malt extract agar plates. Chemical discovery guided by UV and MS led to the isolation of two novel yellow natural products, and their structures were elucidated as aromatic alpha-pyrones homopyrones A (1) and B (2) by HRMS and NMR. Combined investigations including retro-biosynthesis, genome mining, and gene deletions successfully linked both compounds to their related biosynthetic gene clusters. This demonstrated that homopyrones are biosynthesized by using cinnamoyl-CoA as the starter unit, followed by extension with three malonyl-CoA units, and lactonization to give the core hybrid backbone structure. The polyketide synthase AhpA includes a C-methylation domain, which however seems to be promiscuous since only 2 is C-methylated. Altogether, the homopyrones represent a rare case of hybrid phenylpropanoid- and polyketide-derived natural products in filamentous fungi.