Characterization of a Silent Azaphilone Gene Cluster from Aspergillus niger ATCC 1015 Reveals a Hydroxylation-Mediated Pyran-Ring Formation

SummaryAzaphilones are a class of fungal metabolites characterized by a highly oxygenated pyrano-quinone bicyclic core and exhibiting a broad range of bioactivities. Although widespread among various fungi, their biosynthesis has not been thoroughly elucidated. By activation of a silent (aza) gene cluster in Aspergillus niger ATCC 1015, we discovered six azaphilone compounds, azanigerones A–F (1, 3–7). Transcriptional analysis and deletion of a key polyketide synthase (PKS) gene further confirmed the involvement of the aza gene cluster. The biosynthetic pathway was shown to involve the convergent actions of a highly reducing PKS and a non-reducing PKS. Most significantly, in vitro reaction of a key flavin-dependent monooxygenase encoded in the cluster with an early benzaldehyde intermediate revealed its roles in hydroxylation and pyran-ring formation to afford the characteristic bicylic core shared by azaphilones.

Graphical AbstractFigure optionsDownload high-quality image (244 K)Download as PowerPoint slideHighlights
► Discovery of an azaphilone pathway in Aspergillus niger
► Six azaphilone compounds were isolated and characterized
► The biosynthesis involves convergent actions of an HR-PKS and an NR-PKS
► Hydroxylation by a monooxygenase promotes pyran-ring formation