Global survey of secondary metabolism in Aspergillus niger via activation of specific transcription factors

Genomics analysis confirmed the status of the filamentous fungi as a rich source of novel secondary metabolites; however, the discovery of these compounds is hampered by the cryptic nature of their biosynthetic pathways under laboratory conditions. Consequently, despite substantial research effort over the past decades, much of the secondary metabolome remains uncharacterized in fungal organisms. Our manual curation of biosynthetic gene clusters (BGCs) in the Aspergillus niger NRRL3 genome revealed that only 13 of 86 BGCs have had their cognate secondary metabolite products confirmed or reliably inferred. We also identified 60 transcription factors associated with cryptic BGCs. To further characterize A. niger secondary metabolism, we created a collection of strains each overexpressing a single BGC-associated transcription factor. We analyzed the strain collection using a standardized pipeline where we monitored phenotypic changes and compound production using mass spectrometry. Strains showing evidence of secondary metabolism activation were selected for gene expression analysis. Our approach resulted in the production of multiple potentially novel secondary metabolites and linked a specific BGC to tensidol production in A. niger. More broadly, this study found evidence counter to the existing paradigm of BGC expression controlled by co-localized transcription factors, lending credence to the emerging picture of a complex regulatory network governing fungal secondary metabolism. Significance StatementFungi produce an array of chemically diverse compounds that are routinely found to harbour valuable bioactivity. The products of secondary metabolism, these compounds have been a source of antimicrobials, anti-cancer agents, and other biopharmaceutical compounds termed natural products. Despite their demonstrated economic value, much is still unknown about the biosynthesis, regulation, and identities of these compounds. This study adopted a genome-wide approach to improve our understanding of the regulatory mechanisms that control fungal secondary metabolism, improving our ability to investigate the pathways responsible for natural product production.