Biosynthesis of acurin A and B, two novel isomeric fusarin C-like compounds from Aspergillus aculeatus
This study presents the identification and proposed biosynthetic pathway for two stereoisomeric compounds of mixed polyketide-nonribosomal peptide origin that we named acurin A and acurin B. The compounds were discovered in extracts from Aspergillus aculeatus, a filamentous fungus known for the commercial utilization in the production of several enzymes. The structures of acurin A and B highly resemble the mycotoxin fusarin C produced by several Fusarium species. In our work, we used CRISPR-Cas9 to construct a non-homologous end-joining deficient strain of A. aculeatus, which enabled efficient gene deletions in the acurin gene cluster. Using gene-expression analysis in combination with metabolite profiling of gene-deletion strains, the gene cluster responsible for acurin production was delineated, which allowed us to propose a biosynthetic pathway for formation of acurin. Our results show that acurin is biosynthesized by an individual polyketide synthase and non-ribosomal synthetase. Moreover, at least six other enzymatic activities are required to complete the biosynthesis of acurin. This study shows how we exploit the CRISPR-Cas9 system in filamentous fungi for the rapid construction of fungal host strains that can be readily engineered to elucidate biosynthetic pathways.