Biosynthesis of acurin A and B in Aspergillus aculeatus

Aspergillus aculeatus is known for the commercial utilization in production of several enzymes. We have identified two stereoisomeric compounds of mixed polyketide-nonribosomal peptide origin in the extracts of A. aculeatus that we named acurin A and acurin B. The structures of the compounds strongly resemble the structure of the mycotoxin fusarin C produced by several Fusarium species. CRISPR-Cas9 was used to construct a non-homologous end-joining deficient strain of A. aculeatus, which enabled efficient gene deletions in the acurin gene cluster. Using RT-qPCR in combination with metabolite profiling of gene deletion strains, the acurin producing gene cluster was delineated, which allowed us to propose a biosynthetic pathway for formation of acurin. Our results show that acurin, in contrast to fusarin C, is biosynthesized by an individual polyketide synthase and non-ribosomal synthetase. At least six other enzymatic activities are required for the biosynthesis of acurin. This study shows how we exploit the CRISPR-Cas9 system for the rapid construction of fungal host strains that can be readily engineered to generate valuable knowledge.