Genomic analysis indicated that the genomes of ascomycetes might carry dozens of biosynthetic gene clusters (BGCs), yet many clusters have remained enigmatic. The ascomycete genus Epicoccum, belonging to the family Didymellaceae, is ubiquitous that colonizes different types of substrates and is associated with phyllosphere or decaying vegetation. Species of this genus are prolific producers of bioactive substances. The epicoccamides, as biosynthetically distinct mannosylated tetramate, were first isolated in 2003 from Epicoccum sp. In this study, using a combination of genome mining, chemical identification, genetic deletion, and bioinformatic analysis, we identified the required BGC epi responsible for epicoccamide A biosynthesis in Epicoccum sp. CPCC 400996.
The unconventional biosynthetic gene cluster epi was obtained from an endophyte Epicoccum sp. CPCC 400996 through AntiSMASH-based genome mining. The cluster epi includes six putative open reading frames (epiA-epiF) altogether, in which the epiA encodes a tetramate-forming polyketide synthase and nonribosomal peptide synthetases (PKS-NRPS hybrid). Sequence alignments and bioinformatic analysis to other metabolic pathways of fungal tetramates, we proposed that the gene cluster epi could be involved in generating epicoccamides. Genetic knockout of epiA completely abolished the biosynthesis of epicoccamide A (1), thereby establishing the correlation between the BGC epi and biosynthesis of epicoccamide A. Bioinformatic adenylation domain signature analysis of EpiA and other fungal PKS-NRPSs (NRPs) indicated that the EpiA is L-alanine incorporating tetramates megasynthase. Furthermore, based on the molecular structures of epicoccamide A and deduced gene functions of the cluster epi, a hypothetic metabolic pathway for biosynthesizing compound 1 was proposed. The corresponding tetramates releasing during epicoccamide A biosynthesis was catalyzed through Dieckmann-type cyclization, in which the reductive (R) domain residing in terminal module of EpiA accomplished the conversion. These results unveiled the underlying mechanism of epicoccamides biosynthesis and these findings might provide opportunities for derivatization of epicoccamides or generation of new chemical entities.
Genome mining and genetic inactivation experiments unveiled a previously uncharacterized PKS - NRPS hybrid-based BGC epi responsible for the generation of epicoccamide A (1) in endophyte Epicoccum sp. CPCC 400996. In addition, based on the gene cluster data, a hypothetical biosynthetic pathway of epicoccamide A was proposed.