Comparative genome mining and heterologous expression of an orphan NRPS gene cluster direct the production of ashimides.

The ever-increasing bacterial genomic repositories reveal a great number of uncharacterized biosynthetic gene clusters, representing a tremendous resource for natural product discovery. Genome mining of the marine sp. NA03103 indicates the presence of an orphan nonribosomal peptide synthetase (NRPS) gene cluster (), to which there are no homologous gene clusters in the public genome databases. Heterologous expression of the gene cluster in the SBT18 strain led to the discovery of two novel cyclopeptides, ashimides A and B ( and ), with showing cytotoxic activity. In addition, we use bioinformatic analysis, gene inactivation and stable isotope labelling experiments, as well as biochemical assays, to present a coherent and novel assembly line for ashimide biosynthesis, featuring an unusual desaturation, halogenation and cyclization cascade catalyzed by a P450 monooxygenase and a FAD-dependent halogenase.