Decoding and reprogramming fungal iterative nonribosomal peptide synthetases

Nonribosomal peptide synthetases (NRPSs) assemble a large group of structurally and functionally diverse natural products. While the iterative catalytic mechanism of bacterial NRPSs is known, it remains unclear how fungal NRPSs create products of desired length. Here we show that fungal iterative NRPSs adopt an alternate incorporation strategy. Beauvericin and bassianolide synthetases have the same C(1)-A(1)-T(1)-C(2)-A(2)-MT-T(2a)-T(2b)-C(3) domain organization. During catalysis, C(3) and C(2) take turns to incorporate the two biosynthetic precursors into the growing depsipeptide chain that swings between T(1) and T(2a)/T(2b) with C(3) cyclizing the chain when it reaches the full length. We reconstruct the total biosynthesis of beauvericin in vitro by reacting C(2) and C(3) with two SNAC-linked precursors and present a domain swapping approach to reprogramming these enzymes for peptides with altered lengths. These findings highlight the difference between bacterial and fungal NRPS mechanisms and provide a framework for the enzymatic synthesis of non-natural nonribosomal peptides.