Probing Exchange Units for Combining Iterative and Linear Fungal Nonribosomal Peptide Synthetases

A considerable number of complex peptides are synthesized by nonribosomal peptide synthetases (NRPSs). Due to their multimodular architecture and widely understood basic biosynthetic reactions, these synthetases represent a promising target for compound diversification by active reprogramming. Nevertheless, the limited knowledge about mechanistic details such as C domain specificity hampers rational synthetase engineering. Here, we present a systematic investigation of three fungal NRPS exchange units (C-A-Mt-T, C_CTD-A-Mt-T, and A-Mt-T) focusing on the influence of C domains at heterologous domain junctions. By functionally integrating units from linear cyclosporine synthetase into iterative cyclodepsipeptide synthetases in vivo, we demonstrate that fungal NRPSs of different assembly types can be combined using different swapping sites, while respecting the C domain integrity and specificity. Based on 24 hybrid synthetases, we suggest exchange rules for efficient fungal NRPS engineering. The findings are of importance for rational synthetase design and provide a new set of options for combinatorial reprogramming.