An Unexpected Split‐Merge Pathway in the Assembly of the Symmetric Nonribosomal Peptide Antibiotic Closthioamide

Closthioamide (CTA) is a symmetric nonribosomal peptide (NRP) comprised of two diaminopropane‐linked polythioamidated monomers. CTA is biosynthesized by Ruminiclostridium cellulolyticum via an atypical NRP synthetase (NRPS)‐independent biosynthetic pathway. Although the logic for monomer assembly was recently elucidated, the strategy for the biosynthesis and incorporation of the diamine linker remained a mystery. By means of genome editing, synthesis, and in vitro biochemical assays, we demonstrate that the final steps in CTA maturation proceed through a surprising split‐merge pathway involving the dual use of a thiotemplated intermediate. This pathway includes the first examples of an aldo‐keto reductase catalyzing the reductive release of a thiotemplated product, and of a transthioamidating transglutaminase. In addition to clarifying the remaining steps in CTA assembly, our data shed light on largely unexplored pathways for NRPS‐independent peptide biosynthesis. Closthioamide is a symmetric perthioamidated nonribosomal peptide (NRP) produced by an atypical NRP‐synthetase‐independent biosynthetic pathway. Using a combination of genome editing and in vitro biochemical assays, we elucidated the last steps of closthioamide biosynthesis and found an unexpected asymmetrical route involving novel enzyme functions for members of the aldo‐keto reductase and transglutaminase protein families.