Dehydroalanine (Dha) and dehydrobutyrine (Dhb) display considerable flexibility in a variety of chemical and biological reactions. Natural products containing Dha and/or Dhb residues are often found to display diverse biological activities. While the (Z) geometry is predominant in nature, only a handful of metabolites containing (E)‐Dhb have been found thus far. Here we report discovery of a new antimicrobial peptide, albopeptide, through NMR analysis and chemical synthesis, which contains two contiguous unsaturated residues, Dha‐(E)‐Dhb. It displays narrow‐spectrum activity against vancomycin‐resistant Enterococcus faecium. In‐vitro biochemical assays show that albopeptide originates from a noncanonical NRPS pathway featuring dehydration processes and catalysed by unusual condensation domains. Finally, we provide evidence of the occurrence of a previously untapped group of short unsaturated peptides in the bacterial kingdom, suggesting an important biological function in bacteria.

Non-Ribosomal Peptide Synthetases (NRPSs) assemble a diverse range of natural products with important applications in both medicine and agriculture. They consist of several multienzyme subunits that must interact with each other in a highly controlled manner to facilitate efficient chain transfer, thus ensuring biosynthetic fidelity. Several mechanisms for chain transfer are known for NRPSs, promoting structural diversity. Herein, we report the first biochemically characterized example of a type II thioesterase (TEII) domain capable of catalysing aminoacyl chain transfer between thiolation (T) domains on two separate NRPS subunits responsible for installation of a dehydrobutyrine moiety. Biochemical dissection of this process reveals the central role of the TEII-catalysed chain translocation event and expands the enzymatic scope of TEII domains beyond canonical (amino)acyl chain hydrolysis. The apparent co-evolution of the TEII domain with the NRPS subunits highlights a unique feature of this enzymatic cassette, which will undoubtedly find utility in biosynthetic engineering efforts.

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