Ribosomal Formation of Thioamide Bonds in Polypeptide Synthesis.

It has been well established that the ribosome can accept various nucleophiles on the Xacyl-tRNA in the A site during elongation, where X can be amino, N-alkyl-amino, hydroxy, and thiol groups. However, it remains elusive that the ribosome is able to accept an electrophile in the P site other than the carboxyl group during elongation. Here we report ribosomal formation of a thioamide bond in the mRNA-dependent polypeptide synthesis. In this study, amino(carbothio)acyl-tRNA was prepared by flexizyme and used for the expression of peptides containing a thioamide bond in the nascent peptide chain. We give strong evidence that the thioamide-peptide was formed but accompanied by the amide counterpart due to rapid carbo(S-to-O) exchange during the preparation of amino(carbothio)acyl-tRNA. We also demonstrate the ribosomal formation of thioamide and N-methyl-thioamide bonds in linear as well as macrocyclic peptide scaffolds in the mRNA-dependent manner, showing its potential for applications in peptide-based drug discovery and studying peptide/protein structure and function.