Ribosome-catalyzed synthesis of protein/oligopeptides with unnatural backbone

Nonsense suppression method was used to probe the allowable modification of substrate (amino acid) backbone in the prokaryotic ribosomal system. Dihydrofolate reductase (DHFR) with an amber mutation was translated in the RF1-diminished prokaryotic cell free translation system in the presence of chemically-misacylated yeast tRNA(Phe)CUA. The prokaryotic ribosome showed a restricted tolerance to the backbone modification. Although natural-type alpha-amino acid was accepted as a good substrate for the ribosome, incorporation of beta-aminopropionic acid was not detected under our experimental conditions. Interestingly, we found that the homologous beta-hydroxyalkanoic acid with elongated methylene (backbone) chain-length can be a substrate for the ribosome, giving an important implication for the chemical mechanism of the ribosome-catalyzed peptide bond forming reaction.