In vitro selection of mRNA display libraries containing an unnatural amino acid

The incorporation of unnatural amino acid into selectable, amplifiable peptide and protein libraries expands the chemical diversity of such libraries, thus considerably facilitating the process of obtaining ligands with improved properties (affinity, specificity, and function), particularly against therapeutically interesting targets. Here, we report that biocytin, a biotin derivative of lysine, can be inserted into an mRNA-protein fusion molecule through amber stop codon suppression. We also demonstrate that templates containing the codon corresponding to the biocytin tRNA (a UAG stop codon) can be enriched by iterative cycles of selection against a streptavidin agarose matrix.