Evolution of Iron(II)-Finger Peptides by Using a Bipyridyl Amino Acid

We report the engineering of zinc-finger-like motifs containing the unnatural amino acid (2,2-bipyridin-5-yl)alanine (Bpy-Ala). A phage-display library was constructed in which five residues in the N-terminal finger of zif268 were randomized to include both canonical amino acids and Bpy-Ala. Panning of this library against a nine-base-pair DNA binding site identified several Bpy-Ala-containing functional Zif268 mutants. These mutants bind the Zif268 recognition site with affinities comparable to that of the wild-type protein. Further characterization indicated that the mutant fingers bind low-spin Fe-II rather than Zn-II. This work demonstrates that an expanded genetic code can lead to new metal ion binding motifs that can serve as structural, catalytic, or regulatory elements in proteins.