We report the evolution of an RNA aptamer to change
its binding specificity. RNA aptamers that bind the free
amino acid tyrosine were in vitro selected from a degenerate
pool derived from a previously selected dopamine aptamer.
Three independent sequences bind tyrosine in solution,
the winner of the selection binding with a dissociation
constant of 35 μM. Competitive affinity chromatography
with tyrosine-related ligands indicated that the selected
aptamers are highly L-stereo selective and also recognize
L-tryptophan and L-dopa with similar affinity. The binding
site was localized by sequence comparison, analysis of
minimal boundaries, and structural probing upon ligand
binding. Tyrosine-binding sites are characterized by the
presence of both tyrosine (UAU and UAC) and termination
(UAG and UAA) triplets.