Evolvability of the mode of peptide binding by an RNA

The HIV Rev-response element (RRE) RNA binds strongly to two unrelated peptides, the HIV Rev peptide and an RRE-binding aptamer, the RSG-1.2 peptide, at a similar site, but using distinct sets of interactions. In this study, the nucleotide base requirements for the binding of the RRE to the Rev and RSG-1.2 peptides were determined by selection of Rev- and RSG-1.2-binding RRE variants using a bacterial reporter system. As a result, distinct differences in the bases necessary for binding the two peptides were found in the upper stem of the RRE. Strikingly, single nucleotide changes in this region were found to switch the peptide-binding specificity of the RRE from a bifunctional Rev- and RSG-1.2-binding mode to either a Rev-specific or a RSG-1.2- specific mode, demonstrating how an RNA can evolve alternative binding strategies in discrete steps without intermediate loss of function. This evolvability of the mode of peptide binding by an RNA presumably reflects the multidimensionality of conformational space that a given RNA has available for ligand recognition, which may have been utilized in the evolution of RNA-polypeptide complexes.