23S rRNA similarity from selection for peptidyl transferase mimicry

RNAs from a randomized pool were selected by affinity elution for binding to the molecule CCdApPuro, a high-affinity ligand of ribosomal peptidyl transferase designed as a transition-state analogue of peptide formation. The selected RNAs show affinity for CCdApPuro comparable to that of the peptidyl transferase center itself (Kd approximately 10 nM). Chemical modification/protection experiments implicate bases completely conserved among the selected RNAs in CCdApPuro interaction, which appears to involve both CCdA and puromycin moieties, that is, both A- and P-site homologues. The apparent selected binding site shows up to 17 nucleotides with similarity to conserved nucleotides of the peptidyl transferase loop domain of 23S rRNA and is conserved when reselected under mutagenesis. Thus, these nucleotides of 23S rRNA likely provide elements of the peptidyl transferase active center that bind the reactants near the site of peptide bond formation. Binding of CCdApPuro by a peptidyl transferase-like motif in the absence of protein strengthens the hypothesis that peptidyl transfer originated in an RNA world.