An RNA pocket for an aliphatic hydrophobe.

We have isolated an RNA with specific affinity for the L-valine side chain, using selection-amplification. The active RNA secondary structure, identified by repeated selection, is a highly conserved asymmetric (4:10) internal loop adjacent to required G-U pairs. The binding free-energy per methylene is up to 1.5 kcal mol-1, and very dependent on group position. Amino acid binding is L-stereoselective and distinguishes aliphatic sidechains by size and, given the same total size, by configuration. Though aliphatic-RNA interactions have frequently been neglected, their avidity and specificity seem sufficient for a biological role.