AN RNA POCKET FOR AN ALIPHATIC HYDROPHOBE

We have isolated an RNA with specific affinity for the L-valine side c hain, using selection-amplification. The active RNA secondary structur e, identified by repeated selection, is a highly conserved asymmetric (4:10) internal loop adjacent to required C-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 distinguis hes aliphatic sidechains by size and, given the same total size, by co nfiguration. Though aliphatic-RNA interactions have frequently been ne glected, their avidity and specificity seem sufficient for a biologica l role.