RNAS WITH DUAL-SPECIFICITY AND DUAL RNAS WITH SIMILAR SPECIFICITY

The biological role of RNA is delimited by its possible reactions, whi ch can be explored by selection. A comparison of selected RNAs that bi nd one ligand with those that bind two related ligands suggests that a single nucleotide substitution can expand binding specificity. An RNA site with dual (joint) specificity has adenine and cytosine bases who se pK(a)'s appear shifted upward, thereby mimicking an efficient gener al acid-base catalyst. The joint site also contains two conserved, loo ped arginine-coding triplets implicated in arginine site formation. Tw o selected joint RNAs are identical in some regions and distinct in ot hers. The distinct regions, like some peptides, seem to function simil arly without being similar in primary structure.