STRATEGIES FOR RNA FOLDING

RNAs are surprisingly adept at folding into specific shapes capable of ligand recognition and catalysis. Thermodynamic analysis of the unfol ding of several different RNAs suggests that there are at least three strategies an RNA might use to achieve a very stable and compactly fol ded structure: hydrogen bonding between irregular complementary surfac es (as in transfer RNA tertiary structure); monovalent and divalent io ns bound to specific sites (as found in a ribosomal RNA fragment) and pseudoknot folds (exemplified by a messenger RNA fragment with extensi ve noncanonical structure).