RNA FOLDS: Insights from recent crystal structures

An RNA fold is the result of packing together two or more coaxial helical s tacks. To date, four RNA folds have been determined at near-atomic resoluti on by X-ray crystallography: transfer RNA, the hammerhead ribozyme, the PB- PB domain of the Tetrahymena group I intron, and the hepatitis delta virus ribozyme. All four folds result in RNAs that are considerably more compact than isolated A-form duplexes. These structures illustrate, to varying degr ees, three modes of fold stabilization: association of complementary molecu lar surfaces, stabilization of close RNA packing by binding of cations, and stabilization through pseudoknotting.