Role of counterion condensation in folding of the Tetrahymena ribozyme. I.Equilibrium stabilization by cations

Folding of RNA into an ordered, compact structure requires substantial neut ralization of the negatively charged backbone by positively charged counter ions. Using a native gel electrophoresis assay, we have examined the effect s of counterion condensation upon the equilibrium folding of the Tetrahymen a ribozyme. Incubation of the ribozyme in the presence of mono-, di- and tr ivalent ions induces a conformational state that is capable of rapidly form ing the native structure upon brief exposure to Mg2+. The cation concentrat ion dependence of this transition is directly correlated with the charge of the counterion used to induce folding. Substrate cleavage assays confirm t he rapid onset of catalytic activity under these conditions. These results are discussed in terms of classical counter-ion condensation theory. A mode l for folding is proposed which predicts effects of charge, ionic radius an d temperature on counterion-induced RNA folding transitions. (C) 2001 Acade mic Press.