An optimal Mg2+ concentration for kinetic folding of the Tetrahymena ribozyme

Divalent metal ions, such as Mg2+, are generally required for tertiary stru cture formation in RNA. Although the role of Mg2+ binding in RNA-folding eq uilibria has been studied extensively. little is known about the role of Mg 2+ in RNA-folding kinetics. In this paper, we explore the effect of Mg2+ on the rate-limiting step in the kinetic folding pathway of the Tetrahymena r ibozyme. Analysis of these data reveals the presence of a Mg2+-stabilized k inetic trap that slows folding at higher Mg2+ concentrations. Thus, the Tet rahymena ribozyme folds with an optimal rate at 2 mM Mg2+, just above the c oncentration required for stable structure formation. These results suggest that thermodynamic and kinetic folding of RNA are cooptimized at a Mg2+ co ncentration that is sufficient to stabilize the folded form but low enough to avoid kinetic traps and misfolding.