A SPECIFIC MONOVALENT METAL-ION INTEGRAL TO THE AA PLATFORM OF THE RNA TETRALOOP RECEPTOR

Metal ions are essential for the folding and activity of large catalyt ic RNAs. While divalent metal ions have been directly implicated in RN A tertiary structure formation, the role of monovalent ions has been l argely unexplored. Here we report the first specific monovalent metal ion binding site within a catalytic RNA. As seen crystallographically, a potassium ion is coordinated immediately below AA platforms of the Tetrahymena ribozyme P4-P6 domain, including that within the tetraloop receptor. Interference and kinetic experiments demonstrate that potas sium ion binding within the tetraloop receptor stabilizes the folding of the P4-P6 domain and enhances the activity of the Azoarcus group I intron. Since a monovalent ion binding site is integral to the tetralo op receptor, a tertiary structural motif that occurs frequently in RNA , monovalent metal ions are likely to participate in the folding and a ctivity of a wide diversity of RNAs.