Mg 2+ and K+ binding in RNA structure. (a) Divalent magnesium ions (Mg2+) coordinate to phosphate groups and stabilize the close approach of phosphate backbones in the folded structure of the P4–P6 domain of the Tetrahymena group I ribozyme (a catalytic RNA). (b) Monovalent potassium ions (K+) bind to specific sites in the P4–P6 domain, where they favor interactions between both backbone atoms and bases.