The hammerhead cleavage reaction in monovalent cations

Recently, Murray et al. (Chem Biol, 1998, 5:587-595) found that the hammerhead ribozyme does not require divalent metal ions for activity if incubated in high (greater than or equal to1 M) concentrations of monovalent ions. We further characterized the hammerhead cleavage reaction in the absence of divalent metal. The hammerhead is active in a wide range of monovalent ions, and the rate enhancement in 4 M Li+ is only 20-fold less than that in 10 mM Mg2+. Among the Group I monovalent metals, rate correlates in a log-linear manner with ionic radius, The pH dependence of the reaction is similar in 10 mM Mg2+, 4 M Li+, and 4 M Na+. The exchange-inert metal complex Co(NH3)(6)(3+) also supports substantial hammerhead activity. These results suggest that a metal ion does not act as a base in the reaction, and that the effects of different metal ions on hammerhead cleavage rates primarily reflect structural contributions to catalysis.