The hammerhead cleavage reaction in monovalent cations

Recently, Murray et al. (Chem Biol, 1998, 5:587-595) found that the hammerh ead ribozyme does not require divalent metal ions for activity if incubated in high (greater than or equal to1 M) concentrations of monovalent ions. W e 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-line ar manner with ionic radius, The pH dependence of the reaction is similar i n 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 sugge st that a metal ion does not act as a base in the reaction, and that the ef fects of different metal ions on hammerhead cleavage rates primarily reflec t structural contributions to catalysis.