Ribozyme cleavage of a 2′,5′-phosphodiester linkage: Mechanism and a restricted divalent metal-ion requirement

Abstract

The natural substrate cleaved by the hepatitis delta virus (HDV) ribozyme contains a 3′,5′-phosphodiester linkage at the cleavage site; however, a 2′,5′-linked ribose-phosphate backbone can also be cleaved by both trans-acting and self-cleaving forms of the HDV ribozyme. With substrates containing either linkage, the HDV ribozyme generated 2′,3′-cyclic phosphate and 5′-hydroxyl groups suggesting that the mechanisms of cleavage in both cases were by a nucleophilic attack on the phosphorus center by the adjacent hydroxyl group. Divalent metal ion was required for cleavage of either linkage. However, although the 3′,5′-linkage was cleaved slightly faster in Ca²⁺ than in Mg²⁺, the 2′,5′-linkage was cleaved in Mg²⁺ (or Mn²⁺) but not Ca²⁺. This dramatic difference in metal-ion specificity is strongly suggestive of a crucial metal-ion interaction at the active site. In contrast to the HDV ribozymes, cleavage at a 2′,5′-phosphodiester bond was not efficiently catalyzed by the hammerhead ribozyme. The relaxed linkage specificity of the HDV ribozymes may be due in part to lack of a rigid binding site for sequences 5′ to the cleavage site.