Comparison of the hammerhead cleavage reactions stimulated by monovalent and divalent cations

Although the hammerhead reaction proceeds most efficiently in divalent cati ons, cleavage in 4 M LiCl is only similar to 10-fold slower than under stan dard conditions of 10 mM MgCl2 (Murray et al,, Chem Biol, 1998, 5:587-595; Curtis & Bartel, RNA, 2001, this issue, pp. 546-552), To determine if the c atalytic mechanism with high concentrations of monovalent cations is simila r to that with divalent cations, we compared the activities of a series of modified hammerhead ribozymes in the two ionic conditions, Nearly all of th e modifications have similar deleterious effects under both reaction condit ions, suggesting that the hammerhead adopts the same general catalytic stru cture with both monovalent and divalent cations, However, modification of t hree ligands previously implicated in the binding of a functional divalent metal ion have substantially smaller effects on the cleavage rate in Li+ th an in Mg2+. This result suggests that an interaction analogous to the inter action made by this divalent metal ion is absent in the monovalent reaction . Although the contribution of this divalent metal ion to the overall react ion rate is relatively modest, its presence is needed to achieve the full c atalytic rate, The role of this ion appears to be in facilitating formation of the active structure, and any direct chemical role of metal ions in ham merhead catalysis is small.