The structure of a nucleolytic ribozyme that employs a catalytic metal ion

The TS ribozyme (originally called twister-sister) is a nucleolytic catalytic RNA. We present a crystal structure of the ribozyme in a pre-reactive conformation. Two co-axial helical stacks are organized by a three-way junction and two tertiary contacts. Five divalent metal ions are directly coordinated to RNA ligands, making important contributions to the RNA architecture. The scissile phosphate lies in a quasi-helical loop region that is organized by a network of hydrogen bonding. A divalent metal ion is directly bound to the nucleobase 5' to the scissile phosphate, with an innersphere water molecule positioned to interact with the O2' nucleophile. The rate of ribozyme cleavage correlated in a log-linear manner with divalent metal ion pKa, consistent with proton transfer in the transition state, and we propose the bound metal ion is a likely general base for the cleavage reaction. Our data indicate that the TS ribozyme functions predominantly as a metalloenzyme.