Figure 3. The proposed structure of the active site of the twister ribozyme.

This is derived from the crystal structure, with a rotation of U-1 that brings the O2′ (modeled) into an in-line position ³⁴. G33 both acts as a general base (accepting a proton at N1) and stabilizes the transition state (N2 donates a proton to the proR non-bridging O atom of the scissile phosphate). A+1 acts as a general acid, donating a proton from N3. Its N6 donates protons to successive phosphates in the backbone, thereby raising its p K _a.