Figure 7.

Model for transition state interactions within the active site of the Tetrahymena ribozyme. The dashed lines (–––) depict the partial bonds between the reactive phosphorus and the 3′-oxygens of G and U(−1), and δ− depict the partial negative charges on the 3′-oxyanions of S and G, as described in the legend to Fig. 2. Experimental observations from this and previous work have provided evidence for the active site interactions depicted by ••• and |||||| as described in the text. The results described herein strongly suggest the presence of at least three distinct metal ions at the ribozyme active site (M_A, M_B, and M_C; refs. 3, 5, 7, and 9 and this work).