Figure 5.

The proposed model with hydrated metal ion close to the GU pair.

Nucleotides are colored as in Figure 1A,B. C75 in purple, the cleavage site G•U wobble in green, and G2 in blue. The N7 of G1 interacts with a hydrated Mg²⁺ ion (magenta sphere) by inner-sphere coordination (shown as a red dashed line). The distance criteria used for inner-sphere (1.9–2.5Å) and outer-sphere (2.5–3.3Å) are based on the studies of Juneau and Cech(33). 2. Five water molecules (light blue sphere) surround the active site Mg²⁺ and complete the inner-sphere shell (1.9–2.1 Å, shown in black dash lines). The exact positions of the waters in this model are not known, however the water coordinates as drawn to provide a model of the expected radius of the hydration shell. The possible interaction between these coordinated waters and RNA functional groups are denoted by blue dotted lines. Other experimental criteria used to build the model are described in the text.