FIGURE 6.

Scheme showing two alternative pathways of k-turn folding upon binding to L7Ae protein. The Kt-7 RNA is in equilibrium between the extended and folded conformation in free solution, biased toward the extended state at low ionic concentration. In the conformational selection mechanism (the vertical equilibrium shown right), the L7Ae selectively binds to the folded form and thus draws the equilibrium toward this state. Alternatively, the protein could bind the extended state and mechanically force the RNA to adopt the k-turn structure in an induced fit process (left). The conformational selection model requires that L7Ae structure is complementary to that of the folded k-turn, and the structure of the k-turn in the absence of protein is closely similar to that of the bound RNA. The structures determined here (shown far right) show that these requirements are met.