Figure 7. Model for assembly of the 30S 5′ domain.

(a) Free energy diagram illustrating how selective stabilization of a native-like intermediate by S16 depopulates competing I’s and results in more cooperative assembly, 1°, S4+S17+S20. (b) Helices that switch conformation when bound by S16 (blue surface) are highlighted in a structure of the 30S ribosome (2avy). (c) An ensemble of partly folded RNA (I_C) containing different configurations of core helices are bound and further stabilized by primary assembly proteins S4, S17 and S20 (pale pink, green and yellow). In the absence of S16, a native-like (I_N) and a non-native intermediate (I_nC) have similar free energies and are both formed. S16 (light blue) preferentially stabilizes I_N, resulting in depopulation of I_nC and a smoother transition to the native RNP (N). Long-distance communication between the binding site of S16 in H17 (cyan) and H15 (blue) and helix 3 (purple) stabilizes the helix 18 pseudoknot (pink) in the 30S decoding center. Equilibrium data do not distinguish paths from I_nC to N (dashed arrows).