Structural intermediates and folding events in the early assembly of the ribosomal small subunit

Using all-atom explicit solvent molecular dynamics (MD) simulations, we investigated the early structural intermediates of the 5' domain of the 16S rRNA in Escherichia coli upon the removal of the primary binding r-proteins S4, S17, and S20 and the secondary binding r-protein S16. Removal of each r-protein corresponded to the disappearance of subdomains with correlated dynamics. Correlation-based network analysis of the MD trajectories of the naked rRNA showed that the different subdomains are connected via multiple pathways with high betweenness. These pathways cross at the internal loop of helix 17 (h17) in the five-way junction (5WJ). The structure of the internal loop is disrupted by the binding of S17 and rescued by the addition of S16, suggesting an important function of the secondary binding protein in biasing the rRNA folding landscape toward the native basin. Using structure-based Gō simulations, we investigated the folding barriers of the lower four-way junction (4WJ) with h6, which is the primary binding site of S20 and the first to be transcribed. The folding of the 4WJ is consistent with the protection patterns observed in hydroxyl radical footprinting. Results from the all-atom simulations show that the fluctuations in the 5WJ are independent of the fluctuations in the 4WJ, suggesting that the subdomains fold independently and are stabilized by primary r-proteins.