Shape changes and cooperativity in the folding of central domain of the 16S ribosomal RNA

Abstract

Both the small and large subunits of the ribosome, the molecular machine that synthesizes proteins, are complexes of ribosomal RNAs (rRNAs) and a number of proteins. In bacteria, the small subunit has a single 16S rRNA whose folding is the first step in its assembly. The central domain of the 16S rRNA folds independently, driven either by Mg2+ ions or by interaction with ribosomal proteins. In order to provide a quantitative description of ion-induced folding of the [~]350 nucleotide rRNA, we carried out extensive coarse-grained molecular simulations spanning Mg2+ concentration between 0-30 mM. The Mg2+ dependence of the radius of gyration shows that globally the rRNA folds cooperatively. Surprisingly, various structural elements order at different Mg2+ concentrations, indicative of the heterogeneous assembly even within a single domain of the rRNA. Binding of Mg2+ ions is highly specific, with successive ion condensation resulting in nucleation of tertiary structures. We also predict the Mg2+-dependent protection factors, measurable in hydroxyl radical footprinting experiments, which corroborate the specificity of Mg2+-induced folding. The simulations, which agree quantitatively with several experiments on the folding of a three-way junction, show that its folding is preceded by formation of other tertiary contacts in the central junction. Our work provides a starting point in simulating the early events in the assembly of the small subunit of the ribosome. O_TEXTBOXSignificance StatementRibosomes are complexes between ribosomal RNA (rRNA) and a number of proteins. Because ribosome assembly begins with rRNA folding, we simulated the molecular details of Mg2+-driven folding of the central domain of the bacterial rRNA. Good agreement with experiments on the folding of the three-way junction in the center of the rRNA validates the model. Coupling of rRNA folding and Mg2+ binding shows that ions interact with rRNA segments in a coordinated manner. The shape of rRNA changes from a sphere in the unfolded state to a prolate ellipsoid at high Mg2+ concentration, which is the opposite of what transpires when a globular protein folds. Our study pro-vides the needed framework for undertaking ion-driven folding of large RNA molecules. C_TEXTBOX