Alternative Conformations and Motions Adopted by 30S Ribosomal Subunits Visualized by Cryo-Electron Microscopy.

It is only after recent advances in cryo-electron microscopy that is now possible to describe at high resolution structures of large macromolecules that do not crystalize. Purified 30S subunits interconvert between the active and inactive conformations. The active conformation was described by crystallography in the early 2000s, but the structure of the inactive form at high resolution remains unsolved. Here we used cryo-electron microscopy to obtain the structure of the inactive conformation of the 30S subunit to 3.6 Angstrom resolution and study its motions. In the inactive conformation, three nucleotides at the 3 prime end of the 16S rRNA cause the region of helix 44 forming the decoding center to adopt an unlatched conformation and the 3 prime end of the 16S rRNA positions similarly to the mRNA during translation. Incubation of inactive 30S subunits at 42 C reverts these structural changes. The position adopted by helix 44 dictates the most prominent motions of the 30S subunit. We found that extended exposures to low magnesium concentrations induces unfolding of large rRNA structural domains. The air-water interface to which ribosome subuints are exposed during sample preparation also peel off some ribosomal proteins. Overall this study provides new insights about the conformational space explored by the 30S ribosomal subunit when the ribosomal particles are free in solution.