It is only after recent advances in cryo-electron microscopy that it is now possible to describe at high-resolution structures of large macromolecules that do not crystalize. Purified 30S subunits interconvert between an "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 Å resolution and study its motions. In the inactive conformation, alternative base pairing of three nucleotides causes the region of helix 44, forming the decoding center to adopt an unlatched conformation and the 3' end of the 16S rRNA positions similarly to the mRNA during translation. Incubation of inactive 30S subunits at 42 ºC reverts these structural changes. The air-water interface to which ribosome subunits are exposed during sample preparation also peel off some ribosomal proteins. Extended exposures to low magnesium concentrations make the ribosomal particles more susceptible to the air-water interface causing the unfolding of large rRNA structural domains. Overall this study provides new insights about the conformational space explored by the 30S ribosomal subunit when the ribosomal particles are free in solution.

中文翻译：

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冷冻电子显微镜可视化的 30S 核糖体亚基采用的替代构象和运动。

只有在冷冻电子显微镜的最新进展之后，现在才有可能描述不结晶的大分子的高分辨率结构。纯化的 30S 亚基在“活性”和“非活性”构象之间相互转换。在 2000 年代初期，通过晶体学描述了活性构象，但在高分辨率下非活性形式的结构仍未得到解决。在这里，我们使用冷冻电子显微镜获得 30S 亚基的非活性构象的结构，分辨率为 3.6 Å，并研究其运动。在非活性构象中，三个核苷酸的交替碱基配对导致螺旋 44 区域，形成解码中心，采用未锁定的构象，16S rRNA 的 3' 末端位置与翻译过程中的 mRNA 相似。在 42 ºC 下孵育非活性 30S 亚基可恢复这些结构变化。样品制备过程中核糖体亚基所接触的空气-水界面也会剥离一些核糖体蛋白质。长期暴露于低镁浓度会使核糖体颗粒更容易受到空气-水界面的影响，从而导致大 rRNA 结构域的展开。总的来说，当核糖体颗粒在溶液中游离时，这项研究提供了关于 30S 核糖体亚基探索的构象空间的新见解。长期暴露于低镁浓度会使核糖体颗粒更容易受到空气-水界面的影响，从而导致大 rRNA 结构域的展开。总的来说，当核糖体颗粒在溶液中游离时，这项研究提供了关于 30S 核糖体亚基探索的构象空间的新见解。长期暴露于低镁浓度会使核糖体颗粒更容易受到空气-水界面的影响，从而导致大 rRNA 结构域的展开。总的来说，当核糖体颗粒在溶液中游离时，这项研究提供了关于 30S 核糖体亚基探索的构象空间的新见解。