Fluorescence turn-on aptamers,in vitroevolved RNA molecules that bind conditional fluorophores and activate their fluorescence, have emerged as RNA counterparts of the fluorescent proteins. Turn-on aptamers have been selected to bind diverse fluorophores, and they achieve varying degrees of specificity and affinity. These RNA–fluorophore complexes, many of which exceed the brightness of green fluorescent protein and their variants, can be used as tags for visualizing RNA localization and transport in live cells. Structure determination of several fluorescent RNAs revealed that they have diverse, unrelated overall architectures. As most of these RNAs activate the fluorescence of their ligands by restraining their photoexcited states into a planar conformation, their fluorophore binding sites have in common a planar arrangement of several nucleobases, most commonly a G-quartet. Nonetheless, each turn-on aptamer has developed idiosyncratic structural solutions to achieve specificity and efficient fluorescence turn-on. The combined structural diversity of fluorophores and turn-on RNA aptamers has already produced combinations that cover the visual spectrum. Further molecular evolution and structure-guided engineering is likely to produce fluorescent tags custom-tailored to specific applications.

中文翻译：

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用光追踪 RNA：荧光开启 RNA 适配体的选择、结构和设计

荧光开启适体，体外结合条件荧光团并激活其荧光的进化 RNA 分子已成为荧光蛋白的 RNA 对应物。已选择开启适体来结合不同的荧光团，它们实现不同程度的特异性和亲和力。这些 RNA-荧光团复合物，其中许多超过绿色荧光蛋白及其变体的亮度，可用作可视化活细胞中 RNA 定位和运输的标签。几种荧光 rna 的结构测定表明它们具有不同的、不相关的整体结构。由于这些 RNA 中的大多数通过将其光激发状态限制为平面构象来激活其配体的荧光，因此它们的荧光团结合位点共同具有几个核碱基的平面排列，最常见的是 G-四重奏。尽管如此，每个开启适体都开发了特殊的结构解决方案，以实现特异性和有效的荧光开启。荧光团和开启 RNA 适配体的组合结构多样性已经产生了覆盖视觉光谱的组合。进一步的分子进化和结构引导工程可能会产生针对特定应用定制的荧光标签。