Riboswitches are RNAs capable of binding cellular metabolites using a diverse array of secondary and tertiary structures to modulate gene expression. The recent determination of the three-dimensional structures of parts of six different riboswitches illuminates common features that allow riboswitches to be grouped into one of two types. Type I riboswitches, as exemplified by the purine riboswitch, are characterized by a single, localized binding pocket supported by a largely pre-established global fold. This arrangement limits ligand-induced conformational changes in the RNA to a small region. In contrast, Type II riboswitches, such as the thiamine pyrophosphate riboswitch, contain binding pockets split into at least two spatially distinct sites. As a result, binding induces both local changes to the binding pocket and global architecture. Similar organizational themes are found in other noncoding RNAs, making it possible to begin to build a hierarchical classification of RNA structure based on the spatial organization of their active sites and associated secondary structural elements.

中文翻译：

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核糖开关：RNA结构和功能中的新兴主题。

核糖开关是能够使用多种二级和三级结构阵列来调节基因表达的，结合细胞代谢产物的RNA。最近对六个不同核糖开关的零件的三维结构的确定阐明了允许将核糖开关分组为两种类型之一的共同特征。以嘌呤核糖开关为例，I型核糖开关的特征是单个，局部结合袋，其主要由预先确定的整体折叠支撑。这种排列将配体诱导的RNA构象变化限制在一个很小的区域。相反，II型核糖开关，如硫胺素焦磷酸核糖开关，包含结合袋，其分成至少两个空间上不同的位点。结果，绑定既引起绑定袋的局部更改，也引起全局体系结构。