Ligand recognition and discrimation in the S-adenosylhomocysteine (SAH) sensing riboswitch

核糖开关最近被发现是一类全新的基因控制元件，它通过结构的mRNA结构域和小的代谢物分子之间的直接和特异的相互作用来调节细菌、植物和真菌中的基因表达。这些信使核糖核酸结构域以非常高的特异性和亲和力识别它们的同源配体，并为此目的利用复杂的三维结构。此外，它们还能够区分化学上密切相关的分子。在与S-腺苷甲硫氨酸(SAM)或S-腺苷同型半胱氨酸(SAH)结合的核糖开关基团中发现了一个特别显著的例子。这两种代谢物的不同之处仅在于存在或不存在单一甲基和带正电的硫离子。然而，这些核糖开关与SAM或SAH的结合亲和力是SAM或SAH的200-1000倍。然而，对于三种不同类型的SAM结合的核糖开关，核糖开关-配体复合体的结构已经被解决，没有关于SAH结合的核糖开关的结构信息。本项目的目标是利用核磁共振波谱技术研究SAH核糖开关适配子结构域在溶液中与SAH形成的复合体的三维结构，以揭示SAH的识别模式和配体识别的结构基础。

Riboswitches have been very recently identified as a fundamentally new class of genetic control elements that modulate gene expression in bacteria, plants and fungi through direct and specific interactions between structured mRNA domains and small metabolite molecules. These mRNA domains recognize their cognate ligands with remarkably high specificity and affinity and utilize intricate three-dimensional structures to this end. Furthermore, they are able to discriminate against chemically closely related molecules. An especially remarkable example for this discrimation is found in the riboswitch groups which either bind S-adenosylmethionine (SAM) or S-adenosylhomocysteine (SAH). These two metabolites differ only by the presence or absence of a single methyl group and a positively charged sulfonium ion. Nevertheless these riboswitches bind either SAM or SAH with a 200 to > 1000fold higher affinity. Whereas for three different classes of SAM-binding riboswitches structures of the riboswitch-ligand complexes have been solved, no structural information is available for the SAH-binding riboswitch. The goal of this project is to solve the three-dimensional structure of the SAH riboswitch aptamer domain in complex with SAH in solution using NMR-spectroscopy to reveal the mode of SAH recognition and the structural basis for ligand discrimination.