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

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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.