Binding of small molecules, such as guanine, can alter the structure of RNA control elements found in the untranslated region of some genes. These structural alterations usually repress expression of the genes with which they are associated by affecting transcription, translation or mRNA processing. A structural and biochemical study from the Batey lab begins to unravel the nature of the conformational changes induced within one such control element, the guanine-responsive riboswitch, upon

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... action with the biologically relevant substrate hypoxanthine. The structure of the complex, the first of a riboswitch, reveals a highly compact RNA fold in which the ligand sits in a pocket created by the three-way junction. While this junction is directly responsible for hypoxanthine recognition, their data show that a loop-loop interaction is also required for efficient binding. Another striking feature of the structure is that the hypoxanthine is almost completely buried within the RNA and surrounded by conserved nucleotides. Like many RNA-ligand and RNA-protein interactions, binding is accompanied by a large conformational change in the RNA. This change in the structure directs the folding of the riboswitch toward a conformation that allows the surrounding RNA to adopt a classic terminator stem-loop that stops transcription. (Nature 432, [411][412][413][414][415] 2004) EJ