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Metabolite binding flips riboswitch © NATURE 2004 Many bacteria contain RNA sensors that directly control gene expression by binding various small-molecule metabolites. Now, the first structure of one of these so-called riboswitches--which are embedded in the noncoding region of certain messenger RNAs--reveals exactly how metabolite binding influences gene expression [Nature, 432, 411 (2004)]. Robert T. Batey and coworkers at the University of Colorado, Boulder, crystallized hypoxanthine (a purine) bound to the purine-binding portion of a riboswitch that controls the production of bacterial enzymes involved in purine metabolism and transport. Their 1.95-Å X-ray crystal structure of this prototypical riboswitch (shown) reveals that hypoxanthine (red) is almost completely enveloped by the RNA. Binding of hypoxanthine (or guanine) stabilizes the riboswitch's complex fold and triggers the formation of a hairpin-shaped structure in the neighboring RNA that stops the cell's transcription machinery, Batey says. This mechanism allows the riboswitch to single-handedly turn off gene expression in response to an increase in the intracellular concentration of a metabolite and is likely common to all riboswitches, he points out. |
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