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REPORTS for 3-D structure determination by X-ray crystallography, signifying that the obtained X-ray data may help determine the 3-D structure of the preQ1 riboswitch. Further analysis of crystal data suggested stacking conformations of RNA-ligand complexes that were different from previously observed stacking interactions for non-methylated RNA. This suggests that crystallization of RNA with 2’-O-methylated nucleotides generates hydrophobic patches, reorganizing RNA in different conformations. 2’-O-methylations can also be used to acquire phase information for structure determination by replacing the oxygen with a selenium atom. Such a substitution would permit application of single- and multiple-wavelength anomalous dispersion (SAD and MAD) techniques to the selenium atom, simplifying the daunting task of 3-D structure determination.

Conclusions As a unique RNA system for gene expression control, riboswitches stand out as potential targets for antimicrobial drugs because they control essential genes in a wide range of bacteria and can recognize small drug-like molecules. With the help of 3-D structures, these natural metabolites may be redesigned to modern drug standards. The riboswitch-ligand complexes for both SAM-II and preQ1 were successfully crystallized, completing the primary goal of the project. Careful optimization of crystal conditions led to the generation of preQ1 crystals that diffracted at 3.1 Å, a resolution suitable for determination of the 3-D structure. Additional work on the preQ1 riboswitch will likely reveal novel metabolite binding rules, which may contribute to the design of future riboswitch-targeting antimicrobial drugs.


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