The structure of the SAM/SAH-binding riboswitch.

S-adenosylmethionine (SAM) is a central metabolite since it is used as a methyl group donor in many different biochemical reactions. Many bacteria control intracellular SAM concentrations using riboswitch-based mechanisms. A number of structurally different riboswitch families specifically bind to SAM and mainly regulate the transcription or the translation of SAM-biosynthetic enzymes. In addition, a highly specific riboswitch class recognizes S-adenosylhomocysteine (SAH)-the product of SAM-dependent methyl group transfer reactions-and regulates enzymes responsible for SAH hydrolysis. High-resolution structures are available for many of these riboswitch classes and illustrate how they discriminate between the two structurally similar ligands SAM and SAH. The so-called SAM/SAH riboswitch class binds both ligands with similar affinities and is structurally not yet characterized. Here, we present a high-resolution nuclear magnetic resonance structure of a member of the SAM/SAH-riboswitch class in complex with SAH. Ligand binding induces pseudoknot formation and sequestration of the ribosome binding site. Thus, the SAM/SAH-riboswitches are translational 'OFF'-switches. Our results establish a structural basis for the unusual bispecificity of this riboswitch class. In conjunction with genomic data our structure suggests that the SAM/SAH-riboswitches might be an evolutionary late invention and not a remnant of a primordial RNA-world as suggested for other riboswitches.

NMR resonance assignments for the SAM/SAH-binding riboswitch RNA bound to S-adenosylhomocysteine.

Riboswitches are structured RNA elements in the 5'-untranslated regions of bacterial mRNAs that are able to control the transcription or translation of these mRNAs in response to the specific binding of small molecules such as certain metabolites. Riboswitches that bind with high specificity to either S-adenosylmethionine (SAM) or S-adenosylhomocysteine (SAH) are widespread in bacteria. Based on differences in secondary structure and sequence these riboswitches can be grouped into a number of distinct classes. X-ray structures for riboswitch RNAs in complex with SAM or SAH established a structural basis for understanding ligand recognition and discrimination in many of these riboswitch classes. One class of riboswitches-the so-called SAM/SAH riboswitch class-binds SAM and SAH with similar affinity. However, this class of riboswitches is structurally not yet characterized and the structural basis for its unusual bispecificity is not established. In order to understand the ligand recognition mode that enables this riboswitch to bind both SAM and SAH with similar affinities, we are currently determining its structure in complex with SAH using NMR spectroscopy. Here, we present the NMR resonance assignment of the SAM/SAH binding riboswitch (env9b) in complex with SAH as a prerequisite for a solution NMR-based high-resolution structure determination.

Measurement of Ligand-Target Residence Times by 1H Relaxation Dispersion NMR Spectroscopy.

A ligand-observed 1H NMR relaxation experiment is introduced for measuring the binding kinetics of low-molecular-weight compounds to their biomolecular targets. We show that this approach, which does not require any isotope labeling, is applicable to ligand-target systems involving proteins and nucleic acids of variable molecular size. The experiment is particularly useful for the systematic investigation of low affinity molecules with residence times in the micro- to millisecond time regime.

Stable isotope-labeled RNA phosphoramidites to facilitate dynamics by NMR.

Given that Ribonucleic acids (RNAs) are a central hub of various cellular processes, methods to synthesize these RNAs for biophysical studies are much needed. Here, we showcase the applicability of 6-(13)C-pyrimidine phosphoramidites to introduce isolated (13)C-(1)H spin pairs into RNAs up to 40 nucleotides long. The method allows the incorporation of 6-(13)C-uridine and -cytidine residues at any desired position within a target RNA. By site-specific positioning of the (13)C-label using RNA solid phase synthesis, these stable isotope-labeling patterns are especially well suited to resolve resonance assignment ambiguities. Of even greater importance, the labeling pattern affords accurate quantification of important functional transitions of biologically relevant RNAs (e.g., riboswitch aptamer domains, viral RNAs, or ribozymes) in the µs- to ms time regime and beyond without complications of one bond carbon scalar couplings. We outline the chemical synthesis of the 6-(13)C-pyrimidine building blocks and their use in RNA solid phase synthesis and demonstrate their utility in Carr Purcell Meiboom Gill relaxation dispersion, ZZ exchange, and chemical exchange saturation transfer NMR experiments.