SAM riboswitch

Abstract: The S-adenosylmethionine (SAM) riboswitch is one of the most recurrent riboswitches found in bacteria and has three known different natural aptamers. The Bacillus subtilis yitJ SAM riboswitch aptamer is organized around a four-way junction which is characterized by the presence of a pseudoknot and a K-turn motif. By replacing the adenine involved in a Watson−Crick base pair at position 138 in the core region of the aptamer with the fluorescent analogue 2-aminopurine (2AP), we show that the ligand-induced reorganization of the aptamer strongly attenuates 2AP fluorescence. The fluorescence quenching process is specific to SAM on the basis of the observation that the structural analogue S-adenosylhomocysteine does not promote a similar effect. We find that the pseudoknot is important for the reorganization of the core domain and that the K-turn motif also has a marked influence on the core domain reorganization, most probably through its important role in pseudoknot formation. Finally, we show that SAM ribos...

Abstract: Riboswitches are metabolite-sensing, conserved domains located in non-coding regions of mRNA that are central to regulation of gene expression. Here we report the first three-dimensional structure of the recently discovered S-adenosyl-L-methionine responsive SAM-VI riboswitch. SAM-VI adopts a unique fold and ligand pocket that are distinct from all other known SAM riboswitch classes. The ligand binds to the junctional region with its adenine tightly intercalated and Hoogsteen base-paired. Furthermore, we reveal the ligand discrimination mode of SAM-VI by additional X-ray structures of this riboswitch bound to S-adenosyl-L-homocysteine and a synthetic ligand mimic, in combination with isothermal titration calorimetry and fluorescence spectroscopy to explore binding thermodynamics and kinetics. The structure is further evaluated by analysis of ligand binding to SAM-VI mutants. It thus provides a thorough basis for developing synthetic SAM cofactors for applications in chemical and synthetic RNA biology.

Abstract: We investigate the folding energy landscape for a given RNA sequence through Boltzmann ensemble (BE) sampling of RNA secondary structures. The ensemble of sampled structures is used to derive distributions of energies and base-pair distances between two configurations. We identify structural features that can be utilized for RNA gene finding. Characterization of the EL through BE sampling of secondary structures is computationally demanding and has multiple heterogeneous stages. We develop the Distributed Adaptive Runtime Environment to effectively address the computational requirements. Distributed Adaptive Runtime Environment is built upon an extensible and interoperable pilot-job and supports the concurrent execution of a broad range of task sizes across a range of infrastructure. It is used to investigate two RNA systems of different sizes, S-adenosyl methionine (SAM) binding RNA sequences known as SAM-I riboswitches, and the S gene of the bovine corona virus RNA genome. We demonstrate how the implementation lowers the total time to solution for increases in RNA length, the number of sequences investigated, and the number of sampled structures. The distributions of energies and base-pair distances reveal variations in folding dynamics and pathways among the SAM riboswitch sequences. Our results for BCoV RNA genome sequences also indicate sensitivity of folding to coding-neutral variations in sequence. We search for a characteristic motif from within the SAM-I consensus structure – a four-way junction, among BE sampled structures for all 2910 SAM-I sequences identified from Rfam (the curated ncRNA family database). We find that BE sampling provides insight into the variations in conformational distribution among sequences of the same ncRNA family. Therefore, BE sampling of secondary structures is a viable pre-processing or post-processing tool to complement comparative sequence analysis. The understanding gained shows how appropriately designed cyberinfrastructure can provide new insight into RNA folding and structure formation. Copyright © 2011 John Wiley & Sons, Ltd.