Site-specific dual-color labeling of long RNAs for single-molecule spectroscopy.

Labeling of long RNA molecules in a site-specific yet generally applicable manner is integral to many spectroscopic applications. Here we present a novel covalent labeling approach that is site-specific and scalable to long intricately folded RNAs. In this approach, a custom-designed DNA strand that hybridizes to the RNA guides a reactive group to target a preselected adenine residue. The functionalized nucleotide along with the concomitantly oxidized 3'-terminus can subsequently be conjugated to two different fluorophores via bio-orthogonal chemistry. We validate this modular labeling platform using a regulatory RNA of 275 nucleotides, the btuB riboswitch of Escherichia coli, demonstrate its general applicability by modifying a base within a duplex, and show its site-selectivity in targeting a pair of adjacent adenines. Native folding and function of the RNA is confirmed on the single-molecule level by using FRET as a sensor to visualize and characterize the conformational equilibrium of the riboswitch upon binding of its cofactor adenosylcobalamin. The presented labeling strategy overcomes size and site constraints that have hampered routine production of labeled RNA that are beyond 200 nt in length.

The X-ray Structures of Six Octameric RNA Duplexes in the Presence of Different Di- and Trivalent Cations.

Due to the polyanionic nature of RNA, the principles of charge neutralization and electrostatic condensation require that cations help to overcome the repulsive forces in order for RNA to adopt a three-dimensional structure. A precise structural knowledge of RNA-metal ion interactions is crucial to understand the mechanism of metal ions in the catalytic or regulatory activity of RNA. We solved the crystal structure of an octameric RNA duplex in the presence of the di- and trivalent metal ions Ca(2+), Mn(2+), Co(2+), Cu(2+), Sr(2+), and Tb(3+). The detailed investigation reveals a unique innersphere interaction to uracil and extends the knowledge of the influence of metal ions for conformational changes in RNA structure. Furthermore, we could demonstrate that an accurate localization of the metal ions in the X-ray structures require the consideration of several crystallographic and geometrical parameters as well as the anomalous difference map.

The AdoCbl-riboswitch interaction investigated by in-line probing and surface plasmon resonance spectroscopy (SPR).

The unique feature of riboswitches is to control selected gene expression by specific recognition of a cognate ligand. AdoCbl (adenosyl cobalamin, coenzyme B12) riboswitches regulate the expression of enzymes and transporters involved in bacterial AdoCbl biosynthesis or uptake on a transcriptional and/or translational level. The analysis of ligand recognition and the induced conformational changes requires a detailed knowledge of the kinetics and thermodynamics of ligand binding and of the secondary structure rearrangements. This chapter describes the investigation of coenzyme B12 binding to the btuB riboswitch from Escherichia coli by in-line probing assays and surface plasmon resonance spectroscopy. The experimental conditions, requirements, and performance of both methods are presented together with the evaluation of the experimental data to determine the associated conformational changes of the RNA and the kinetic and thermodynamic parameters of ligand binding. Owing to the light sensitivity of the cobalt(I)'carbon bond, these methods were specifically modified to ensure the chemical integrity of AdoCbl.