Effect of cations on tRNA structure

Fluorescence characterization of the transfer RNA-like domain of transfer messenger RNA in complex with small binding protein B

Transfer messenger RNA (tmRNA) and small binding protein B (SmpB) are the main components of the trans-translation rescue machinery that releases stalled ribosomes from defective mRNAs. Little is known about how SmpB binding affects the conformation of the tRNA-like domain (TLD) of tmRNA. It has been previously hypothesized that the absence of a D stem in the TLD provides flexibility in the elbow region of tmRNA, which can be stabilized by its interaction with SmpB. Here, we have used Förster resonance energy transfer to characterize the global structure of the tRNA-like domain of tmRNA in the presence and absence of SmpB and as a function of Mg(2+) concentration. Our results show tight and specific binding of SmpB to tmRNA. Surprisingly, our data show that the global conformation and flexibility of tmRNA do not change upon SmpB binding. However, Mg(2+) ions induce an 11 Å compaction in the tmRNA structure, suggesting that the flexibility in the H2a stem may allow different conformations of tmRNA as the TLD and mRNA-like domain need to be positioned differently while moving through the ribosome.

Calculation of the binding free energy for magnesium-RNA interactions

The nature of the interaction between nucleic acids and divalent ions in solution is complex. It includes long-range electrostatic and short-range nonelectrostatic forces. Water molecules can be in an inner coordination shell that intervenes between the ion and its binding site. This work describes a method for calculating the binding free energy and applies it to a specific Mg-RNA system in the presence of monovalent salt. The approach combines high-level ab initio theory with Poisson-Boltzmann calculations and provides an accurate description of all terms of the binding free energy for magnesium ions located at the RNA surface (including nonelectrostatic interactions). Some alternative macroscopic approaches are also discussed.

On the role of magnesium ions in RNA stability

Divalent cations, like magnesium, are crucial for the structural integrity and biological activity of RNA. In this article, we present a picture of how magnesium stabilizes a particular folded form of RNA. The overall stabilization of RNA by Mg2+ is given by the free energy of transferring RNA from a reference univalent salt solution to a mixed salt solution. This term has favorable energetic contributions from two distinct modes of binding: diffuse binding and site binding. In diffuse binding, fully hydrated Mg ions interact with the RNA via nonspecific long-range electrostatic interactions. In site binding, dehydrated Mg2+ interacts with anionic ligands specifically arranged by the RNA fold to act as coordinating ligands for the mental ion. Each of these modes has a strong coulombic contribution to binding; however, site binding is also characterized by substantial changes in ion solvation and other nonelectrostatic contributions. We will show how these energetic differences can be exploited to experimentally distinguish between these two classes of ions using analyses of binding polynomials. We survey a number of specific systems in which Mg(2+)-RNA interactions have been studied. In well-characterized systems such as certain tRNAs and some rRNA fragments these studies show that site-bound ions can play an important role in RNA stability. However, the crucial role of diffusely bound ions is also evident. We emphasize that diffuse binding can only be described rigorously by a model that accounts for long-range electrostatic forces. To fully understand the role of magnesium ions in RNA stability, theoretical models describing electrostatic forces in systems with complicated structures must be developed.

Influence of the polyamines spermine and spermidine on yeast tRNAPhe as revealed from its imino proton NMR spectrum

A comparison of imino proton NMR spectra of yeast tRNAPhe recorded at various solution conditions indicates, that polyamines have a limited effect on the structure of this tRNA molecule. Polyamines are found to catalyse the solvent exchange of several imino protons in yeast tRNAPhe not only of non hydrogen bonded imino protons, but also of imino protons of the GU and of some AU and tertiary base pairs. It is concluded that at low levels of catalysing components the exchange rates of the latter protons are not determined by the base pair lifetime. In the presence of high levels of spermidine the solvent exchange rates of imino protons of several base pairs in the molecule were assessed as a function of the temperature. Apparent activation energies derived from these rates were found to be less than 80 kJ/mol, which is indicative for (transient) independent opening of the corresponding base pairs. In the acceptor helix the GU base pair acts as a dynamic dislocation. The AU base pairs at one side of the GU base pair exhibit faster transient opening than the GC base pairs on the other side of this wobble pair. The base pairs m2GC10 and GC11 from the D stem and GC28 from the anticodon stem show relatively slow opening up to high temperatures. Model studies suggest that 1-methyladenosine, an element of tRNA itself, catalyses imino proton solvent exchange in a way similar to polyamines.

Crystallization of transfer ribonucleic acids

A compilation of crystallization experiments of tRNAs published in literature as well as original results are given and discussed in this paper. Up to now 17 different tRNA species originating from Escherichia coli and from the yeast Saccharomyces cerevisiae have been crystallized. All structural tRNA families are represented, namely the tRNAs with large or small extra-loops and among them the initiator tRNAs. The tRNAs with small variable loops (4 to 5 nucleotides), e.g. tRNAAsp and tRNAPhe, yield the best diffracting crystals. Crystalline polymorphism is a common feature; about 100 different crystal forms have been observed, but only 6 among them enabled structure determination studies by X-ray diffraction. Crystallization strongly depends upon experimental parameters such as the presence of polyamines and magnesium as well as upon the purity and the molecular integrity of the tRNAs. Crystals are usually obtained by vapour diffusion methods using salts (e.g. ammonium sulfate), organic solvents (e.g. isopropanol, dioxane or 2-methyl-2,4-pentane diol) or polyethylene glycol as precipitants. A methodological strategy for crystallyzing new tRNA species is described.

Proton exchange rates in transfer RNA as a function of spermidine and magnesium

Solvent exchange rates of selected protons were measured by NMR saturation recovery for E. coli tRNAVal, E. colifMet and yeast tRNAPhe, at temperatures from 20 to 40 degrees C, in the presence of 0.12M Na+ and various levels of added spermidine. tRNAVal was also studied with added Mg++. The exchange rates in zero spermidine and Mg++ indicate early melting of the U8 A14 interaction, in accord with thermodynamic melting studies. Exchange rates for secondary protons suggest early melting of the T stem in tRNAfMet and the acceptor stem in tRNAPhe, in contradiction with melting transition assignments from thermodynamic work. Addition of 10 spermidines per tRNA stabilizes the secondary and tertiary interactions more effectively than added Na+, but less so than Mg++. Added spermidine has the curious effect of increasing the exchange rate of the psi 55 N1 proton, while protecting the psi 55 N3 proton from exchange in all three tRNA's. Added Mg++ has the same effect on tRNAVal.

Study of transfer ribonucleic acid unfolding by dynamic nuclear magnetic resonance

Nuclear magnetic resonance (NMR) measurements of proton exchange were performed on yeast tRNAPhe, and in much less detail on Escherichia coli tRNAfMet, over a range of Mg2+ concentrations and temperatures, at neutral pH and 0.1 M NaCl. The resonances studied were those of ring nitrogen protons, resonating between 10 and 15 ppm downfield from sodium 3-(trimethylsilyl)-1-propanesulfonate, which partake in hydrogen bonding between bases of secondary and tertiary pairs. Methods include saturation--recovery, line width, and real-time observation after a change to deuterated solvent. The relevant theory is briefly reviewed. We believe that most of the higher temperature rates reflect major unfolding of the molecule. For E. coli tRNAfMet, the temperature dependence of the rate for the U8--A14 resonance maps well onto previous optical T-jump studies for a transition assigned to tertiary melting. For yeast tRNAPhe, exchange rates of several resolved protons could be studied from 30 to 45 degrees C in zero Mg2+ concentration and had activation energies on the order of 40 kcal/mol. Initially, the tertiary structure melts, followed shortly by the acceptor stem. At high Mg2+ concentration, relatively few exchange rates are measurable below the general cooperative melt at about 60 degrees C; these are attributed to tertiary changes. Real-time observations suggest a change in the exchange mechanism at room temperature with a lower activation energy. The results are compared with those obtained by other methods directed toward assaying ribonucleic acid dynamics.