Unusual mechanical stability of a minimal RNA kissing complex

Single-molecule mechanical unfolding and folding of a pseudoknot in human telomerase RNA

RNA unfolding and folding reactions in physiological conditions can be facilitated by mechanical force one molecule at a time. By using force-measuring optical tweezers, we studied the mechanical unfolding and folding of a hairpin-type pseudoknot in human telomerase RNA in a near-physiological solution, and at room temperature. Discrete two-state folding transitions of the pseudoknot are seen at approximately 10 and approximately 5 piconewtons (pN), with ensemble rate constants of approximately 0.1 sec(-1), by stepwise force-drop experiments. Folding studies of the isolated 5'-hairpin construct suggested that the 5'-hairpin within the pseudoknot forms first, followed by formation of the 3'-stem. Stepwise formation of the pseudoknot structure at low forces are in contrast with the one-step unfolding at high forces of approximately 46 pN, at an average rate of approximately 0.05 sec(-1). In the constant-force folding trajectories at approximately 10 pN and approximately 5 pN, transient formation of nonnative structures were observed, which is direct experimental evidence that folding of both the hairpin and pseudoknot takes complex pathways. Possible nonnative structures and folding pathways are discussed.

An RNA toolbox for single-molecule force spectroscopy studies

Precise, controllable single-molecule force spectroscopy studies of RNA and RNA-dependent processes have recently shed new light on the dynamics and pathways of RNA folding and RNA-enzyme interactions. A crucial component of this research is the design and assembly of an appropriate RNA construct. Such a construct is typically subject to several criteria. First, single-molecule force spectroscopy techniques often require an RNA construct that is longer than the RNA molecules used for bulk biochemical studies. Next, the incorporation of modified nucleotides into the RNA construct is required for its surface immobilization. In addition, RNA constructs for single-molecule studies are commonly assembled from different single-stranded RNA molecules, demanding good control of hybridization or ligation. Finally, precautions to prevent RNase- and divalent cation-dependent RNA digestion must be taken. The rather limited selection of molecular biology tools adapted to the manipulation of RNA molecules, as well as the sensitivity of RNA to degradation, make RNA construct preparation a challenging task. We briefly illustrate the types of single-molecule force spectroscopy experiments that can be performed on RNA, and then present an overview of the toolkit of molecular biology techniques at one's disposal for the assembly of such RNA constructs. Within this context, we evaluate the molecular biology protocols in terms of their effectiveness in producing long and stable RNA constructs.

Effects of nucleic acid local structure and magnesium ions on minus-strand transfer mediated by the nucleic acid chaperone activity of HIV-1 nucleocapsid protein

HIV-1 nucleocapsid protein (NC) is a nucleic acid chaperone, which is required for highly specific and efficient reverse transcription. Here, we demonstrate that local structure of acceptor RNA at a potential nucleation site, rather than overall thermodynamic stability, is a critical determinant for the minus-strand transfer step (annealing of acceptor RNA to (−) strong-stop DNA followed by reverse transcriptase (RT)-catalyzed DNA extension). In our system, destabilization of a stem-loop structure at the 5′ end of the transactivation response element (TAR) in a 70-nt RNA acceptor (RNA 70) appears to be the major nucleation pathway. Using a mutational approach, we show that when the acceptor has a weak local structure, NC has little or no effect. In this case, the efficiencies of both annealing and strand transfer reactions are similar. However, when NC is required to destabilize local structure in acceptor RNA, the efficiency of annealing is significantly higher than that of strand transfer. Consistent with this result, we find that Mg²⁺ (required for RT activity) inhibits NC-catalyzed annealing. This suggests that Mg²⁺ competes with NC for binding to the nucleic acid substrates. Collectively, our findings provide new insights into the mechanism of NC-dependent and -independent minus-strand transfer.

Real-time control of the energy landscape by force directs the folding of RNA molecules

The rugged folding-energy landscapes of RNAs often display many competing minima. How do RNAs discriminate among competing conformations in their search for the native state? By using optical tweezers, we show that the folding-energy landscape can be manipulated to control the fate of an RNA: individual RNA molecules can be induced into either native or misfolding pathways by modulating the relaxation rate of applied force and even be redirected during the folding process to switch from misfolding to native folding pathways. Controlling folding pathways at the single-molecule level provides a way to survey the manifold of folding trajectories and intermediates, a capability that previously was available only to theoretical studies.

Hairpin extensions enhance the efficacy of mycolyl transferase-specific antisense oligonucleotides targeting Mycobacterium tuberculosis

We have investigated the efficacy of modifying gene-specific antisense phosphorothioate oligodeoxyribonucleotides (PS-ODNs) by the addition of 5' and 3' hairpin extensions. As a model system, we have targeted the Mycobacterium tuberculosis 30/32-kDa mycolyl transferase protein complex genes encoding three highly related enzymes (antigens 85 A, B, and C). Whereas the addition of a hairpin extension at only one end of the PS-ODNs did not improve their inhibitory capacity, the addition of hairpin extensions at both ends enhanced their capacity to inhibit M. tuberculosis multiplication in comparison with unmodified PS-ODNs. A combination of three 5'-, 3'-hairpin-modified PS-ODNs (HPS-ODNs) targeting each of the three mycolyl transferase transcripts inhibited bacterial growth in broth culture by approximately 1.75 log units (P < 0.0001) and in human THP-1 macrophages by approximately 0.4 log units (P < 0.0001), which to our knowledge has not previously been demonstrated for any PS-ODN; reduced target gene transcription by > or =90%; caused approximately 90% reduction in mycolyl transferase expression; and increased bacterial sensitivity to isoniazid by 8-fold. The growth-inhibitory effect of the HPS-ODNs was gene-specific. Mismatched HPS-ODNs had no growth-inhibitory capacity. This study demonstrates that 5'- and 3'-HPS-ODNs are highly efficacious against M. tuberculosis and supports the further development of antisense technology as a therapeutic modality against tuberculosis.

LNA derivatives of a kissing aptamer targeted to the trans-activating responsive RNA element of HIV-1

We previously identified an RNA aptamer targeted to the trans-activating responsive (TAR) element of the HIV-1 genome [F. Ducongé, J.J. Toulmé, In vitro selection identifies key determinants for loop--loop interactions: RNA aptamers selective for the TAR RNA element of HIV--1. RNA 5 (1999) 1605--1614]. This hairpin aptamer binds to its target through loop-loop interactions. We derived chemically modified R06 aptamers that show improved nuclease resistance and affinity for TAR. We review here the results obtained with chimeric aptamers containing locked nucleic acid (LNA) residues. Chimeras containing 2 to 4 LNA residues in an RNA or 2'-O-methyl,RNA context display binding properties of interest and compete with the viral protein Tat for binding to TAR. NMR studies have shown that these properties are modulated by the conformation of the loop-loop helix depending on the presence of LNA residues.