Search

VirF is the master activator of virulence genes of Shigella and its expression is required for the invasion of the human intestinal mucosa by pathogenic bacteria. VirF was shown to directly activate the transcription of virB and icsA, which encode two essential proteins involved in the pathogenicity process, by binding their promoter regions. In this study, we demonstrate by band shift, enzymatic probing and cross-linking experiments that VirF, in addition to DNA, can also bind the icsA transcript and RnaG, an antisense non-coding small RNA that promotes the premature termination of icsA mRNA through a transcriptional attenuation mechanism. Furthermore, we show that VirF binds in vitro also other species of RNAs, although with lower specificity. The existence of VirF-RnaG and VirF-icsA mRNA complexes is confirmed in a pulldown assay carried out under experimental conditions that very close reproduce the in vivo conditions and that allows immobilized VirF to “fish” out RnaG and icsA mRNA from a total RNA extract. The VirF binding sites identified on both icsA mRNA and RnaG contain a 13 nucleotides stretch (5’-UUUUaGYcUuUau-3’) that is the RNA-converted consensus sequence previously proposed for the VirF-DNA interaction. Band-shift assays with a synthetic RNA molecule whose sequence perfectly matches the consensus indicate that this signature plays a key role also in the VirF-RNA interaction, in particular when exposed in a stem-loop structure. To further explore the icsA-RnaG-VirF regulatory system, we developed an in vitro test (RNA-RNA Pairing Assay) in which pairing between icsA mRNA and synthetic RNAs that reproduce the individual stem-loop motifs of RnaG, was analyzed in the presence of VirF. This assay shows that this protein can prevent the formation of the kissing complex, defined as the initial nucleation points for RNA heteroduplex formation, between RnaG and icsA mRNA. Consistently, VirF alleviates the RnaG-mediated repression of icsA transcription in vitro. Therefore VirF, by hindering the icsA transcript-RnaG interaction, exhibits an activity opposed to that usually displayed by proteins, which generally assist the RNA-RNA interaction; this quite uncommon and new function and the regulatory implications of VirF as a potential RNA-binding protein are discussed.

International audience; In the RNA realm, non‐Watson Crick base pairs are abundant and affect either the RNA 3D structure and/or its function. Here we investigated the formation of RNA kissing complexes where the loop‐loop interaction is modulated by non‐Watson‐Crick pairs. Mass spectrometry, surface plasmon resonance and UV‐melting experiments show that the G • U wobble base pair favors the kissing complex formation only when placed at specific positions. We tried to rationalize this effect by molecular modelling, including molecular mechanics Poisson–Boltzmann surface area (MMPBSA) thermodynamics calculation and PBSA calculation of the electrostatic potential surfaces. Modeling reveals that the G • U stabilization is due to a specific electrostatic environment defined by the base pairs of the entire loop‐loop region. The loop is not symmetric, and therefore the identity and position of each base pair matters. Predicting and visualizing the electrostatic environment created by a given sequence can help designing specific kissing complexes with high affinity, for potential therapeutic, nanotechnology or analytical applications.

International audience; In HIV-1, trans-activation of transcription of the viral genome is regulated by an imperfect hairpin, the trans-activating responsive (TAR) RNA element, located at the 5' untranslated end of all viral transcripts. TAR acts as a binding site for viral and cellular proteins. In an attempt to identify RNA ligands that would interfere with the virus life-cycle by interacting with TAR, an in vitro selection was previously carried out. RNA hairpins that formed kissing-loop dimers with TAR were selected [Ducong?. and Toulm?J (1999) RNA, 5:1605-1614]. We describe here the crystal structure of TAR bound to a high-affinity RNA aptamer. The two hairpins form a kissing complex and interact through six Watson-Crick base pairs. The complex adopts an overall conformation with an inter-helix angle of 28.1 degrees , thus contrasting with previously reported solution and modelling studies. Structural analysis reveals that inter-backbone hydrogen bonds between ribose 2' hydroxyl and phosphate oxygens at the stem-loop junctions can be formed. Thermal denaturation and surface plasmon resonance experiments with chemically modified 2'-O-methyl incorporated into both hairpins at key positions, clearly demonstrate the involvement of this intermolecular network of hydrogen bonds in complex stability.

Riboswitches are cis-acting regulatory RNA elements prevalently located in the leader sequences of bacterial mRNA. An adenine sensing riboswitch cis-regulates adeninosine deaminase gene (add) in Vibrio vulnificus. The structural mechanism regulating its conformational changes upon ligand binding mostly remains to be elucidated. In this open framework it has been suggested that the ligand stabilizes the interaction of the distal "kissing loop" complex. Using accurate full-atom molecular dynamics with explicit solvent in combination with enhanced sampling techniques and advanced analysis methods it could be possible to provide a more detailed perspective on the formation of these tertiary contacts. In this work, we used umbrella sampling simulations to study the thermodynamics of the kissing loop complex in the presence and in the absence of the cognate ligand. We enforced the breaking/formation of the loop-loop interaction restraining the distance between the two loops. We also assessed the convergence of the results by using two alternative initialization protocols. A structural analysis was performed using a novel approach to analyze base contacts. Our simulations qualitatively indicated that the ligand could stabilize the kissing loop complex. We also compared with previously published simulation studies. Kissing complex stabilization given by the ligand was compatible with available experimental data. However, the dependence of its value on the initialization protocol of the umbrella sampling simulations posed some questions on the quantitative interpretation of the results and called for better converged enhanced sampling simulations., Comment: Accepted for publication on BMC Bioinformatics

SL1 is a stem-loop RNA sequence from the genome of HIV-1 thought to be the initiation site for the dimerization of the retroviral genomic RNA. The aim of this study is to check the stability in solution of different experimental dimeric structures available in the literature. Two kinds of dimer have been evidenced: an extended duplex looking like a double helix with two internal bulges and a kissing complex in which the monomers with a stem/loop conformation are linked by intermolecular loop-loop interactions. Two divergent experimental structures of the kissing complex from the Lai isolate are reported in the literature, one obtained from NMR (Mujeeb et al., Nature Structural Biology, 1998, Vol. 5, pp. 432-436) and the other one from x-ray crystallography (Ennifar et al., Nature Structural Biology, 2001, Vol. 8, pp. 1064-1068). A crystallographic structure of the Mal isolate was also reported (Ennifar et al., Nature Structure Biology, 2001, Vol. 8, pp. 1064-1068). Concerning the extended duplex, a NMR structure is available for Lai (Girard et al., Journal of Biomolecular Structure and Dynamics, 1999, Vol. 16, pp. 1145-1157) and a crystallographic structure for Mal (Ennifar et al., Structure, 1999, Vol. 7, pp. 1439-1449). Using a molecular dynamics technique, all these experimental structures have been simulated in solution with explicit water and counterions. We show that both extended duplex structures are stable. On the contrary, the crystallographic structures of the Lai and Mal kissing complexes are rapidly destabilized in aqueous environment. Finally, the NMR structure of the Lai loop-loop kissing complex remains globally stable over a 20 ns MD simulation, although large rearrangements occur at the level of the stem/loop junctions that are flexible, as shown from free energy calculations. These results are compared to electrophoresis experiments on dimer formation.