FourU: a novel type of RNA thermometer in Salmonella

Direct observation of the temperature-induced melting process of the Salmonella fourU RNA thermometer at base-pair resolution

In prokaryotes, RNA thermometers regulate a number of heat shock and virulence genes. These temperature sensitive RNA elements are usually located in the 5'-untranslated regions of the regulated genes. They repress translation initiation by base pairing to the Shine-Dalgarno sequence at low temperatures. We investigated the thermodynamic stability of the temperature labile hairpin 2 of the Salmonella fourU RNA thermometer over a broad temperature range and determined free energy, enthalpy and entropy values for the base-pair opening of individual nucleobases by measuring the temperature dependence of the imino proton exchange rates via NMR spectroscopy. Exchange rates were analyzed for the wild-type (wt) RNA and the A8C mutant. The wt RNA was found to be stabilized by the extraordinarily stable G14-C25 base pair. The mismatch base pair in the wt RNA thermometer (A8-G31) is responsible for the smaller cooperativity of the unfolding transition in the wt RNA. Enthalpy and entropy values for the base-pair opening events exhibit linear correlation for both RNAs. The slopes of these correlations coincide with the melting points of the RNAs determined by CD spectroscopy. RNA unfolding occurs at a temperature where all nucleobases have equal thermodynamic stabilities. Our results are in agreement with a consecutive zipper-type unfolding mechanism in which the stacking interaction is responsible for the observed cooperativity. Furthermore, remote effects of the A8C mutation affecting the stability of nucleobase G14 could be identified. According to our analysis we deduce that this effect is most probably transduced via the hydration shell of the RNA.

Small stress response proteins in Escherichia coli: proteins missed by classical proteomic studies

Proteins of 50 or fewer amino acids are poorly characterized in all organisms. The corresponding genes are challenging to reliably annotate, and it is difficult to purify and characterize the small protein products. Due to these technical limitations, little is known about the abundance of small proteins, not to mention their biological functions. To begin to characterize these small proteins in Escherichia coli, we assayed their accumulation under a variety of growth conditions and after exposure to stress. We found that many small proteins accumulate under specific growth conditions or are stress induced. For some genes, the observed changes in protein levels were consistent with known transcriptional regulation, such as ArcA activation of the operons encoding yccB and ybgT. However, we also identified novel regulation, such as Zur repression of ykgMO, cyclic AMP response protein (CRP) repression of azuC, and CRP activation of ykgR. The levels of 11 small proteins increase after heat shock, and induction of at least 1 of these, YobF, occurs at a posttranscriptional level. These results show that small proteins are an overlooked subset of stress response proteins in E. coli and provide information that will be valuable for determining the functions of these proteins.

Mining regulatory 5'UTRs from cDNA deep sequencing datasets

Regulatory 5' untranslated regions (r5'UTRs) of mRNAs such as riboswitches modulate the expression of genes involved in varied biological processes in both bacteria and eukaryotes. New high-throughput sequencing technologies could provide powerful tools for discovery of novel r5'UTRs, but the size and complexity of the datasets generated by these technologies makes it difficult to differentiate r5'UTRs from the multitude of other types of RNAs detected. Here, we developed and implemented a bioinformatic approach to identify putative r5'UTRs from within large datasets of RNAs recently identified by pyrosequencing of the Vibrio cholerae small transcriptome. This screen yielded only approximately 1% of all non-overlapping RNAs along with 75% of previously annotated r5'UTRs and 69 candidate V. cholerae r5'UTRs. These candidates include several putative functional homologues of diverse r5'UTRs characterized in other species as well as numerous candidates upstream of genes involved in pathways not known to be regulated by r5'UTRs, such as fatty acid oxidation and peptidoglycan catabolism. Two of these novel r5'UTRs were experimentally validated using a GFP reporter-based approach. Our findings suggest that the number and diversity of pathways regulated by r5'UTRs has been underestimated and that deep sequencing-based transcriptomics will be extremely valuable in the search for novel r5'UTRs.

Regulatory RNAs in prokaryotes: here, there and everywhere

A recent meeting on 'Regulatory RNAs in prokaryotes' reflected the growing interest in this research topic. Almost 200 scientists met to discuss the identification, structure, function and mechanistic details of regulatory RNAs in bacteria and archaea. The topics included small regulatory RNAs, riboswitches, RNA thermosensors and CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) elements.

RNA phosphodiester backbone dynamics of a perdeuterated cUUCGg tetraloop RNA from phosphorus-31 NMR relaxation analysis

We have analyzed the relaxation properties of all (31)P nuclei in an RNA cUUCGg tetraloop model hairpin at proton magnetic field strengths of 300, 600 and 900 MHz in solution. Significant H, P dipolar contributions to R (1) and R (2) relaxation are observed in a protonated RNA sample at 600 MHz. These contributions can be suppressed using a perdeuterated RNA sample. In order to interpret the (31)P relaxation data (R (1), R (2)), we measured the (31)P chemical shift anisotropy (CSA) by solid-state NMR spectroscopy under various salt and hydration conditions. A value of 178.5 ppm for the (31)P CSA in the static state (S (2) = 1) could be determined. In order to obtain information about fast time scale dynamics we performed a modelfree analysis on the basis of our relaxation data. The results show that subnanosecond dynamics detected around the phosphodiester backbone are more pronounced than the dynamics detected for the ribofuranosyl and nucleobase moieties of the individual nucleotides (Duchardt and Schwalbe, J Biomol NMR 32:295-308, 2005; Ferner et al., Nucleic Acids Res 36:1928-1940, 2008). Furthermore, the dynamics of the individual phosphate groups seem to be correlated to the 5' neighbouring nucleobases.

Microbial thermosensors

Temperature is among the most important of the parameters that free-living microbes monitor. Microbial physiology needs to be readjusted in response to sudden temperature changes. When the ambient temperature rises or drops to potentially harmful levels, cells mount protective stress responses--so-called heat or cold shock responses, respectively. Pathogenic microorganisms often respond to a temperature of around 37 degrees C by inducing virulence gene expression. There are two main ways in which temperature can be measured. Often, the consequences of a sudden temperature shift are detected. Such indirect signals are known to be the accumulation of denatured proteins (heat shock) or stalled ribosomes (cold shock). However, this article focuses solely on direct thermosensors. Since the conformation of virtually every biomolecule is susceptible to temperature changes, primary sensors include DNA, RNA, proteins and lipids.

HNHC: a triple resonance experiment for correlating the H2, N1(N3) and C2 resonances in adenine nucleobases of 13C-, 15N-labeled RNA oligonucleotides

A novel NMR pulse sequence has been developed that correlates the H2 resonances with the C2 and the N1 (N3) resonances in adenine nucleobases of 13C, 15N labeled oligonucleotides. The pulse scheme of the new 3D-HNHC experiment is composed of a 2J-15N-HSQC and a 1J-13C-HSQC and utilizes large 2J(H2, N1(N3)) and 1J(H2, C2) couplings. The experiment was applied to a medium-size 13C, 15N-labeled 36mer RNA. It is useful to resolve assignment ambiguities occurring especially in larger RNA molecules due to resonance overlap in the 1H-dimension. Therefore, the missing link in correlating the imino H3 resonances of the uracils across the AU base pair to the H8 resonances of the adenines via the novel pulse sequence and the TROSY relayed HCCH-COSY (Simon et al. in J Biomol NMR 20:173-176 2001) is provided.

Ribosomal initiation complexes probed by toeprinting and effect of trans-acting translational regulators in bacteria

Toeprinting was developed to study the formation of ribosomal initiation complexes in bacteria. This approach, based on the inhibition of reverse transcriptase elongation, was used to monitor the effect of ribosomal components and translational factors on the formation of the active ribosomal initiation complex. Moreover, this method offers an easy way to study in vitro how mRNA conformational changes alter ribosome binding at the initiation site. These changes can be induced either by environmental cues (temperature, ion concentration), or by the binding of metabolites, regulatory proteins, and trans-acting RNAs. An experimental guide is given to follow the different steps of the formation of ribosomal initiation complexes in Escherichia coli and Staphylococcus aureus, and to monitor the mechanism of action of several regulators on translation initiation in vitro. Protocols to prepare the ribosome and the subunits are also given for Thermus thermophilus, Staphylococcus aureus, and Escherichia coli.

Generation of synthetic RNA-based thermosensors

Structured RNAs with fundamental sensory and regulatory potential have been discovered in all kingdoms of life. Bacterial RNA thermometers are located in the 5'-untranslated region of certain heat shock and virulence genes. They regulate translation by masking the Shine-Dalgarno sequence in a temperature-dependent manner. To engineer RNA-based thermosensors, we used a combination of computer-based rational design and in vivo screening. After only two rounds of selection, several RNA thermometers that are at least as efficient as natural thermometers were obtained. Structure probing experiments revealed temperature-dependent conformational changes in these translational control elements. Our study demonstrates that temperature-controlled RNA elements can be designed by a simple combined computational and experimental approach.

Probing mRNA structure and sRNA-mRNA interactions in bacteria using enzymes and lead(II)

Enzymatic probing and lead(II)-induced cleavages have been developed to study the secondary structure of RNA molecules either free or engaged in complex with different ligands. Using a combination of probes with different specificities (unpaired vs. paired regions), it is possible to get information on the accessibility of each nucleotide, on the binding site of a ligand (noncoding RNAs, protein, metabolites), and on RNA conformational changes that accompanied ligand binding or environmental conditions (temperature, pH, ions, etc.). The detection of the cleavages can be conducted by two different ways, which are chosen according to the length of the studied RNA. The first method uses end-labeled RNA molecules and the second one involves primer extension by reverse transcriptase. We provide here an experimental procedure that was designed to map the structure of mRNA and mRNA-sRNA interaction in vitro.

A rough guide to the non-coding RNA world of Salmonella

Salmonella species are enterobacterial pathogens that have been exceptionally well investigated with respect to virulence mechanisms, microbial pathogenesis, genome evolution and many fundamental pathways of gene expression and metabolism. While these studies have traditionally focused on protein functions, Salmonella has also become a model organism for RNA-mediated regulation. The present review is dedicated to the non-coding RNA world of Salmonella: it covers small RNAs (sRNAs) that act as post-transcriptional regulators of gene expression, novel Salmonella cis-regulatory RNA elements that sense metabolite and metal ion concentrations (or temperature), and globally acting RNA-binding proteins such as CsrA or Hfq (inactivation of which cause drastic phenotypes and virulence defects). Owing to mosaic genome structure, some of the Salmonella sRNAs are widely conserved in bacteria whereas others are very specific to Salmonella species. Intriguingly, sRNAs of either type (CsrB/C, InvR, SgrS) facilitate cross-talk between the Salmonella core genome and its laterally acquired virulence regions. Work in Salmonella also identified physiological functions (and mechanisms thereof) of RNA that had remained unknown in Escherichia coli, and pioneered the use of high-throughput sequencing technology to identify the sRNA and mRNA targets of bacterial RNA-binding proteins.

Design of simple synthetic RNA thermometers for temperature-controlled gene expression in Escherichia coli

RNA thermometers are thermosensors that regulate gene expression by temperature-induced changes in RNA conformation. Naturally occurring RNA thermometers exhibit complex secondary structures which are believed to undergo a series of gradual structural changes in response to temperature shifts. Here, we report the de novo design of considerably simpler RNA thermometers that provide useful RNA-only tools to regulate bacterial gene expression by a shift in the growth temperature. We show that a single small stem-loop structure containing the ribosome binding site is sufficient to construct synthetic RNA thermometers that work efficiently at physiological temperatures. Our data suggest that the thermometers function by a simple melting mechanism and thus provide minimum size on/off switches to experimentally induce or repress gene expression by temperature.

RNA switches regulate initiation of translation in bacteria

A large variety of RNA-based mechanisms have been uncovered in all living organisms to regulate gene expression in response to internal and external changes, and to rapidly adapt cell growth in response to these signals. In bacteria, structural elements in the 5′ leader regions of mRNAs have direct effects on translation initiation of the downstream coding sequences. The docking and unfolding of these mRNAs on the 30S subunit are critical steps in the initiation process directly modulating and timing translation. Structural elements can also undergo conformational changes in response to environmental cues (i.e., temperature sensors) or upon binding of a variety oftrans-acting factors, such as metabolites, non-coding RNAs or regulatory proteins. These RNA switches can temporally regulate translation, leading either to repression or to activation of protein synthesis.

Simulating RNA folding kinetics on approximated energy landscapes

We present a general computational approach to simulate RNA folding kinetics that can be used to extract population kinetics, folding rates and the formation of particular substructures that might be intermediates in the folding process. Simulating RNA folding kinetics can provide unique insight into RNA whose functions are dictated by folding kinetics and not always by nucleotide sequence or the structure of the lowest free-energy state. The method first builds an approximate map (or model) of the folding energy landscape from which the population kinetics are analyzed by solving the master equation on the map. We present results obtained using an analysis technique, map-based Monte Carlo simulation, which stochastically extracts folding pathways from the map. Our method compares favorably with other computational methods that begin with a comprehensive free-energy landscape, illustrating that the smaller, approximate map captures the major features of the complete energy landscape. As a result, our method scales to larger RNAs. For example, here we validate kinetics of RNA of more than 200 nucleotides. Our method accurately computes the kinetics-based functional rates of wild-type and mutant ColE1 RNAII and MS2 phage RNAs showing excellent agreement with experiment.

Genome-wide bioinformatic prediction and experimental evaluation of potential RNA thermometers

Only recently, the fundamental role of regulatory RNAs in prokaryotes and eukaryotes has been appreciated. We developed a pipeline from bioinformatic prediction to experimental validation of new RNA thermometers. Known RNA thermometers are located in the 5'-untranslated region of certain heat shock or virulence genes and control translation by temperature-dependent base pairing of the ribosome binding site. We established the searchable database RNA-SURIBA (Structures of Untranslated Regions In BActeria). A structure-based search pattern reliably recognizes known RNA thermometers and predicts related structures upstream of annotated genes in complete genome sequences. The known ROSE(1) (Repression Of heat Shock gene Expression) thermometer and several other functional ROSE-like elements were correctly predicted. For further investigation, we chose a new candidate upstream of the phage shock gene D (pspD) in the pspABCDE operon of E. coli. We established a new reporter gene system that measures translational control at heat shock temperatures and we demonstrated that the upstream region of pspD does not confer temperature control to the phage shock gene. However, translational efficiency was modulated by a point mutation stabilizing the predicted hairpin. Testing other candidates by this structure prediction and validation process will lead to new insights into the requirements for biologically active RNA thermometers. The database is available on http://www.ruhr-uni-bochum.de/mikrobiologie/ .