Structural probing and damage selection of citrulline- and arginine-specific RNA aptamers identify base positions required for binding.

In a recent study, an RNA aptamer for the specific recognition of the amino acid L-arginine was evolved from an in vitro selected L-citrulline binding parent sequence [M. Famulok (1994) J. Am. Chem. Soc. 116, 1698-1706]. We have now carried out a structural analysis of these aptamers by using chemical modification experiments. Footprinting experiments and a damage selection approach were performed to identify those positions protected from modification in the presence of the amino acids and modifications that interfere with the binding of the ligand. It is shown that of the two bulged regions present in both aptamers one can be modified without loss of binding activity whereas in the other bulge nearly every position is shown to be involved in the recognition of the ligands. This might be indicative for non-canonical base pairing to occur within the non-Watson-Crick paired regions which might be stabilized by the complexed amino acid. Binding to the cognate amino acid significantly enhances the conformational stability of the RNA. We also tested the sensitivity of both aptamers towards lead (II) ion induced cleavage and identified a hypersensitive cleavage site within the invariant bulged region. Lead cleavage is inhibited by the complexed amino acid, indicating a conformational change of the aptamer upon ligand binding. NMR titration data obtained with both aptamers and their cognate ligands confirm the proposed conformational changes and indicate the formation of a 1:1 complex of RNA:amino acid.     

The affinity ratio--its pivotal role in gold nanoparticle-based competitive colorimetric aptasensor

We present an important role of the ratio of affinities in unmodified gold nanoparticles-based colorimetric aptasensor reactions. An affinity ratio, representing the competitive interactions among aptamers, targets, and unmodified gold nanoparticles (umAuNPs), was found to be an important factor for the sensitivity (the performance), where the affinity ratio is the affinity of the aptamer to targets divided by the affinity to umAuNPs (K(dAuNP)/K(dTarget)). In this study, the five different aptamers having different affinity ratios to both umAuNPs and targets are used, and the degree of color change is well correlated with its affinity ratio. This result is verified by using a tetracycline binding aptamer (TBA) showing different affinities to its three derivatives, tetracycline, oxytetracycline and doxycycline. Based on this model, the sensitivity of umAuNPs based colorimetric detection for ibuprofen can be enhanced simply through reducing the ibuprofen binding aptamer's affinity to umAuNP by using bis (p-sulfonatophenyl) phenylphosphine as an AuNP-capping ligand, instead of using the citrate. As a result, a clear color change is observed even at a 20-fold less amount of ibuprofen. This study presents that the performance (detection sensitivity) of umAuNPs-based colorimetric aptasensors could be improved by simply adjusting the affinity ratio of the aptamers to targets and umAuNPs, without knowing the conformational changes of aptamers upon the target binding or needing any modification of aptamer sequences.     

A tetracycline-binding RNA aptamer

Aptamers are perfect tools to study the interaction of small ligands with RNA. To study the mode of interaction of tetracycline with RNA, we isolated aptamers with high affinity to this antibiotic via in vitro selection. One of the selected aptamers, cb28, which has a comparable affinity to tetracycline as the small ribosomal subunit, was characterised in more detail. Cb28 binds only to typical tetracyclines, while atypical tetracyclines are not recognised. The hydroxyl group at position 6 is an essential determinant for recognition, while modifications at positions 4, 5 and 7 do not interfere with RNA binding. Binding of tetracycline to cb28 is magnesium dependent. The secondary structure of cb28 was determined by lead cleavage and DMS modification. Upon tetracycline binding, nucleotides in J2/3 and the P5 stem-loop are protected from cleavage by lead, indicating a conformational change in the RNA. This conformational change was confirmed by tetracycline dependent changes in the DMS modification pattern. Photo-induced affinity incorporation of tetracycline into cb28 resulted in a crosslink to position G76, a residue in L5. The mode of binding of tetracycline to the cb28 aptamer resembles its interaction with the primary binding site on the small ribosomal subunit.     

Properties of an LNA-modified ricin RNA aptamer

'Locked nucleic acids' (LNAs) are sugar modified nucleic acids containing the 2'-O-4'C-methylene-β-D-ribofuranoses. The substitution of RNAs with LNAs leads to an enhanced thermostability. Aptamers are nucleic acids, which are selected for specific target binding from a large library pool by the 'SELEX' method. Introduction of modified nucleic acids into aptamers can improve their stability. The stem region of a ricin A chain RNA aptamer was substituted by locked nucleic acids. Different constructs of the LNA-substituted aptamers were examined for their thermostability, binding activity, folding and RNase sensitivity as compared to the natural RNA counterpart. The LNA-modified aptamers were active in target binding, while the loop regions and the adjacent stem nucleotides remained unsubstituted. The thermostability and RNase resistance of LNA substituted aptamers were enhanced as compared to the native RNA aptamer. This study supports the approach to substitute the aptamer stem region by LNAs and to leave the loop region unmodified, which is responsible for ligand binding. Thus, LNAs possess an encouraging potential for the development of new stabilized nucleic acids and will promote future diagnostic and therapeutic applications.     

Molecular recognition of amino acids by RNA aptamers: the evolution into an L-tyrosine binder of a dopamine-binding RNA motif

We report the evolution of an RNA aptamer to change its binding specificity. RNA aptamers that bind the free amino acid tyrosine were in vitro selected from a degenerate pool derived from a previously selected dopamine aptamer. Three independent sequences bind tyrosine in solution, the winner of the selection binding with a dissociation constant of 35 microM. Competitive affinity chromatography with tyrosine-related ligands indicated that the selected aptamers are highly L-stereo selective and also recognize L-tryptophan and L-dopa with similar affinity. The binding site was localized by sequence comparison, analysis of minimal boundaries, and structural probing upon ligand binding. Tyrosine-binding sites are characterized by the presence of both tyrosine (UAU and UAC) and termination (UAG and UAA) triplets.     

Direct selection of RNA beacon aptamers

A method for the direct selection of RNA molecules that can be easily converted into beacon aptamers is presented. Beacon aptamers are fluorescently labeled nucleic acids that signal the presence of a specific ligand through changes in fluorescence intensity. Typically, ligand binding causes an increase in fluorescence intensity by inducing a conformational change that separates a fluorophore/quencher pair. The method presented here simultaneously selects for ligand binding and induction of an appropriate conformational change. The method was tested by selecting RNA molecules that can detect the aminoglycoside antibiotic tobramycin. After 14 rounds of selection, two sequence families emerged. Upon conversion into beacon aptamers, representatives of the two selected sequence families specifically detected tobramycin, while a negative control RNA that did not survive the selection protocol did not function as a tobramycin beacon aptamer.     

Solution structure of an informationally complex high-affinity RNA aptamer to GTP

Higher-affinity RNA aptamers to GTP are more informationally complex than lower-affinity aptamers. Analog binding studies have shown that the additional information needed to improve affinity does not specify more interactions with the ligand. In light of those observations, we would like to understand the structural characteristics that enable complex aptamers to bind their ligands with higher affinity. Here we present the solution structure of the 41-nt Class I GTP aptamer (K(d) = 75 nM) as determined by NMR. The backbone of the aptamer forms a reverse-S that shapes the binding pocket. The ligand nucleobase stacks between purine platforms and makes hydrogen bonds with the edge of another base. Interestingly, the local modes of interaction for the Class I aptamer and an RNA aptamer that binds ATP with a K(d) of 6 microM are very much alike. The aptamers exhibit nearly identical levels of binding specificity and fraction of ligand sequestered from the solvent (81%-85%). However, the GTP aptamer is more informationally complex (approximately 45 vs. 35 bits) and has a larger recognition bulge (15 vs. 12 nucleotides) with many more stabilizing base-base interactions. Because the aptamers have similar modes of ligand binding, we conclude that the stabilizing structural elements in the Class I aptamer are responsible for much of the difference in K(d). These results are consistent with the hypothesis that increasing the number of intra-RNA interactions, rather than adding specific contacts to the ligand, is the simplest way to improve binding affinity.     

Fluorescent aptasensors based on conformational adaptability of abasic site-containing aptamers in combination with abasic site-binding ligands

Aptamers are nucleic acids that can selectively bind to a variety of targets. Aptamers usually undergo conformational transitions from a flexible or disordered structure into a rigid or ordered structure upon target-binding. This study describes a detection method for l-argininamide (l-Arm) and adenosine based on the conformational adaptability of nucleic acid aptamers. An abasic site (AP site) was formed in the stem and close to the target-binding site of a stem-loop aptamer as an anchoring pocket for a fluorescent ligand. 3,5-Diamino-6-chloro-2-pyrazine carbonitrile (DCPC), which can bind to AP site-containing DNA duplexes by pseudo-base pairing, was utilized as a signaling reporter for the target-binding. The binding of a target to an aptamer induces the tight pairing of bases flanking the AP site, so that DCPC can effectively bind to the stem. The binding of DCPC is accompanied by a significant enhancement of its fluorescence. This new sensing method without an antisense DNA strand was demonstrated by using l-Arm and its aptamer as a model. It was confirmed that the method can sensitively detect l-Arm with a detection limit of 2.1 μM. The proposed method was also applied to adenosine detection, where the reported sequence of an adenosine aptamer was slightly modified. The method based on an AP site-containing aptamer and an AP site-binding ligand was applicable to detection of a target in horse serum.     

Detection system based on the conformational change in an aptamer and its application to simple bound/free separation

Aptamers are good molecular recognition elements for biosensors. Especially, their conformational change, which is induced by the binding to the target molecule, enables the development of several types of useful detection systems. We applied this property to bound/free separation, which is a crucial process for highly sensitive detection. We designed aptamers which change their conformation upon binding to the target molecule and thereby expose a single-strand bearing the complementary sequence to the capture probe immobilized onto the support. We named the designed aptamers "capturable aptamers" and the capture probe "capture DNA". Three capturable aptamers were designed based on the PrP aptamer, which binds to prion protein. One of these capturable aptamers was demonstrated to recognize prion protein and change its conformation upon binding to it. A detection system using this designed capturable aptamer for prion protein was developed. Capturable aptamers and capture DNA allow us to perform simple bound/free separation with only one target ligand.     

NMR resonance assignments for the tetramethylrhodamine binding RNA aptamer 3 in complex with the ligand 5-carboxy-tetramethylrhodamine

RNA aptamers are used in a wide range of biotechnological or biomedical applications. In many cases the high resolution structures of these aptamers in their ligand-complexes have revealed fundamental aspects of RNA folding and RNA small molecule interactions. Fluorescent RNA-ligand complexes in particular find applications as optical sensors or as endogenous fluorescent tags for RNA tracking in vivo. Structures of RNA aptamers and aptamer ligand complexes constitute the starting point for rational function directed optimization approaches. Here, we present the NMR resonance assignment of an RNA aptamer binding to the fluorescent ligand tetramethylrhodamine (TMR) in complex with the ligand 5-carboxy-tetramethylrhodamine (5-TAMRA) as a starting point for a high-resolution structure determination using NMR spectroscopy in solution.     

Crystal structure of an RNA aptamer bound to thrombin

Aptamers, an emerging class of therapeutics, are DNA or RNA molecules that are selected to bind molecular targets that range from small organic compounds to large proteins. All of the determined structures of aptamers in complex with small molecule targets show that aptamers cage such ligands. In structures of aptamers in complex with proteins that naturally bind nucleic acid, the aptamers occupy the nucleic acid binding site and often mimic the natural interactions. Here we present a crystal structure of an RNA aptamer bound to human thrombin, a protein that does not naturally bind nucleic acid, at 1.9 A resolution. The aptamer, which adheres to thrombin at the binding site for heparin, presents an extended molecular surface that is complementary to the protein. Protein recognition involves the stacking of single-stranded adenine bases at the core of the tertiary fold with arginine side chains. These results exemplify how RNA aptamers can fold into intricate conformations that allow them to interact closely with extended surfaces on non-RNA binding proteins.     

Molecular sensing by the aptamer domain of the FMN riboswitch: a general model for ligand binding by conformational selection

Understanding the nature of the free state of riboswitch aptamers is important for illuminating common themes in gene regulation by riboswitches. Prior evidence indicated the flavin mononucleotide (FMN)-binding riboswitch aptamer adopted a ‘bound-like’ structure in absence of FMN, suggesting only local conformational changes upon ligand binding. In the scope of pinpointing the general nature of such changes at the nucleotide level, we performed SHAPE mapping experiments using the aptamer domain of two phylogenetic variants, both in absence and in presence of FMN. We also solved the crystal structures of one of these domains both free (3.3 Å resolution) and bound to FMN (2.95 Å resolution). Our comparative study reveals that structural rearrangements occurring upon binding are restricted to a few of the joining regions that form the binding pocket in both RNAs. This type of binding event with minimal structural perturbations is reminiscent of binding events by conformational selection encountered in other riboswitches and various RNAs.     

Conditional gene expression by controlling translation with tetracycline-binding aptamers

We present a conditional gene expression system in Saccharomyces cerevisiae which exploits direct RNA–metabolite interactions as a mechanism of genetic control. We inserted preselected tetracycline (tc) binding aptamers into the 5′-UTR of a GFP encoding mRNA. While aptamer insertion generally reduces GFP expression, one group of aptamers displayed an additional, up to 6-fold, decrease in fluorescence upon tc addition. Regulation is observed for aptamers inserted cap-proximal or near the start codon, but is more pronounced from the latter position. Increasing the thermodynamic stability of the aptamer augments regulation but reduces expression of GFP. Decreasing the stability leads to the opposite effect. We defined nucleotides which influence the regulatory properties of the aptamer. Exchanging a nucleotide probably involved in tc binding only influences regulation, while mutations at another position alter expression in the absence of tc, without affecting regulation. Thus, we have developed and characterized a regulatory system which is easy to establish and controlled by a non-toxic, small ligand with good cell permeability.     

Screening and Characterization of a Novel RNA Aptamer That Specifically Binds to Human Prostatic Acid Phosphatase and Human Prostate Cancer Cells

Prostatic acid phosphatase (PAP) expression increases proportionally with prostate cancer progression, making it useful in prognosticating intermediate to high-risk prostate cancers. A novel ligand that can specifically bind to PAP would be very helpful for guiding prostate cancer therapy. RNA aptamers bind to target molecules with high specificity and have key advantages such as low immunogenicity and easy synthesis. Here, human PAP-specific aptamers were screened from a 2′-fluoropyrimidine (FY)-modified RNA library by SELEX. The candidate aptamer families were identified within six rounds followed by analysis of their sequences and PAP-specific binding. A gel shift assay was used to identify PAP binding aptamers and the 6N aptamer specifically bound to PAP with a Kd value of 118 nM. RT-PCR and fluorescence labeling analyses revealed that the 6N aptamer bound to PAP-positive mammalian cells, such as PC-3 and LNCaP. IMR-90 negative control cells did not bind the 6N aptamer. Systematic minimization analyses revealed that 50 nucleotide sequences and their two hairpin structures in the 6N 2′-FY RNA aptamer were equally important for PAP binding. Renewed interest in PAP combined with the versatility of RNA aptamers, including conjugation of anti-cancer drugs and nano-imaging probes, could open up a new route for early theragnosis of prostate cancer.     

An adaptable pentaloop defines a robust neomycin-B RNA aptamer with conditional ligand-bound structures

Aptamers can be highly specific for their targets, which implies precise molecular recognition between aptamer and target. However, as small polymers, their structures are more subject to environmental conditions than the more constrained longer RNAs such as those that constitute the ribosome. To understand the balance between structural and environmental factors in establishing ligand specificity of aptamers, we examined the RNA aptamer (NEO1A) previously reported as specific for neomycin-B. We show that NEO1A can recognize other aminoglycosides with similar affinities as for neomycin-B and its aminoglycoside specificity is strongly influenced by ionic strength and buffer composition. NMR and 2-aminopurine (2AP) fluorescence studies of the aptamer identified a flexible pentaloop and a stable binding pocket. Consistent with a well-structured binding pocket, docking analysis results correlated with experimental measures of the binding energy for most ligands. Steady state fluorescence studies of 2AP-substituted aptamers confirmed that A16 moves to a more solvent accessible position upon ligand binding while A14 moves to a less solvent accessible position, which is most likely a base stack. Analysis of binding affinities of NEO1A sequence variants showed that the base in position 16 interacts differently with each ligand and the interaction is a function of the buffer constituents. Our results show that the pentaloop provides NEO1A with the ability to adapt to external influences on its structure, with the critical base at position 16 adjusting to incorporate each ligand into a stable pocket by hydrophobic interactions and/or hydrogen bonds depending on the ligand and the ionic environment.     

Characterisation of aptamer–target interactions by branched selection and high-throughput sequencing of SELEX pools

Nucleic acid aptamer selection by systematic evolution of ligands by exponential enrichment (SELEX) has shown great promise for use in the development of research tools, therapeutics and diagnostics. Typically, aptamers are identified from libraries containing up to 10¹⁶ different RNA or DNA sequences by 5–10 rounds of affinity selection towards a target of interest. Such library screenings can result in complex pools of many target-binding aptamers. New high-throughput sequencing techniques may potentially revolutionise aptamer selection by allowing quantitative assessment of the dynamic changes in the pool composition during the SELEX process and by facilitating large-scale post-SELEX characterisation. In the present study, we demonstrate how high-throughput sequencing of SELEX pools, before and after a single round of branched selection for binding to different target variants, can provide detailed information about aptamer binding sites, preferences for specific target conformations, and functional effects of the aptamers. The procedure was applied on a diverse pool of 2′-fluoropyrimidine-modified RNA enriched for aptamers specific for the serpin plasminogen activator inhibitor-1 (PAI-1) through five rounds of standard selection. The results demonstrate that it is possible to perform large-scale detailed characterisation of aptamer sequences directly in the complex pools obtained from library selection methods, thus without the need to produce individual aptamers.     

What defines a synthetic riboswitch? – Conformational dynamics of ciprofloxacin aptamers with similar binding affinities but varying regulatory potentials

Among the many in vitro-selected aptamers derived from SELEX protocols, only a small fraction has the potential to be applied for synthetic riboswitch engineering. Here, we present a comparative study of the binding properties of three different aptamers that bind to ciprofloxacin with similar K_D values, yet only two of them can be applied as riboswitches. We used the inherent ligand fluorescence that is quenched upon binding as the reporter signal in fluorescence titration and in time-resolved stopped-flow experiments. Thus, we were able to demonstrate differences in the binding kinetics of regulating and non-regulating aptamers. All aptamers studied underwent a two-step binding mechanism that suggests an initial association step followed by a reorganization of the aptamer to accommodate the ligand. We show that increasing regulatory potential is correlated with a decreasing back-reaction rate of the second binding step, thus resulting in a virtually irreversible last binding step of regulating aptamers. We suggest that a highly favoured structural adaption of the RNA to the ligand during the final binding step is essential for turning an aptamer into a riboswitch. In addition, our results provide an explanation for the fact that so few aptamers with regulating capacity have been found to date. Based on our data, we propose an adjustment of the selection protocol for efficient riboswitch detection.     

Protein dynamics and monomer-monomer interactions in AntR activation by electron paramagnetic resonance and double electron-electron resonance

The Anthracis repressor (AntR) is a Mn(II)-activated DNA binding protein that is involved in the regulation of Mn(II) homeostasis in Bacillus anthracis. AntR is structurally and functionally homologous to Mn(II)-activated repressor from Bacillus subtillis (MntR). Our studies on AntR focus on metal-regulated activation of the protein. Line shape analysis of continuous wave electron paramagnetic resonance (EPR) spectra showed that metal binding resulted in a general reduction of backbone dynamics and that there were no further changes in backbone motion upon DNA binding. Double electron-electron resonance (DEER) pulsed EPR spectroscopy was used to measure distances between nitroxide spin labels strategically placed in dimeric AntR. The DEER data were analyzed assuming Gaussian distributions for discrete populations of spins. A structural model for AntR was built from homology to MntR, and the experimentally measured distances were simulated to distinguish between spin label and backbone motions. Together with the computational analysis, the DEER results for apo-AntR indicated relatively narrow conformational distributions for backbone residues at the dimer interface and near the metal binding site. No significant changes were observed on these sites in the presence of metal or DNA. On the other hand, the distribution of the conformers and the distances between the putative DNA binding helices decreased upon metal binding. These results suggest that the DNA binding region of AntR shows large amplitude backbone motions in the absence of metal, which may preclude sequence-specific binding to promoter sites. Metal binding narrows the range of conformations accessible in this region and shortens the mean distance between the DNA binding helices, probably resulting in alignment that optimizes promoter recognition and binding.