DNA-catalyzed transfer of a reporter group

Amplification by nucleic acid-templated reactions

Nucleic acid-templated reactions that proceed with turnover provide a means for signal amplification, which facilitates the use and detection of biologically occurring DNA/RNA molecules.

Fluorescent probes for nucleic Acid visualization in fixed and live cells

This review analyses the literature concerning non-fluorescent and fluorescent probes for nucleic acid imaging in fixed and living cells from the point of view of their suitability for imaging intracellular native RNA and DNA. Attention is mainly paid to fluorescent probes for fluorescence microscopy imaging. Requirements for the target-binding part and the fluorophore making up the probe are formulated. In the case of native double-stranded DNA, structure-specific and sequence-specific probes are discussed. Among the latest, three classes of dsDNA-targeting molecules are described: (i) sequence-specific peptides and proteins; (ii) triplex-forming oligonucleotides and (iii) polyamide oligo(N-methylpyrrole/N-methylimidazole) minor groove binders. Polyamides seem to be the most promising targeting agents for fluorescent probe design, however, some technical problems remain to be solved, such as the relatively low sequence specificity and the high background fluorescence inside the cells. Several examples of fluorescent probe applications for DNA imaging in fixed and living cells are cited. In the case of intracellular RNA, only modified oligonucleotides can provide such sequence-specific imaging. Several approaches for designing fluorescent probes are considered: linear fluorescent probes based on modified oligonucleotide analogs, molecular beacons, binary fluorescent probes and template-directed reactions with fluorescence probe formation, FRET donor-acceptor pairs, pyrene excimers, aptamers and others. The suitability of all these methods for living cell applications is discussed.

Reducing product inhibition in nucleic acid-templated ligation reactions: DNA-templated cycligation

Programmable interactions allow nucleic acid molecules to template chemical reactions by increasing the effective molarities of appended reactive groups. DNA/RNA-triggered reactions can proceed, in principle, with turnover in the template. The amplification provided by the formation of many product molecules per template is a valuable asset when the availability of the DNA or RNA target is limited. However, turnover is usually impeded by reaction products that block access to the template. Product inhibition is most severe in ligation reactions, where products after ligation have dramatically increased template affinities. We introduce a potentially generic approach to reduce product inhibition in nucleic acid-programmed ligation reactions. A DNA-triggered ligation-cyclization sequence ("cycligation") of bifunctional peptide nucleic acid (PNA) conjugates affords cyclic ligation products. Melting experiments revealed that product cyclization is accompanied by a pronounced decrease in template affinity compared to linear ligation products. The reaction system relies upon haloacetylated PNA-thioesters and isocysteinyl-PNA-cysteine conjugates, which were ligated on a DNA template according to a native chemical ligation mechanism. Dissociation of the resulting linear product-template duplex (induced by, for example, thermal cycling) enabled product cyclization through sulfur-halide substitution. Both ligation and cyclization are fast reactions (ligation: 86 % yield after 20 min, cyclization: quantitative after 5 min). Under thermocycling conditions, the DNA template was able to trigger the formation of new product molecules when fresh reactants were added. Furthermore, cycligation produced 2-3 times more product than a conventional ligation reaction with substoichiometric template loads (0.25-0.01 equiv). We believe that cyclization of products from DNA-templated reactions could ultimately afford systems that completely overcome product inhibition.

Fluorescent silver nanoclusters as DNA probes

Fluorescent silver nanoclusters (few atoms, quantum sized) have attracted much attention as promising substitutes for conventional fluorophores. Due to their unique environmental sensitivities, new fluorescent probes have been developed based on silver nanoclusters for the sensitive and specific detection of DNA. In this review we present the recent discoveries of activatable and color-switchable properties of DNA-templated silver nanoclusters and discuss the strategies to use these new properties in DNA sensing.

Very rapid DNA-templated reaction for efficient signal amplification and its steady-state kinetic analysis of the turnover cycle

Oligonucleotide-templated reactions are powerful tools for the detection of nucleic acid sequences. One of the major scientific challenges associated with this technique is the rational design of non-enzyme-mediated catalytic templated reactions capable of multiple turnovers that provide high levels of signal amplification. Herein, we report the development of a nucleophilic aromatic substitution reaction-triggered fluorescent probe. The probe underwent a rapid templated reaction without any of the undesired background reactions. The fluorogenic reaction conducted in the presence of a template provided a 223-fold increase in fluorescence after 30 s compared with the nontemplated reaction. The probe provided an efficient level of signal amplification that ultimately enabled particularly sensitive levels of detection. Assuming a simple model for the templated reactions, it was possible to estimate the rate constants of the chemical reaction in the presence and in the absence of the template. From these kinetic analyses, it was possible to confirm that an efficient turnover cycle had been achieved, on the basis of the dramatic enhancement in the rate of the chemical reaction considered to be the rate-determining step. With maximized turnover efficiency, it was demonstrated that the probe could offer a high turnover number of 1500 times to enable sensitive levels of detection with a detection limit of 0.5 pM in the catalytic templated reactions.

Long-lived luminogenic probe for detection of RNA in a crude solution of living bacterial cells

A pre-type sensitizer for a lanthanide complex on an oligonucleotide was successfully converted to a perfect final structure in a target DNA/RNA-templated reaction, without any chemical reagent or enzyme, under neutral conditions. The final form of the lanthanide-oligonucleotide provided a long-lived luminescence signal, appropriate for time-gated luminescence analysis and signal amplification. Target DNA/RNA-assisted time-gated luminescence analysis is a powerful tool for elimination of autofluorescence and detection of target RNA in living bacterial cells.

Reactions templated by nucleic acids: more ways to translate oligonucleotide-based instructions into emerging function

The programmability of oligonucleotide recognition offers an attractive platform to direct the assembly of reactive partners that can engage in chemical reactions. Recently, significant progress has been made in both the breadth of chemical transformations and in the functional output of the reaction. Herein we summarize these recent progresses and illustrate their applications to translate oligonucleotide instructions into functional materials and novel architectures (conductive polymers, nanopatterns, novel oligonucleotide junctions); into fluorescent or bioactive molecule using cellular RNA; to interrogate secondary structures or oligonucelic acids; or a synthetic oligomer.

Oligonucleotide-templated chemical reactions: pushing the boundaries of a nature-inspired process

Widespread in nature, oligonucleotide-templated reactions of phosphodiester bond formation have inspired chemists who are now applying this elegant strategy to the catalysis of a broad range of otherwise inefficient reactions. This review highlights the increasing diversity of chemical reactions that can be efficiently catalysed by an oligonucleotide template, using Watson-Crick base-pairing to bring both reagents in close enough proximity to react, thus increasing significantly their effective molarity. The applications of this elegant concept for nucleic acid sensing and controlled organic synthesis will also be discussed.

The role of reactivity in DNA templated native chemical PNA ligation during PCR

DNA templated fluorogenic reactions have been used as a diagnostic tool for the sequence specific detection of nucleic acids; and it has been shown that the native chemical ligation between thioester- and 1,2-aminothiol-modified PNA probes is amongst the most selective DNA detection methods reported. We explored whether a DNA templated reaction can be interfaced with the polymerase chain reaction (PCR). This endeavor posed a significant challenge. The reactive groups involved must be sufficiently stable to tolerate the high temperature applied in the PCR process. Nevertheless, the ligation reaction must proceed very rapidly and sequence specifically within the short time available in the annealing and primer extension steps before denaturation is used after approx. 1 min to commence the next PCR cycle. This required a careful optimization of the ternary complex architecture as well as adjustments of probe length and probe reactivities. Our results point to the prime importance of the ligation architecture. We show that once suitable annealing sites have been identified less reactive probe sets may be preferable if sequence specificity is of major concern. The reactivity tuning enabled the development of an in-PCR ligation, which was used for the single nucleotide specific typing of the V600E (T1799A) point mutation in the human BRaf gene. Showcasing the efficiency and sequence specificity of native chemical PNA ligation, attomolar template proofed sufficient to trigger signal while a 1000-fold higher load of single mismatched template failed to induce appreciable signal.

Detection of miRNA in live cells by using templated RuII-catalyzed unmasking of a fluorophore

Reactions templated by cellular nucleic acids are attractive for nucleic acid sensing or responsive systems. Herein we report the use of a photocatalyzed reductive cleavage of an immolative linker to unmask a rhodamine fluorophore, and its application to miRNA imaging. The reaction was found to proceed with a very high turnover (>4000) and provided reliable detection down to 5 pM of template by using γ-serine-modified peptide nucleic acid (PNA) probes. The reaction was used for the selective detection of miR-21 in BT474 cells and miR-31 in HeLa cells following irradiation for 30 min. The probes were introduced by using reversible permeation with streptolysin-O (SLO) or a transfection technique.

Nucleic acid sensing by an orthogonal reporter system based on homo-DNA

We have developed an assay for single strand DNA or RNA detection which is based on the homo-DNA templated Staudinger reduction of the profluorophore rhodamine-azide. The assay is based on a three component system, consisting of a homo-DNA/DNA hybrid probe, a set of homo-DNA reporter strands and the target DNA or RNA. We present two different formats of the assay (Omega probe and linear probe) in which the linear probe was found to perform best with catalytic turnover of the reporter strands (TON: 8) and a match/mismatch discrimination of up to 19. The advantage of this system is that the reporting (homo-DNA) and sensing (DNA) domain are decoupled from each other since the two pairing systems are bioorthogonal. This allows independent optimization of either domain which may lead to higher selectivity in in vivo imaging.

Promoting strand exchange in a DNA-templated transfer reaction

Most DNA-templated reactions suffer from product inhibition. We explored a DNA-triggered fluorophor transfer reaction and demonstrated that comb-type polylysine-polydextran copolymers increase the turnover in template by promoting strand exchange.

Oligonucleotide Conjugates for Detection of Specific Nucleic Acid Sequences

In this chapter, we summarise the designs of fluorophore-modified nucleic acids used as probes for the detection of target DNA/RNA. Recently, there has been an increasing demand for the sequence-specific detection of DNA and RNA in biology and biotechnology. Fluorescent probes based on nucleic acids are useful because of their simplicity and ease of handling. Here, we described three types of fluorescent probe: 1) linear probes, 2) binary probes, and 3) molecular beacons. Each can have one or more fluorophores. Mechanisms for the fluorescence responses of these probes are also discussed in detail. These fluorescent probes have been used in real-time polymerase chain reaction (PCR), genetic analyses, and messenger RNA (mRNA) imaging in living cells. Improvements in sensitivity, selectivity, and nuclease resistance of these probes will lead to more widespread applications in chemical biology, biotechnology, and medicine.

PNA FIT-probes for the dual color imaging of two viral mRNA targets in influenza H1N1 infected live cells

Fluorogenic hybridization probes that allow RNA imaging provide information as to how the synthesis and transport of particular RNA molecules is orchestrated in living cells. In this study, we explored the peptide nucleic acid (PNA)-based FIT-probes in the simultaneous imaging of two different viral mRNA molecules expressed during the replication cycle of the H1N1 influenza A virus. PNA FIT-probes are non-nucleotidic, nonstructured probes and contain a single asymmetric cyanine dye which serves as a fluorescent base surrogate. The fluorochrome acts as a local intercalator probe and reports hybridization of target DNA/RNA by enhancement of fluorescence. Though multiplexed hybridization probes are expected to facilitate the analysis of RNA expression, there are no previous reports on the dual color imaging of two different viral mRNA targets. In this work, we developed a set of two differently colored PNA FIT-probes that allow the spectrally resolved imaging of mRNA coding for neuraminidase (NA) and matrix protein 1 (M1); proteins which execute distinct functions during the replication of the influenza A virus. The probes are characterized by a wide range of applicable hybridization temperatures. The same probe sequence enabled live-cell RNA imaging (at 37 °C) as well as real-time PCR measurements (at 60 °C annealing temperature). This facilitated a comprehensive analysis of RNA expression by quantitative (qPCR) and qualitative (imaging) means. Confocal laser scanning microscopy showed that the viral-RNA specific PNA FIT-probes neither stained noninfected cells nor cells infected by a control virus. The joint use of differently colored PNA FIT-probes in this feasibility study revealed significant differences in the expression pattern of influenza H1N1 mRNAs coding for NA or M1. These experiments provide evidence for the usefulness of PNA FIT-probes in investigations on the temporal and spatial progression of mRNA synthesis in living cells for two mRNA species.

Templated chemistry for monitoring damage and repair directly in duplex DNA

We report the fluorogenic detection of the product of base excision repair (an abasic site) in a specific sequence of duplex DNA. This is achieved by DNA-templated chemistry, employing triple helix-forming probes that contain unnatural nucleobases designed to selectively recognize the site of a missing base. Light-up signals of up to 36-fold were documented, and probes could be used to monitor enzymatic removal of a damaged base.

Oligonucleotide-templated reactions for sensing nucleic acids

Oligonucleotide-templated reactions are useful for applying nucleic acid sensing. Various chemistries for oligonucleotide-templated reaction have been reported so far. Major scientific interests are focused on the development of signal amplification systems and signal generation systems. We introduce the recent advances of oligonucleotide-templated reaction in consideration of the above two points.

Programmable one-pot multistep organic synthesis using DNA junctions

A system for multistep DNA-templated synthesis is controlled by the sequential formation of DNA junctions. Reactants are attached to DNA adapters which are brought together by hybridization to DNA template strands. This process can be repeated to allow sequence-controlled oligomer synthesis while maintaining a constant reaction environment, independent of oligomer length, at each reaction step. Synthesis can take place in a single pot containing all required reactive monomers. Different oligomers can be synthesized in parallel in the same vessel, and the products of parallel synthesis can be ligated, reducing the number of reaction steps required to produce an oligomer of a given length.

Cleavage of oligonucleotides containing a P3'→N5' phosphoramidate linkage mediated by single-stranded oligonucleotide templates

Double-stranded DNA (dsDNA) templates can hybridize to and accelerate cleavage of oligonucleotides containing a P3'→N5' phosphoramidate (P-N) linkage. This dsDNA-templated cleavage of P-N linkages could be due to conformational strain placed on the linkage upon triplex formation. To determine whether duplex formation also induced conformational strain, we examined the reactivity of the oligonucleotides with a P-N linkage in the presence of single-stranded templates, and compared these reactions to those with dsDNA templates. P-N oligonucleotides that are cleaved upon duplex formation could be used as probes to detect single-stranded nucleic acids.

Control of the photocatalytic activity of bimetallic complexes of pyropheophorbide-a by nucleic acids

Photocatalytic activity of a photosensitizer (PS) in an oligodeoxyribonucleotide duplex 5'-PS~ODN1/ODN2~Q-3' is inhibited because of close proximity of a quencher Q. The ODN2 in this duplex is selected to be longer than the ODN1. Therefore, in the presence of a nucleic acid (analyte), which is fully complementary to the ODN2 strand, the duplex is decomposed with formation of an analyte/ODN2~Q duplex and a catalytically active, single stranded PS~ODN1. In this way the catalytic activity of the PS can be controlled by the specific nucleic acids. We applied this reaction earlier for the amplified detection of ribonucleic acids in live cells (Arian, D.; Cló, E.; Gothelf, K.; Mokhir, A. Chem.-Eur. J.2010, 16(1), 288). As a photosensitizer (PS) we used In(3+)(pyropheophorbide-a)chloride and as a quencher (Q)--Black-Hole-Quencher-3 (BHQ-3). The In(3+) complex is a highly active photocatalyst in aqueous solution. However, it can coordinate additional ligands containing thiols (e.g., proteins, peptides, and aminoacids), that modulate properties of the complex itself and of the corresponding bio- molecules. These possible interactions can lead to undesired side effects of nucleic acid controlled photocatalysts (PS~ODN1/ODN2∼Q) in live cells. In this work we explored the possibility to substitute the In(3+) complex for those ones of divalent metal ions, Zn(2+) and Pd(2+), which exhibit lower or no tendency to coordinate the fifth ligand. We found that one of the compounds tested (Pd(pyropheophorbide-a) is as potent and as stable photosensitizer as its In(3+) analogue, but does not coordinate additional ligands that makes it more suitable for cellular applications. When the Pd complex was introduced in the duplex PS~ODN1/ODN2~Q as a PS, its photocatalytic activity could be controlled by nucleic acids as efficiently as that of the corresponding In(3+) complex.

Improved templated fluorogenic probes enhance the analysis of closely related pathogenic bacteria by microscopy and flow cytometry

Templated fluorescence activation has recently emerged as a promising molecular approach to detect and differentiate nucleic acid sequences in vitro and in cells. Here, we describe the application of a reductive quencher release strategy to the taxonomic analysis of Gram-negative bacteria by targeting a single nucleotide difference in their 16S rRNA in a two-color assay. For this purpose, it was necessary to develop a release linker containing a quencher suitable for red and near-infrared fluorophores, and to improve methods for the delivery of probes into cells. A cyanine-dye labeled oligonucleotide probe containing the new quencher-release linker showed unprecedentedly low background signal and high fluorescence turn-on ratios. The combination of a fluorescein-containing and a near-IR emitting probe discriminated E. coli from S. enterica despite nearly identical ribosomal target sequences. Two-color analysis by microscopy and the first successful discrimination of bacteria by two-color flow cytometry with templated reactive probes are described.

DNA-instructed acyl transfer reactions for the synthesis of bioactive peptides

We present a method which allows for the translation of nucleic acid information into the output of molecules that interfere with disease-related protein-protein interactions. The method draws upon a nucleic acid-templated reaction, in which adjacent binding of reactive conjugates triggers the transfer of an aminoacyl or peptidyl group from a donating thioester-linked PNA-peptide hybrid to a peptide-PNA acceptor. We evaluated the influence of conjugate structures on reactivity and sequence specificity. The DNA-triggered peptide synthesis proceeded sequence specifically and showed catalytic turnover in template. The affinity of the formed peptide conjugates for the BIR3 domain of the X-linked inhibitor of apoptosis protein (XIAP) is discussed.

Peptidomimetic bond formation by DNA-templated acyl transfer

The efficiencies of DNA-templated acyl transfer reactions between a thioester modified oligonucleotide and a series of amine and thiol based nucleophiles are directly compared. The reactivity of the nucleophile, reaction conditions (solvent, buffer, pH) and linker length all play important roles in determining the efficiency of the transfer reaction. Careful optimisation of the system enables the use of DNA-templated synthesis to form stable peptide-like bonds under mild aqueous conditions close to neutral pH.

DNA as a versatile chemical component for catalysis, encoding, and stereocontrol

DNA (deoxyribonucleic acid) is the genetic material common to all of Earth's organisms. Our biological understanding of DNA is extensive and well-exploited. In recent years, chemists have begun to develop DNA for nonbiological applications in catalysis, encoding, and stereochemical control. This Review summarizes key advances in these three exciting research areas, each of which takes advantage of a different subset of DNA's useful chemical properties.

Two successive reactions on a DNA template: a strategy for improving background fluorescence and specificity in nucleic acid detection

We report a new strategy for template-mediated fluorogenic chemistry that results in enhanced performance for the fluorescence detection of nucleic acids. In this approach, two successive templated reactions are required to induce a fluorescence signal, rather than only one. These novel fluorescein-labeled oligonucleotide probes, termed 2-STAR (STAR = Staudinger-triggered α-azidoether release) probes, contain two quencher groups tethered by separate reductively cleavable linkers. When a 2-STAR quenched probe successively binds adjacent to two mono-triphenylphosphine-(TPP)-DNAs or one dual-TPP-DNA, the two quenchers are released, resulting in a fluorescence signal. Because of the requirement for two consecutive reactions, 2-STAR probes display an unprecedented level of sequence specificity for template-mediated probe designs. At the same time, background emission generated by off-template reactions or incomplete quenching is among the lowest of any fluorogenic reactive probes for the detection of DNA or RNA.

An autocatalytic chromogenic and fluorogenic photochemical reaction controlled by nucleic acids

The autocatalytic photochemical reaction, which is potentially controlled by any selected nucleic acid, is highly sequence specific and not inhibited by its products, was developed. This reaction generates colored and fluorescent products, which can be monitored by the naked eye.

The ODN probes conjugating the Cu(II) complex enhance the luminol chemiluminescence by assembling on the DNA template

Potent peroxidase-like activity of the β-ketoenamine (1)-dicopper (II) complex (2) for the chemiluminescence (CL) of luminol either in the presence or absence of H(2)O(2) has been previously demonstrated by our group. In this study, the β-ketoenamine (1) as the ligand unit for copper(II) was incorporated into the oligonucleotide (ODN) probes. It has been shown that the catalytic activity of the ODN probes conjugating the ligand-Cu(II) complex is activated by hybridization with the target DNA with the complementary sequence. Thus, this study has successfully demonstrated the basic concept for the sensitive detection of nucleic acids by CL based on the template-inductive activation of the catalytic unit for CL.

Enzyme-assisted binary probe for sensitive detection of RNA and DNA

The new enzyme-assisted assay for DNA/RNA detection provides real-time fluorescent signal readout along with low limit of detection and high discrimination power toward a single-base substitution. Requiring only two new unmodified DNA oligonucleotides for the detection of each new analyte, the assay is an efficient tool for low-cost analysis of multiple analytes.

Templated chemistry for sequence-specific fluorogenic detection of duplex DNA

We describe the development of templated fluorogenic chemistry for detection of specific sequences of duplex DNA in solution. In this approach, two modified homopyrimidine oligodeoxynucleotide probes are designed to bind by triple-helix formation at adjacent positions on a specific purine-rich target sequence of duplex DNA. One fluorescein-labeled probe contains an α-azidoether linker to a fluorescence quencher; the second (trigger) probe carries a triarylphosphine group that is designed to reduce the azide and cleave the linker. The data showed that at pH 5.6 these probes yielded a strong fluorescence signal within minutes on addition to a complementary homopurine duplex DNA target. The signal increased by a factor of about 60, and was completely dependent on the presence of the target DNA. Replacement of cytosine in the probes with pseudoisocytosine allowed the templated chemistry to proceed readily at pH 7. Single nucleotide mismatches in the target oligonucleotide slowed the templated reaction considerably; this demonstrated high sequence selectivity. The use of templated fluorogenic chemistry for detection of duplex DNAs has not been previously reported and could allow detection of double-stranded DNA, at least for homopurine-homopyrimidine target sites, under native and nondenaturing conditions.