Highly specific unnatural base pair systems as a third base pair for PCR amplification.

Toward the expansion of the genetic alphabet of DNA, we present highly efficient unnatural base pair systems as an artificial third base pair for PCR. Hydrophobic unnatural base pair systems between 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds) and 2-nitro-4-propynylpyrrole (Px) were fine-tuned for efficient PCR, by assessing the amplification efficiency and fidelity using different polymerases and template sequence contexts and modified Px bases. Then, we found that some modifications of the Px base reduced the misincorporation rate of the unnatural base substrates opposite the natural bases in templates without reducing the Ds-Px pairing selectivity. Under optimized conditions using Deep Vent DNA polymerase, the misincorporation rate was extremely low (0.005%/bp/replication), which is close to that of the natural base mispairings by the polymerase. DNA fragments with different sequence contexts were amplified ∼10(10)-fold by 40 cycles of PCR, and the selectivity of the Ds-Px pairing was >99.9%/replication, except for 99.77%/replication for unfavorable purine-Ds-purine motifs. Furthermore, >97% of the Ds-Px pair in DNA survived in the 10(28)-fold amplified products after 100-cycle PCR (10 cycles repeated 10 times). This highly specific Ds-Px pair system provides a framework for new biotechnology.

Monitoring the site-specific incorporation of dual fluorophore-quencher base analogues for target DNA detection by an unnatural base pair system.

We developed intramolecular dual fluorophore-quencher base analogues for site-specific incorporation into DNA by an unnatural base pair replication system. An unnatural base pair between 7-(2-thienyl)-imidazo[4,5-b]pyridine (Ds) and 2-nitro-4-propynylpyrrole (Px) exhibits high fidelity in PCR amplification, and the 2-nitropyrrole moiety of Px acts as a quencher. Deoxyribonucleoside triphosphates of Px linked with a fluorophore (Cy3, Cy5 or FAM) were chemically synthesized, and the fluorescent properties and the enzymatic incorporation of the fluorophore-linked dPxTPs into DNA were examined in PCR amplification. The fluorophore-linked dPxTPs were site-specifically incorporated by PCR into DNA, opposite Ds in templates, with high selectivity. Furthermore, we found that the fluorescence of the triphosphates was partially quenched, but increased upon their incorporation into DNA. These dual fluorophore-quencher base analogues would be useful for site-specific DNA labeling and for monitoring the amplification products of target nucleic acid molecules with a specific sequence. We have demonstrated the utility of the fluorophore-linked Px substrates and the Ds-Px pairing in real-time quantitative PCR for target DNA molecule detection.

A new unnatural base pair system between fluorophore and quencher base analogues for nucleic acid-based imaging technology.

In the development of orthogonal extra base pairs for expanding the genetic alphabet, we created novel, unnatural base pairs between fluorophore and quencher nucleobase analogues. We found that the nucleobase analogue, 2-nitropyrrole (denoted by Pn), and its 4-substitutions, such as 2-nitro-4-propynylpyrrole (Px) and 4-[3-(6-aminohexanamido)-1-propynyl]-2-nitropyrrole (NH(2)-hx-Px), act as fluorescence quenchers. The Pn and Px bases specifically pair with their pairing partner, 7-(2,2'-bithien-5-yl)imidazo[4,5-b]pyridine (Dss), which is strongly fluorescent. Thus, these unnatural Dss-Pn and Dss-Px base pairs function as reporter-quencher base pairs, and are complementarily incorporated into DNA by polymerase reactions as a third base pair in combination with the natural A-T and G-C pairs. Due to the static contact quenching, the Pn and Px quencher bases significantly decreased the fluorescence intensity of Dss by the unnatural base pairings in DNA duplexes. In addition, the Dss-Px pair exhibited high efficiency and selectivity in PCR amplification. Thus, this new unnatural base pair system would be suitable for detection methods of target nucleic acid sequences, and here we demonstrated the applications of the Dss-Pn and Dss-Px pairs as molecular beacons and in real-time PCR. The genetic alphabet expansion system with the replicable, unnatural fluorophore-quencher base pair will be a useful tool for sensing and diagnostic applications, as well as an imaging tool for basic research.

A unique fluorescent base analogue for the expansion of the genetic alphabet.

Fluorescent nucleobase analogues are useful in a wide variety of biology and biotechnology tools as molecular probes and reporters for nucleic acids. Here we present a novel fluorescent purine analogue, 7-(2,2'-bithien-5-yl)-imidazo[4,5-b]pyridine (denoted as Dss). The nucleoside triphosphates of Dss can be site-specifically incorporated into DNA and RNA by polymerases, opposite its pairing partner, pyrrole-2-carbaldehyde (Pa), in DNA templates. Despite its high specificity in replication and transcription, Dss in oligonucleotides functions as a universal base that pairs with all four natural bases with nearly equal thermal stabilities. Thus, Dss would be a powerful tool for fluorescent base replacements at specific positions in functional DNA and RNA molecules.

An unnatural base pair system for efficient PCR amplification and functionalization of DNA molecules.

Toward the expansion of the genetic alphabet, we present an unnatural base pair system for efficient PCR amplification, enabling the site-specific incorporation of extra functional components into DNA. This system can be applied to conventional PCR protocols employing DNA templates containing unnatural bases, natural and unnatural base triphosphates, and a 3'-->5' exonuclease-proficient DNA polymerase. For highly faithful and efficient PCR amplification involving the unnatural base pairing, we identified the natural-base sequences surrounding the unnatural bases in DNA templates by an in vitro selection technique, using a DNA library containing the unnatural base. The system facilitates the site-specific incorporation of a variety of modified unnatural bases, linked with functional groups of interest, into amplified DNA. DNA fragments (0.15 amol) containing the unnatural base pair can be amplified 10(7)-fold by 30 cycles of PCR, with <1% total mutation rate of the unnatural base pair site. Using the system, we demonstrated efficient PCR amplification and functionalization of DNA fragments for the extremely sensitive detection of zeptomol-scale target DNA molecules from mixtures with excess amounts (pmol scale) of foreign DNA species. This unnatural base pair system will be applicable to a wide range of DNA/RNA-based technologies.

Sequences around the unnatural base pair in DNA templates for efficient replication.

Unnatural base pairs, compatible with PCR amplification, could potentially increase the versatility of nucleic acids. We recently reported an unnatural base pair, between 7-(2-thienyl)-imidazo[4,5-b]pyridine (denoted by Ds) and 2-nitropyrrole (denoted by Pn), which specifically and efficiently functions in PCR. Toward the efficient incorporation of extra, functional components into DNA fragments, we examined the influence of the sequences around the unnatural base pair and the dependence of the substrate concentrations on the selectivity and efficiency of replication by DNA polymerase.

Efficient PCR amplification by an unnatural base pair system.

Expansion of the genetic alphabet by an unnatural base pair system enables the site-specific incorporation of extra functional components into nucleic acids and proteins. In this system, PCR amplification of DNA templates containing unnatural base pairs is essential for modern biotechnology. We present a new unnatural base pair system, in which DNA duplexes containing the unnatural base pairs can be efficiently amplified by PCR. The system also provides a method for the site-specific incorporation of functional components into amplified DNA fragments by PCR, using unnatural base substrates linked with functional groups of interest.

Development of an unnatural base pair for efficient PCR amplification.

An unnatural base pair system could expand the genetic alphabet, enabling the site-specific incorporation of extra, functional components into nucleic acids and proteins. We developed an unnatural base pair between 7-(2-thienyl)imidazo[4,5-b]pyridine (denoted by Ds) and 2-nitropyrrole (denoted by Pn), which specifically and efficiently functions in DNA amplification by PCR. After 20 cycles of PCR, the mutation rate of the Ds-Pn pair in an amplified DNA fragment was approximately 1%. The Ds-Pn pair system in combination with other unnatural base pairs could be useful for DNA/RNA-based biotechnology.

An efficient unnatural base pair for PCR amplification.

Expansion of the genetic alphabet by an unnatural base pair system provides a powerful tool for modern biotechnology. As an alternative to previous unnatural base pairs, we have developed a new pair between 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds) and 2-nitropyrrole (Pn), which functions in DNA amplification. Pn more selectively pairs with Ds in replication than another previously reported pairing partner, pyrrole-2-carbaldehyde (Pa). The nitro group of Pn efficiently prevented the mispairing with A. High efficiency and selectivity of the Ds-Pn pair in PCR amplification were achieved by using a substrate mixture of the gamma-amidotriphosphate of Ds and the usual triphosphates of Pn and the natural bases, with Vent DNA polymerase as a 3' to 5' exonuclease-proficient polymerase. After 20 cycles of PCR, the total mutation rate of the Ds-Pn site in an amplified DNA fragment was approximately 1%. PCR amplification of DNA fragments containing the unnatural Ds-Pn pair would be useful for expanded genetic systems in DNA-based biotechnology.

Fluorescent probing for RNA molecules by an unnatural base-pair system.

Fluorescent labeling of nucleic acids is widely used in basic research and medical applications. We describe the efficient site-specific incorporation of a fluorescent base analog, 2-amino-6-(2-thienyl)purine (s), into RNA by transcription mediated by an unnatural base pair between s and pyrrole-2-carbaldehyde (Pa). The ribonucleoside 5'-triphosphate of s was site-specifically incorporated into RNA, by T7 RNA polymerase, opposite Pa in DNA templates. The fluorescent intensity of s in RNA molecules changes according to the structural environment. The site-specific s labeling of RNA hairpins and tRNA molecules provided characteristic fluorescent profiles, depending on the labeling sites, temperature and Mg2+ concentration. The Pa-containing DNA templates can be amplified by PCR using 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds), another pairing partner of Pa. This site-specific fluorescent probing by the unnatural pair system including the s-Pa and Ds-Pa pairs provides a powerful tool for studying the dynamics of the local structural features of 3D RNA molecules and their intra- and intermolecular interactions.

An unnatural base pair system for in vitro replication and transcription.

The development of unnatural base pairs that function in replication, transcription, and translation could expand the genetic alphabet and enable the site-specific incorporation of functional components into nucleic acids and proteins. We present an unnatural base pair between 7-(2-thienyl)-imidazo[4,5-b]pyridine (denoted by Ds) and pyrrole-2-carbaldehyde (denoted by Pa). In replication, the Ds-Pa pair exhibits high selectivity in combination with the usual and modified triphosphate substrates and exonuclease-proficient DNA polymerases. In transcription, the Ds-Pa pair mediates the site-specific incorporation of the substrates of both Ds and Pa into RNA by T7 RNA polymerase. This unnatural base pair system could facilitate the specific incorporation of functional components into RNA molecules at desired positions using DNA templates containing the unnatural base pair, which can be amplified by PCR.

An unnatural hydrophobic base pair system: site-specific incorporation of nucleotide analogs into DNA and RNA.

Methods for the site-specific incorporation of extra components into nucleic acids can be powerful tools for creating DNA and RNA molecules with increased functionality. We present an unnatural base pair system in which DNA containing an unnatural base pair can be amplified and function as a template for the site-specific incorporation of base analog substrates into RNA via transcription. The unnatural base pair is formed by specific hydrophobic shape complementation between the bases, but lacks hydrogen bonding interactions. In replication, this unnatural base pair exhibits high selectivity in combination with the usual triphosphates and modified triphosphates, gamma-amidotriphosphates, as substrates of 3' to 5' exonuclease-proficient DNA polymerases, allowing PCR amplification. In transcription, the unnatural base pair complementarity mediates the incorporation of these base substrates and their analogs, such as a biotinylated substrate, into RNA by T7 RNA polymerase (RNAP). With this system, functional components can be site-specifically incorporated into a large RNA molecule.

Site-specific fluorescent labeling of RNA molecules by specific transcription using unnatural base pairs.

Site-specific fluorescent labeling of RNA molecules was achieved by specific transcription using an unnatural base pair system. The unnatural base pairs between 2-amino-6-(2-thienyl)purine (s) and 2-oxo(1H)pyridine (y), and 2-amino-6-(2-thiazolyl)purine (v) and y function in transcription, and the substrates of y and 5-modified y bases can be site-specifically incorporated into RNA, opposite s or v in DNA templates, by T7 RNA polymerase. Ribonucleoside 5'-triphosphates of 5-fluorophore-linked y bases were chemically synthesized from the nucleoside of y. These fluorescent substrates were site-specifically incorporated into RNA by transcription mediated by the s-y and v-y pairs. By using this fluorescent labeling method, specific positions of Raf-binding and theophylline-binding RNA aptamers were fluorescently labeled, and the specific binding to their target molecules was detected by their fluorescent intensities. This site-specific labeling method using an unnatural base pair system will be useful for analyzing conformational changes of RNA molecules and for detecting interactions between RNA and its binding species.

Site-specific biotinylation of RNA molecules by transcription using unnatural base pairs.

Direct site-specific biotinylation of RNA molecules was achieved by specific transcription mediated by unnatural base pairs. Unnatural base pairs between 2-amino-6-(2-thienyl)purine (denoted by s) and 2-oxo(1H)pyridine (denoted by y), or 2-amino-6-(2-thiazolyl)purine (denoted as v) and y specifically function in T7 transcription. Using these unnatural base pairs, the substrate of biotinylated-y (Bio-yTP) was selectively incorporated into RNA, opposite s or v in the DNA templates, by T7 RNA polymerase. This method was applied to the immobilization of an RNA aptamer on sensor chips, and the aptamer accurately recognized its target protein. This direct site-specific biotinylation will provide a tool for RNA-based biotechnologies.

An efficient unnatural base pair for a base-pair-expanded transcription system.

For the site-specific incorporation of artificial components into RNA by transcription, an efficient, unnatural base pair between 2-amino-6-(2-thiazolyl)purine (denoted as v) and 2-oxo(1H)pyridine (denoted as y) was developed. The substrates of y and 5-substituted y were site-specifically incorporated into RNA by T7 RNA polymerase opposite v in templates. The efficiency and fidelity of the v-y pairing in transcription were as high as those of the natural A-T(U) and G-C pairings. Furthermore, RNAs containing two adjacent y bases were also transcribed from DNA templates containing two v bases. This specific transcription allows the large-scale preparation of artificial RNAs and can be combined with other systems to simultaneously incorporate several different components into a transcript.

Non-hydrogen-bonded base pairs for specific transcription.

Specific transcription mediated by unnatural base pairs could create novel RNA molecules with increased functionality and expand the genetic code. Here, we report an unnatural base pair between pyrrole-2-carbaldehyde (Pa) and 2-amino-6-(2-thienyl)purine (s) or 6-(2-thienyl)purine (s') for the site-specific incorporation of s or s' into RNA by T7 RNA polymerase, using DNA templates containing Pa. Despite the absence of significant hydrogen bonding interactions between the unnatural bases, the efficiency and fidelity of the s-Pa pairing in transcription were as high as those of the natural base pairings. As shown in replication, this indicates the importance of shape complementarity between pairing bases in transcription. Since the s base is fluorescent, this transcription mediated by the s-Pa pair provides a useful tool for site-specific fluorescence probing of RNA molecules.

Fluorescent properties of an unnatural nucleobase, 2-amino-6-(2-thienyl)purine, in DNA and RNA fragments.

Nucleoside derivatives of 2-amino-6-(2-thienyl)purine (s) are fluorescent and can be site-specifically incorporated into RNA by transcription mediated by unnatural base pairs between s and its complementary bases. To utilize the fluorescent s base as a probe, we examined the fluorescent properties of s in DNA and RNA fragments. The nucleoside of s exhibited a fluorescence emission centered at 432 nm, characterized by two major excitation maxima (299 and 352 nm), and its quantum yield was 0.41 at pH 7.0. The fluorescence intensity of s in DNA fragments (12-mer) differed depending on circumstances such as the base components, stacking, and pairings. The s base was introduced into a tRNA at specific positions by specific transcription using the unnatural base pairs. The fluorescence intensity of s at each position reflected the local structural features of the tRNA molecules.

Site-specific incorporation of fluorescent probes into RNA by specific transcription using unnatural base pairs.

To analyze the local conformational changes of RNA molecules, we developed a site-specific fluorescent labeling method for RNA fragments by T7 transcription, using unnatural base pairs between 2-amino-6-(2-thienyl)purine (s) and 2-oxo(1H)pyridine (y) and between 2-amino-6-(2-thiazolyl)purine (v) and y. Ribonucleoside 5'-triphosphates of 5-fluorescence-linked y derivatives can be site-specifically incorporated into RNA, opposite s or v in DNA templates, by T7 RNA polymerase. Using this specific transcription, the substrate of a fluorescein-linked y was introduced into a theophylline-binding RNA aptamer. The replacement of U6 by the fluorescein-linked y maintained both the binding ability and selectivity of the aptamer to theophylline. Furthermore, the fluorescence intensity was increased upon theophylline binding, but was not changed by the addition of caffeine.

A two-unnatural-base-pair system toward the expansion of the genetic code.

Toward the site-specific incorporation of amino acid analogues into proteins, a two-unnatural-base-pair system was developed for coupled transcription-translation systems with the expanded genetic code. A previously designed unnatural base pair between 2-amino-6-(2-thienyl)purine (denoted by s) and pyridin-2-one (denoted by y) was used for the site-specific incorporation of yTP into RNA opposite s in templates by T7 RNA polymerase. For the site-specific incorporation of sTP into RNA, a newly developed unnatural base, imidazolin-2-one (denoted by z), is superior to y as a template base for pairing with s in T7 transcription. The combination of the s-y and s-z pairs provides a powerful tool to prepare both y-containing mRNA and s-containing tRNA for efficient coupled transcription-translation systems, in which the genetic code is expanded by the codon-anticodon interactions mediated by the s-y pair. In addition, the nucleoside of s is strongly fluorescent, and thus the s-z pair enables the site-specific fluorescent labeling of RNA molecules. These unnatural-base-pair studies provide valuable information for understanding the mechanisms of replication and transcription.

Unnatural base pairs mediate the site-specific incorporation of an unnatural hydrophobic component into RNA transcripts.

Site-specific incorporation of a hydrophobic nucleotide analog into RNA, by T7 transcription mediated by unnatural base pairs, was developed. The nucleotide analog, 5-phenylethynyl-3-(beta-D-ribofuranosyl)pyridin-2-one 5-triphosphate (denoted by Ph-yTP), was chemically synthesized and then site-specifically incorporated by T7 RNA polymerase into RNA opposite the pairing partner, 2-amino-6-(2-thienyl)purine (denoted by s) in DNA templates. The introduction of Ph-y into a theophylline-binding RNA aptamer, in which a uridine in the internal loop was replaced by Ph-y, raised the thermal stability of the aptamer. Thus, this unnatural nucleotide analog would be useful for stabilizing RNA tertiary structures and complexes between RNA and other molecules.