Combining recombinase polymerase amplification with tyrosine modified 2'-deoxyuridine-5'-triphosphate for direct voltammetric detection of double-stranded DNA: Application to potato pathogen Dickeya solani

The approach based on a combination of isothermal recombinase polymerase amplification (RPA), 2'-deoxyuridine-5'-triphosphate modified with tyrosine aromatic group (dUTP-Y1), and direct voltammetric detection of RPA product carrying electroactive labels was successfully applied to the potato pathogen Dickeya solani. The artificial nucleotide dUTP-Y1 demonstrated a good compatibility with RPA, enabling by targeting a section of D. solani genome with a unique sequence to produce the full-size modified products at high levels of substitution of dTTP by dUTP-Y1 (up to 80-90 %) in the reaction mixture. The optimized procedure of square wave voltammetry allowed to reliably detect the product generated by RPA at 80 % substitution of dTTP by dUTP-Y1 (dsDNA-Y1) in microliter sample volumes on the surface of disposable carbon screen printed electrodes at the potential of about 0.6 V. The calibration curve for the amplicon detection was linear in coordinates 'Ip, A vs. Log (c, M)' within the 0.05-1 μM concentration range. The limit of detection for dsDNA-Y1 was estimated as 8 nM. The sensitivity of the established electrochemical approach allowed to detect amplicons generated in a single standard 50 μL RPA reaction after their purification with silica-coated magnetic beads. The overall detectability of D. solani with the suggested combination of RPA and voltammetric registration of dsDNA-Y1 can be as low as a few copies of bacterial genome per standard reaction. In total, amplification, purification, and electrochemical detection take about 120-150 min. Considering the potential of direct electrochemical analysis for miniaturization, as well as compliance with low-cost and low-power requirements, the findings provide grounds for future development of microfluidic devices integrating isothermal amplification, amplicon purification and detection based on the tyrosine modified nucleotide for the purpose of 'on-site' detection of various pathogens.

Polymerase incorporation of 4-nitrophenyl modified 2'-deoxyuridine-5'-triphosphates into double-stranded DNA for direct electrochemical detection

Three novel 2'-deoxyuridine-5'-triphosphates modified with 4-nitrophenyl groups via various linkers (dUTP-N1, dUTP-N2, and dUTP-N3) were tested as bearers of reducible electroactive labels as well as substrates suitable for enzymes used in polymerase chain reaction (PCR) and recombinase polymerase amplification (RPA) with a potential application to direct electrochemical detection of double-stranded deoxyribonucleic acid (dsDNA). In cyclic and square wave voltammograms on carbon screen printed electrodes, the labeled dUTP have demonstrated distinct reduction peaks at potentials of -0.7 V to -0.9 V (phosphate buffer, pH 7.4). The reduction peak currents of dUTP-N derivatives were found to increase with their molar concentrations. The dUTP-N3 with a double bond in the linker had the lowest reduction potential (about 100 mV less negative) among the derivatives studied. Further, dUTP-N nucleotides were tested as substrates in PCR and RPA to incorporate the electroactive labels into 90, 210, or 206 base pair long dsDNA amplicons. However, only a dUTP-N1 derivative with a shorter linker without the double bond demonstrated satisfactory compatibility with both PCR and RPA, though with a low reaction output of modified dsDNA amplicons (at 100% substitution of dTTP). The dsDNA amplicons produced by PCR with 85% substitution of dTTP by the dUTP-N1 in the reaction mixture were successfully detected by square wave voltammetry at micromolar concentrations at high square wave frequency.

Polymerase incorporation of fluorescein or rhodamine modified 2'-deoxyuridine-5'-triphosphates into double-stranded DNA for direct electrochemical detection

The 2'-deoxyuridine-5'-triphosphates modified with fluorescein (dUTP-Fl) or rhodamine (dUTP-Rh) were tested as bearers of electroactive labels and as proper substrates for polymerases used in polymerase chain reaction (PCR) and isothermal recombinase polymerase amplification (RPA) with the aim of electrochemical detection of double-stranded DNA (dsDNA) amplification products. For this purpose, electrochemical behavior of free fluorescein and rhodamine as well as the modified nucleotides, dUTP-Fl and dUTP-Rh, was studied by cyclic (CV) and square wave (SWV) voltammetry on carbon screen printed electrodes. Both free fluorescein and dUTP-Fl underwent a two-step oxidation at the peak potentials (Ep) of 0.6-0.7 V and 0.8-0.9 V (phosphate buffer, pH 7.4). The reduction peaks of fluorescein and dUTP-Fl were registered between -0.9 V and -1 V, but they did not depend on concentration. The free rhodamine and dUTP-Rh have demonstrated the well-defined oxidation peaks at 0.8-0.9 V. In addition, the distinct reduction peaks at Ep between -0.8 V and -0.9 V were registered for both rhodamine and dUTP-Rh. The dUTP-Fl and dUTP-Rh were further tested as substrates to incorporate an electroactive label into 210 or 206 base pair long dsDNA amplicons generated either by PCR or RPA. Among two dUTP derivatives tested, dUTP-Fl revealed significantly better compatibility with PCR and RPA, producing the full-size amplicons at 50-90% substitution of dTTP in the reaction mixture. In the PCR, the best compromise between amplicon output and labeling was achieved at the dUTP-Fl : dTTP and dUTP-Rh : dTTP molar ratios of 70% : 30% and 20% : 80% in the PCR mixture, respectively, allowing the direct electrochemical detection of amplicons at micromolar concentrations. Alongside with fluorescence DNA assays, the fluorescein and rhodamine modified dUTP appear as promising electroactive labels to develop direct electrochemical DNA assays for detecting PCR and RPA products.

The effect of γ phosphate modified deoxynucleotide substrates on PCR activity and fidelity

Controlling PCR fidelity is an important issue for molecular biology and high-fidelity PCR is essential for gene cloning. In general, fidelity control is achieved by protein engineering of polymerases. In contrast, only a few studies have reported controlling fidelity using chemically modified nucleotide substrates. In this report, we synthesized nucleotide substrates possessing a modification on Pγ and evaluated the effect of this modification on PCR fidelity. One of the substrates, nucleotide tetraphosphate, caused a modest decrease in Taq DNA polymerase activity and the effect on PCR fidelity was dependent on the type of mutation. The use of deoxyadenosine tetraphosphate enhanced the A:T→G:C mutation dramatically, which is common when using Taq polymerase. Conversely, deoxyguanosine tetraphosphate (dG4P) suppressed this mutation but increased the G:C→A:T mutation during PCR. Using an excess amount of dG4P suppressed both mutations successfully and total fidelity was improved.

Optimization of nucleotides dephosphorylation for RNA structural characterization by tandem mass spectrometry hyphenated with separation methods

As part of RNA characterization, the identification of post-transcriptional modifications can be performed using hyphenation of separation methods with mass spectrometry. To identify RNA modifications with those methods, a first total digestion followed by a dephosphorylation step are usually required to reduce RNA to nucleosides. Even though effective digestion and dephosphorylation are essential to avoid further complications in analysis and data interpretation, to our knowledge, no standard protocol is yet referenced in the literature. Therefore, the aim of this work is to optimize the dephosphorylation step using a total extract of transfer RNA (tRNA)¹ from B. taurus as a model and to determine and fix two protocols, leading to complete dephosphorylation, based on time and bacterial alkaline phosphatase (BAP)² consumptions. Capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) was used to estimate the dephosphorylation efficiency of both protocols on many canonical and modified nucleotides. For a timesaving protocol, we established that full dephosphorylation was obtained after a 4-hour incubation at 37 °C with 7.5 U of BAP for 1 µg of tRNA. And for a BAP-saving protocol, we established that full dephosphorylation was obtained 3.0 U of BAP after an overnight incubation at 37 °C. Both protocols are suitable for quantitative analyses as no loss of analytes is expected. Moreover, they can be widely used for all other RNA classes, including messenger RNA or ribosomal RNA.

Immuno-Northern Blotting: Detection of Modified RNA Using Gel Separation and Antibodies to Modified Nucleosides

Immuno-northern blotting is a method for detecting modified RNAs using gel separation and specific antibodies to modified nucleosides. This method was developed by combining two commonly used molecular biology techniques: western blotting and northern blotting. In this method, urea-polyacrylamide (or agarose) gel-separated RNAs are transferred to positively charged nylon membrane and then immune detection is performed with specific antibodies to modified nucleosides: such as 1-methyladenosine, N6-methyladenosine, and pseudouridine. This highly sensitive and relatively simple method, which uses widely available laboratory equipment, enables small laboratories to compare the abundance of modified nucleic acids across samples.

Rapid multiplex nucleic acid amplification test developed using paper chromatography chip and azobenzene-modified oligonucleotides

Although nucleic acid amplification test (NAT) is widely used for pathogen detection, rapid NAT systems that do not require special and expensive instruments must be developed in order to enable point of care (POC)-NATs, which contribute to early initiation of treatment. As a POC-NAT system, Kaneka DNA chromatography chip (KDCC), developed using DNA tag-bound primer through modified substance, was shown to be suitable for POC testing, due to the rapid detection time, simple procedures, and low manufacturing costs. However, owing to some modifications in primer, the detection performance and amplification speed were shown to be reduced when using KDCC, counteracting the increased speed of detection. To solve these issues and improve the speed of this NAT system, we investigated a better modification substance for KDCC. Here, azobenzene-modified primers were shown to have the highest amplification speed and detection performance in KDCC, of all modifications tested in this study, showing 10-100-fold lower detection limit but maintaining the same reaction time. Additionally, rapid herpes simplex virus detection system with azobenzene modified primers was developed. We believed that this breakthrough will contribute toward enabling greater utilization of POC-NATs for medical care, especially in developing countries and clinics.

Structural effects of modified ribonucleotides and magnesium in transfer RNAs

Modified nucleotides are ubiquitous and important to tRNA structure and function. To understand their effect on tRNA conformation, we performed a series of molecular dynamics simulations on yeast tRNA^Phe and tRNA_init, Escherichia coli tRNA_init and HIV tRNA^Lys. Simulations were performed with the wild type modified nucleotides, using the recently developed CHARMM compatible force field parameter set for modified nucleotides (J. Comput. Chem.2016, 37, 896), or with the corresponding unmodified nucleotides, and in the presence or absence of Mg²⁺. Results showed a stabilizing effect associated with the presence of the modifications and Mg²⁺ for some important positions, such as modified guanosine in position 37 and dihydrouridines in 16/17 including both structural properties and base interactions. Some other modifications were also found to make subtle contributions to the structural properties of local domains. While we were not able to investigate the effect of adenosine 37 in tRNA_init and limitations were observed in the conformation of E. coli tRNA_init, the presence of the modified nucleotides and of Mg²⁺ better maintained the structural features and base interactions of the tRNA systems than in their absence indicating the utility of incorporating the modified nucleotides in simulations of tRNA and other RNAs.

DNA aptamer selection in methanolic media: Adenine-aptamer as proof-of-concept

The major objective of this study is to investigate the usefulness of aptamers as in situ detection tool in organic solvents, which are often used for environmental extraction. But two problems related to the use of methanol-containing buffers have to be addressed. Firstly, the folding of nucleic acids can be impaired, because of weaker hydrogen bonding interactions. Secondly, the affinity of aptamers selected in aqueous buffers can be altered by the presence of methanol. Thus, in order to improve hydrophobicity of the DNA pool, nucleotide with hydrophobic modification 5-(octa1,7-diynyl)-2'-deoxyuridine (ODT) has been chosen instead of thymidine. As a proof of concept, an adenine aptamer operating in presence 25% of methanol has been selected. We have shown that the modified nucleotide is essential for target binding in organic media, in addition to essential structural pattern as proposed through analysing truncated sequences analysis. The strategy described in this paper offers preliminary insight on the adaptability of the implementation of aptamers as key instrument for in situ detection. It could be broaden to identify other aptamers directed against other chemical species after alcoholic extraction or for monitoring by-product traces in drugs production.

Synthesis of 3'-O-fluorescently mono-modified reversible terminators and their uses in sequencing-by-synthesis

Next-generation sequencing (NGS) technologies recently developed are now used for study of genomes from various organisms. Sequencing-by-synthesis (SBS) is a key strategy in the NGS. The SBS uses nucleotides so-called dual-modified reversible terminators (DRTs) in which bases are labeled with fluorophores and 3'-OH is protected with a reversibly cleavable chemical group, respectively. In this study, we examined the possibility of performing SBS with mono-modified reversible terminators (MRTs), in which the reversible blocking group on the 3'-OH plays a dual role as a fluorescent signal report as well as a chemical protection. We studied cyclic reversible termination by using two MRTs (dA and dT), wherein the modifications were two different fluorophores and cleavable to regenerate a free 3'-OH. We here demonstrated that SBS could be achieved with incorporation of MRTs by a DNA polymerase and correct base-calls based on the two different colors from the fluorophores.

Directed polymerase evolution

Polymerases evolved in nature to synthesize DNA and RNA, and they underlie the storage and flow of genetic information in all cells. The availability of these enzymes for use at the bench has driven a revolution in biotechnology and medicinal research; however, polymerases did not evolve to function efficiently under the conditions required for some applications and their high substrate fidelity precludes their use for most applications that involve modified substrates. To circumvent these limitations, researchers have turned to directed evolution to tailor the properties and/or substrate repertoire of polymerases for different applications, and several systems have been developed for this purpose. These systems draw on different methods of creating a pool of randomly mutated polymerases and are differentiated by the process used to isolate the most fit members. A variety of polymerases have been evolved, providing new or improved functionality, as well as interesting new insight into the factors governing activity.

Identification of a T7 RNA polymerase variant that permits the enzymatic synthesis of fully 2'-O-methyl-modified RNA

T7 RNA polymerase is an important biocatalyst that is used in diverse biotechnological applications such as in vitro transcription or protein expression. The enzyme displays high substrate specificity which is payed by significant limitations regarding incorporation of synthetic nucleotide analogs. Of specific interest is enzymatic synthesis of 2'-O-methyl-modified RNA as these nucleic acids exhibit improved biochemical and pharmacokinetic properties that make them attractive for diagnostic and therapeutic purposes. We report here on the development of an activity-based selection/screening approach for assessing polymerase activities in the presence of 2'-O-methyl-modified nucleotides, and on the identification of one variant T7 RNA polymerase which is capable of synthesizing all-2'-O-methyl RNA.