Current Challenges in Nucleic Acid Synthesis

Direct dsRNA preparation by promoter-free RCT and RNase H cleavage using one circular dsDNA template with a mismatched bubble

Double-stranded RNA (dsRNA) has aroused widespread interest due to its effects on immunity and applications based on RNAi. However, the in vitro preparation of dsRNA is costly and laborious. In this study, we have developed a novel and interesting method designated as pfRCT (promoter-free rolling-circle transcription) for direct, facile, and efficient dsRNA preparation. This method generates equal amounts of sense and antisense strands simultaneously from a single circular dsDNA template. To initiate transcription by T7 RNA polymerase without directional preference, a 9-15-bp bubble (mismatched duplex with strong sequence symmetry) is introduced into the template. During RCT, all the necessary reagents, including the template, NTPs, RNA polymerase, RNase H, and Helpers, are present in one pot; and the just-transcribed RNA is immediately truncated by RNase H to monomers with the desired size. The ends of the dsRNA product can also be simply sealed by T4 RNA ligase 1 after pfRCT. This new approach is expected to promote the applications of dsRNA.

Recent advancements in design of nucleic acid nanocarriers for controlled drug delivery

Research of nanoscale nucleic acid carriers has garnered attention in recent years due to their distinctive and controllable properties. However, current knowledge is limited in how we can efficiently utilize these systems for clinical applications. Several researchers have pioneered new and innovative nanocarrier drug delivery systems, but understanding physiochemical properties and behavior in vivo is vital to implementing them as clinical drug delivery platforms. In this review, we outline the most significant innovations in the synthesis, physical properties, and utilization of nucleic acid nanocarriers in the past 5 years, addressing the crucial properties which improve nanocarrier characteristics, delivery, and drug release. The challenges of controlling the transport of nucleic acid nanocarriers and therapeutic release for biological applications are outlined. Barriers which inhibit effective transport into tissue are discussed with emphasis on the modifications needed to overcome such obstacles. The novel strategies discussed in this work summarize the pivotal features of modern nucleic nanocarriers and postulate where future developments could revolutionize the translation of these tools into a clinical setting.

Synthesis of a Peptidoyl RNA Hairpin via a Combination of Solid-Phase and Template-Directed Chain Assembly

Peptidoyl RNAs are the products of ribosome‐free, single‐nucleotide translation. They contain a peptide in the backbone of the oligoribonucleotide and are interesting from a synthetic and a bioorganic point of view. A synthesis of a stabilized version of peptidoyl RNA, with an amide bond between the C‐terminus of a peptide and a 3′‐amino‐2′,3′‐dideoxynucleoside in the RNA chain was developed. The preferred synthetic route used an N‐Teoc‐protected aminonucleoside support and involved a solution‐phase coupling of the amino‐terminal oligonucleotide to a dipeptido dinucleotide. Exploratory UV‐melting and NMR analysis of the hairpin 5′‐UUGGCGAAAGCdC‐LeuLeu‐AA‐3′ indicated that the peptide‐linked RNA segments do not fold in a cooperative fashion. The synthetic access to doubly RNA‐linked peptides on a scale sufficient for structural biology opens the door to the exploration of their structural and biochemical properties.

Direct adenylation from 5'-OH-terminated oligonucleotides by a fusion enzyme containing Pfu RNA ligase and T4 polynucleotide kinase

5′-Adenylated oligonucleotides (AppOligos) are widely used for single-stranded DNA/RNA ligation in next-generation sequencing (NGS) applications such as microRNA (miRNA) profiling. The ligation between an AppOligo adapter and target molecules (such as miRNA) no longer requires ATP, thereby minimizing potential self-ligations and simplifying library preparation procedures. AppOligos can be produced by chemical synthesis or enzymatic modification. However, adenylation via chemical synthesis is inefficient and expensive, while enzymatic modification requires pre-phosphorylated substrate and additional purification. Here we cloned and characterized the Pfu RNA ligase encoded by the PF0353 gene in the hyperthermophilic archaea Pyrococcus furiosus. We further engineered fusion enzymes containing both Pfu RNA ligase and T4 polynucleotide kinase. One fusion enzyme, 8H-AP, was thermostable and can directly catalyze 5′-OH-terminated DNA substrates to adenylated products. The newly discovered Pfu RNA ligase and the engineered fusion enzyme may be useful tools for applications using AppOligos.

Programmable nano-reactors for stochastic sensing

Chemical reactions of single molecules, caused by rapid formation or breaking of chemical bonds, are difficult to observe even with state-of-the-art instruments. A biological nanopore can be engineered into a single molecule reactor, capable of detecting the binding of a monatomic ion or the transient appearance of chemical intermediates. Pore engineering of this type is however technically challenging, which has significantly restricted further development of this technique. We propose a versatile strategy, "programmable nano-reactors for stochastic sensing" (PNRSS), by which a variety of single molecule reactions of hydrogen peroxide, metal ions, ethylene glycol, glycerol, lactic acid, vitamins, catecholamines or nucleoside analogues can be observed directly. PNRSS presents a refined sensing resolution which can be further enhanced by an artificial intelligence algorithm. Remdesivir, a nucleoside analogue and an investigational anti-viral drug used to treat COVID-19, can be distinguished from its active triphosphate form by PNRSS, suggesting applications in pharmacokinetics or drug screening.

Probing of Nucleic Acid Structures, Dynamics, and Interactions With Environment-Sensitive Fluorescent Labels

Fluorescence labeling and probing are fundamental techniques for nucleic acid analysis and quantification. However, new fluorescent probes and approaches are urgently needed in order to accurately determine structural and conformational dynamics of DNA and RNA at the level of single nucleobases/base pairs, and to probe the interactions between nucleic acids with proteins. This review describes the means by which to achieve these goals using nucleobase replacement or modification with advanced fluorescent dyes that respond by the changing of their fluorescence parameters to their local environment (altered polarity, hydration, flipping dynamics, and formation/breaking of hydrogen bonds).

Electrophilic oligodeoxynucleotide synthesis using dM-Dmoc for amino protection

Solid-phase synthesis of electrophilic oligodeoxynucleotides (ODNs) was achieved using dimethyl-Dmoc (dM-Dmoc) as amino protecting group. Due to the high steric hindrance of the 2-(propan-2-ylidene)-1,3-dithiane side product from deprotection, the use of excess nucleophilic scavengers such as aniline to prevent Michael addition of the side product to the deprotected ODN during ODN cleavage and deprotection was no longer needed. The improved technology was demonstrated by the synthesis and characterization of five ODNs including three modified ones. The modified ODNs contained the electrophilic groups ethyl ester, α-chloroamide, and thioester. Using the technology, the sensitive groups can be installed at any location within the ODN sequences without using any sequence- or functionality-specific conditions and procedures.

Enzymatic synthesis of base-modified RNA by T7 RNA polymerase. A systematic study and comparison of 5-substituted pyrimidine and 7-substituted 7-deazapurine nucleoside triphosphates as substrates

We synthesized a small library of eighteen 5-substituted pyrimidine or 7-substituted 7-deazapurine nucleoside triphosphates bearing methyl, ethynyl, phenyl, benzofuryl or dibenzofuryl groups through cross-coupling reactions of nucleosides followed by triphosphorylation or through direct cross-coupling reactions of halogenated nucleoside triphosphates. We systematically studied the influence of the modification on the efficiency of T7 RNA polymerase catalyzed synthesis of modified RNA and found that modified ATP, UTP and CTP analogues bearing smaller modifications were good substrates and building blocks for the RNA synthesis even in difficult sequences incorporating multiple modified nucleotides. Bulky dibenzofuryl derivatives of ATP and GTP were not substrates for the RNA polymerase. In the case of modified GTP analogues, a modified procedure using a special promoter and GMP as initiator needed to be used to obtain efficient RNA synthesis. The T7 RNA polymerase synthesis of modified RNA can be very efficiently used for synthesis of modified RNA but the method has constraints in the sequence of the first three nucleotides of the transcript, which must contain a non-modified G in the +1 position.

Efficient Genome Engineering of a Virulent Klebsiella Bacteriophage Using CRISPR-Cas9

Klebsiella pneumoniae is one of the most common nosocomial opportunistic pathogens and usually exhibits multiple-drug resistance. Phage therapy, a potential therapeutic to replace or supplement antibiotics, has attracted much attention. However, very few Klebsiella phages have been well characterized because of the lack of efficient genome-editing tools. Here, Cas9 from Streptococcus pyogenes and a single guide RNA (sgRNA) were used to modify a virulent Klebsiella bacteriophage, phiKpS2. We first evaluated the distribution of sgRNA activity in phages and proved that it is largely inconsistent with the predicted activity from current models trained on eukaryotic cell data sets. A simple CRISPR-based phage genome-editing procedure was developed based on the discovery that homologous arms as short as 30 to 60 bp were sufficient to introduce point mutation, gene deletion, and swap. We also demonstrated that weak sgRNAs could be used for precise phage genome editing but failed to select random recombinants, possibly because inefficient cleavage can be tolerated through continuous repair by homologous recombination with the uncut genomes. Small frameshift deletion was proved to be an efficient way to evaluate the essentiality of phage genes. By using the abovementioned strategies, a putative promoter and nine genes of phiKpS2 were successfully deleted. Interestingly, the holin gene can be deleted with little effect on phiKpS2 infection, but the reason is not yet clear. This study established an efficient, time-saving, and cost-effective procedure for phage genome editing, which is expected to significantly promote the development of bacteriophage therapy.IMPORTANCE In the present study, we have addressed efficient, time-saving, and cost-effective CRISPR-based phage genome editing of Klebsiella phage, which has the potential to significantly expand our knowledge of phage-host interactions and to promote applications of phage therapy. The distribution of sgRNA activity was first evaluated in phages. Short homologous arms were proven to be enough to introduce point mutation, small frameshift deletion, gene deletion, and swap into phages, and weak sgRNAs were proven useful for precise phage genome editing but failed to select random recombinants, all of which makes the CRISPR-based phage genome-editing method easier to use.

An orthogonal photolabile linker for the complete "on-support" synthesis/fast deprotection/hybridization of RNA

The preparation of a polystyrene solid support decorated with a photolabile linker is described. The entire post-synthetic processing of RNA can be carried out in the solid phase in a minimum amount of time. The deprotected RNA is available for "on-support" hybridization and photolysis releases siRNA duplexes under mild, neutral conditions.

Phosphine-free Stille-Migita chemistry for the mild and orthogonal modification of DNA and RNA

An optimized catalyst system of [Pd2 (dba)3 ] and AsPh3 efficiently catalyzes the Stille reaction between a diverse set of functionalized stannanes and halogenated mono-, di- and oligonucleotides. The methodology allows for the facile conjugation of short and long nucleic acid molecules with moieties that are not compatible with conventional chemical or enzymatic synthesis, among them acid-, base-, or fluoride-labile protecting groups, fluorogenic and synthetically challenging moieties with good to near-quantitative yields. Notably, even azides can be directly introduced into oligonucleotides and (deoxy)nucleoside triphosphates, thereby giving direct access to "clickable" nucleic acids.

Regulatory RNAs: charming gene management styles for synthetic biology applications

RNAs have many important functional properties, including that they are independently controllable and highly tunable. As a result of these advantageous properties, their use in a myriad of sophisticated devices has been widely explored. Yet, the exploitation of RNAs for synthetic applications is highly dependent on the ability to characterize the many new molecules that continue to be discovered by large-scale sequencing and high-throughput screening techniques. In this review, we present an exhaustive survey of the most recent synthetic bacterial riboswitches and small RNAs while emphasizing their virtues in gene expression management. We also explore the use of these RNA components as building blocks in the RNA synthetic biology toolbox and discuss examples of synthetic RNA components used to rewire bacterial regulatory circuitry. We anticipate that this field will expand its catalog of smart devices by mimicking and manipulating natural RNA mechanisms and functions.