Terminal hairpin in oligonucleotide dominantly prioritizes intramolecular cyclization by T4 ligase over intermolecular polymerization: an exclusive methodology for producing ssDNA rings

Circular Single-Stranded DNA: Discovery, Biological Effects, and Applications

The field of nucleic acid therapeutics has witnessed a significant surge in recent times, as evidenced by the increasing number of approved genetic drugs. However, current platform technologies containing plasmids, lipid nanoparticle-mRNAs, and adeno-associated virus vectors encounter various limitations and challenges. Thus, we are devoted to finding a novel nucleic acid vector and have directed our efforts toward investigating circular single-stranded DNA (CssDNA), an ancient form of nucleic acid. CssDNAs are ubiquitous, but generally ignored. Accumulating evidence suggests that CssDNAs possess exceptional properties as nucleic acid vectors, exhibiting great potential for clinical applications in genetic disorders, gene editing, and immune cell therapy. Here, we comprehensively review the discovery and biological effects of CssDNAs as well as their applications in the field of biomedical research for the first time. Undoubtedly, as an ancient form of DNA, CssDNA holds immense potential and promises novel insights for biomedical research.

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.

Construction of ssDNA-Attached LR-Chimera Involving Z-DNA for ZBP1 Binding Analysis

The binding of proteins to Z-DNA is hard to analyze, especially for short non-modified DNA, because it is easily transferred to B-DNA. Here, by the hybridization of a larger circular single-stranded DNA (ssDNA) with a smaller one, an LR-chimera (involving a left-handed part and a right-handed one) with an ssDNA loop is produced. The circular ssDNAs are prepared by the hybridization of two ssDNA fragments to form two nicks, followed by nick sealing with T4 DNA ligase. No splint (a scaffold DNA for circularizing ssDNA) is required, and no polymeric byproducts are produced. The ssDNA loop on the LR-chimera can be used to attach it with other molecules by hybridization with another ssDNA. The gel shift binding assay with Z-DNA specific binding antibody (Z22) or Z-DNA binding protein 1 (ZBP1) shows that stable Z-DNA can form under physiological ionic conditions even when the extra ssDNA part is present. Concretely, a 5'-terminal biotin-modified DNA oligonucleotide complementary to the ssDNA loop on the LR-chimera is used to attach it on the surface of a biosensor inlaid with streptavidin molecules, and the binding constant of ZBP1 with Z-DNA is analyzed by BLI (bio-layer interferometry). This approach is convenient for quantitatively analyzing the binding dynamics of Z-DNA with other molecules.

Preferential production of RNA rings by T4 RNA ligase 2 without any splint through rational design of precursor strand

Rings of single-stranded RNA are promising for many practical applications, but the methods to prepare them in preparative scale have never been established. Previously, RNA circularization was achieved by T4 RNA ligase 2 (Rnl2, a dsRNA ligase) using splints, but the yield was low due to concurrent intermolecular polymerization. Here, various functional RNAs (siRNA, miRNA, ribozyme, etc.) are dominantly converted by Rnl2 to the rings without significant limitations in sizes and sequences. The key is to design a precursor RNA, which is highly activated for the efficient circularization without any splint. First, secondary structure of target RNA ring is simulated by Mfold, and then hypothetically cut at one site so that a few intramolecular base pairs are formed at the terminal. Simply by treating this RNA with Rnl2, the target ring was selectively and efficiently produced. Unexpectedly, circular RNA can be obtained in high yield (>90%), even when only 2 bp form in the 3'-OH side and no full match base pair forms in the 5'-phosphate side. Formation of polymeric by-products was further suppressed by diluting conventional Rnl2 buffer to abnormally low concentrations. Even at high-RNA concentrations (e.g. 50 μM), enormously high selectivity (>95%) was accomplished.