Transcription of Click-Linked DNA in Human Cells

In vivo assembly and expression of DNA containing non-canonical bases in the yeast Saccharomyces cerevisiae

Chemically modified nucleic acids are of utmost interest in synthetic biology to create a regulable and sophisticated synthetic system with tailor-made properties. Implanting chemically modified nucleic acids in microorganisms might serve biotechnological applications, while using them in human cells might lead to new advanced medicines. Previously, we reported that a fully modified DNA sequence (called DZA) composed of the four base-modified nucleotides - 7-deaza-adenine, 5-chlorouracil, 7-deaza-guanine and 5-fluorocytosine - could function as a genetic template in prokaryotic cells, Escherichia coli . Here, we report the synthesis of long, partially or fully modified DZA fragments that encode the yeast-enhanced red fluorescence protein (yEmRFP). The DZA sequences were directly introduced in the genome of the eukaryotic cells, Saccharomyces cerevisiae , via the yeast natural homologous recombination. The simple and straightforward DZA cloning strategy reported herein might be of interest to scientists working in the field of xenobiology in yeast.

Azide-modified Nucleosides as Versatile Tools for Bioorthogonal Labeling and Functionalization

Azide-modified nucleosides are important building blocks for RNA and DNA functionalization by click chemistry based on azide-alkyne cycloaddition. This has put demand on synthetic chemistry to develop approaches for the preparation of azide-modified nucleoside derivatives. We review here the available methods for the synthesis of various nucleosides decorated with azido groups at the sugar residue or nucleobase, their incorporation into oligonucleotides and cellular RNAs, and their application in azide-alkyne cycloadditions for labelling and functionalization.

Completely Chemically Synthesized Long DNA Can be Transcribed in Human Cells

Chemical ligation reaction of DNA is useful for the construction of long functional DNA using oligonucleotide fragments that are prepared by solid phase chemical synthesis. However, the unnatural linkage structure formed by the ligation reaction generally impairs the biological function of the resulting ligated DNA. We achieved the complete chemical synthesis of 78 and 258 bp synthetic DNAs via multiple chemical ligation reactions with phosphorothioate and haloacyl-modified DNA fragments. The latter synthetic DNA, coding shRNA for luciferase genes with a designed truncated SV promoter sequence, successfully induced the expected gene silencing effect in HeLa cells.

In Vivo Expression of Genetic Information from Phosphoramidate-DNA

Chemically modified genes and genomes with customized properties will become a valuable tool in numerous fields, including synthetic biology, biotechnology, and medicine. These genetic materials are meant to store and exchange information with DNA and RNA while tuning their functionality. Herein, we outline the development of an alternative genetic system carrying phosphoramidate linkages that successfully propagates genetic information in bacteria and at the same time is labile to acidic conditions. The P3'→N5' phosphoramidate-containing DNA (PN-DNA) was enzymatically synthesized by using 5'-amino-2',5'-deoxycytidine 5'-N-triphosphates (NH-dCTPs) as substrates for DNA polymerases and employed to encode antibiotic resistance in Escherichia coli. The resulting PN-DNA can be efficiently destroyed by mild acidic conditions, whereas an unmodified counterpart remains intact. A cloning strategy was proposed for assembling modified fragments into a genome. This method can be of interest to scientists working in the field of orthogonal nucleic acid genes and genomes.

A Click Chemistry Approach to Developing Molecularly Targeted DNA Scissors

Nucleic acid click chemistry was used to prepare a family of chemically modified triplex forming oligonucleotides (TFOs) for application as a new gene-targeted technology. Azide-bearing phenanthrene ligands-designed to promote triplex stability and copper binding-were 'clicked' to alkyne-modified parallel TFOs. Using this approach, a library of TFO hybrids was prepared and shown to effectively target purine-rich genetic elements in vitro. Several of the hybrids provide significant stabilisation toward melting in parallel triplexes (>20 °C) and DNA damage can be triggered upon copper binding in the presence of added reductant. Therefore, the TFO and 'clicked' ligands work synergistically to provide sequence-selectivity to the copper cutting unit which, in turn, confers high stabilisation to the DNA triplex. To extend the boundaries of this hybrid system further, a click chemistry-based di-copper binding ligand was developed to accommodate designer ancillary ligands such as DPQ and DPPZ. When this ligand was inserted into a TFO, a dramatic improvement in targeted oxidative cleavage is afforded.

Oligonucleotide analogues with cationic backbone linkages

Their unique ability to selectively bind specific nucleic acid sequences makes oligonucleotides promising bioactive agents. However, modifications of the nucleic acid structure are an essential prerequisite for their application in vivo or even in cellulo. The oligoanionic backbone structure of oligonucleotides mainly hampers their ability to penetrate biological barriers such as cellular membranes. Hence, particular attention has been given to structural modifications of oligonucleotides which reduce their overall number of negative charges. One such approach is the site-specific replacement of the negatively charged phosphate diester linkage with alternative structural motifs which are positively charged at physiological pH, thus resulting in zwitterionic or even oligocationic backbone structures. This review provides a general overview of this concept and summarizes research on four according artificial backbone linkages: aminoalkylated phosphoramidates (and related systems), guanidinium groups, S-methylthiourea motifs, and nucleosyl amino acid (NAA)-derived modifications. The synthesis and properties of the corresponding oligonucleotide analogues are described.

Oligonucleotides with Cationic Backbone and Their Hybridization with DNA: Interplay of Base Pairing and Electrostatic Attraction

Non‐natural oligonucleotides represent important (bio)chemical tools and potential therapeutic agents. Backbone modifications altering hybridization properties and biostability can provide useful analogues. Here, we employ an artificial nucleosyl amino acid (NAA) motif for the synthesis of oligonucleotides containing a backbone decorated with primary amines. An oligo‐T sequence of this cationic DNA analogue shows significantly increased affinity for complementary DNA. Notably, hybridization with DNA is still governed by Watson–Crick base pairing. However, single base pair mismatches are tolerated and some degree of sequence‐independent interactions between the cationic NAA backbone and fully mismatched DNA are observed. These findings demonstrate that a high density of positive charges directly connected to the oligonucleotide backbone can affect Watson–Crick base pairing. This provides a paradigm for the design of therapeutic oligonucleotides with altered backbone charge patterns.

Chemical Morphing of DNA Containing Four Noncanonical Bases

The ability of alternative nucleic acids, in which all four nucleobases are substituted, to replicate in vitro and to serve as genetic templates in vivo was evaluated. A nucleotide triphosphate set of 5-chloro-2'-deoxyuridine, 7-deaza-2'-deoxyadenosine, 5-fluoro-2'-deoxycytidine, and 7-deaza-2'deoxyguanosine successfully underwent polymerase chain reaction (PCR) amplification using templates of different lengths (57 or 525mer) and Taq or Vent (exo-) DNA polymerases as catalysts. Furthermore, a fully morphed gene encoding a dihydrofolate reductase was generated by PCR using these fully substituted nucleotides and was shown to transform and confer trimethoprim resistance to E. coli. These results demonstrated that fully modified templates were accurately read by the bacterial replication machinery and provide the first example of a long fully modified DNA molecule being functional in vivo.

Chimeric RNA Oligonucleotides with Triazole and Phosphate Linkages: Synthesis and RNA Interference

Chimeric RNA oligonucleotides with an artificial triazole linker were synthesized using solution-phase click chemistry and solid-phase automated synthesis. Scalable synthesis methods for jointing units for the chimeric structure have been developed, and after click-coupling of the jointing units with triazole linkers, a series of chimeric oligonucleotides was prepared by utilizing the well-established phosphoramidite method for the elongation. The series of chimeric 21-mer oligonucleotides that possessed the triazole linker at different strands and positions allowed for a screening study of the RNA interference to clarify the preference of the triazole modifications in small-interfering RNA molecules.