Combinatorial nucleoside-deletion-scanning mutagenesis of functional DNA

Tailored Tolane-Perfluorotolane Assembly as Supramolecular Base Pair Replacement in DNA

Arene-fluoroarene interactions offer outstanding possibilities for engineering of supramolecular systems, including nucleic acids. Here, we implement the tolane-perfluorotolane interaction as base pair replacement in DNA. Tolane (THH) and perfluorotolane (TFF) moieties were connected to acyclic backbone units, comprising glycol nucleic acid (GNA) or butyl nucleic acid (BuNA) building blocks, that were incorporated via phosphoramidite chemistry at opposite positions in a DNA duplex. Thermodynamic analyses by UV thermal melting revealed a compelling stabilization by THH/TFF heteropairs only when connected to the BuNA backbone, but not with the shorter GNA linker. Detailed NMR studies confirmed the preference of the BuNA backbone for enhanced polar π-stacking. This work defines how orthogonal supramolecular interactions can be tailored by small constitutional changes in the DNA backbone, and it inspires future studies of arene-fluoroarene-programmed assembly of DNA.

New Deoxyribozymes for the Native Ligation of RNA

Deoxyribozymes (DNAzymes) are small, synthetic, single-stranded DNAs capable of catalyzing chemical reactions, including RNA ligation. Herein, we report a novel class of RNA ligase deoxyribozymes that utilize 5'-adenylated RNA (5'-AppRNA) as the donor substrate, mimicking the activated intermediates of protein-catalyzed RNA ligation. Four new DNAzymes were identified by in vitro selection from an N₄₀ random DNA library and were shown to catalyze the intermolecular linear RNA-RNA ligation via the formation of a native 3'-5'-phosphodiester linkage. The catalytic activity is distinct from previously described RNA-ligating deoxyribozymes. Kinetic analyses revealed the optimal incubation conditions for high ligation yields and demonstrated a broad RNA substrate scope. Together with the smooth synthetic accessibility of 5'-adenylated RNAs, the new DNA enzymes are promising tools for the protein-free synthesis of long RNAs, for example containing precious modified nucleotides or fluorescent labels for biochemical and biophysical investigations.

Large Scale Mutational and Kinetic Analysis of a Self-Hydrolyzing Deoxyribozyme

Deoxyribozymes are catalytic DNA sequences whose atomic structures are generally difficult to elucidate. Mutational analysis remains a principal approach for understanding and engineering deoxyribozymes with diverse catalytic activities. However, laborious preparation and biochemical characterization of individual sequences severely limit the number of mutants that can be studied biochemically. Here, we applied deep sequencing to directly measure the activities of self-hydrolyzing deoxyribozyme sequences in high throughput. First, all single and double mutants within the 15-base catalytic core of the deoxyribozyme I-R3 were assayed to unambiguously determine the tolerated and untolerated mutations at each position. Subsequently, 4096 deoxyribozyme variants with tolerated base substitutions at seven positions were kinetically assayed in parallel. We identified 533 active mutants whose first-order rate constants and activation energies were determined. The results indicate an isolated and narrow peak in the deoxyribozyme sequence space and provide a quantitative view of the effects of multiple mutations on the deoxyribozyme activity for the first time.

Extensive libraries of gene truncation variants generated by in vitro transposition

The detailed analysis of the impact of deletions on proteins or nucleic acids can reveal important functional regions and lead to variants with improved macromolecular properties. We present a method to generate large libraries of mutants with deletions of varying length that are randomly distributed throughout a given gene. This technique facilitates the identification of crucial sequence regions in nucleic acids or proteins. The approach utilizes in vitro transposition to generate 5΄ and 3΄ fragment sub-libraries of a given gene, which are then randomly recombined to yield a final library comprising both terminal and internal deletions. The method is easy to implement and can generate libraries in three to four days. We used this approach to produce a library of >9000 random deletion mutants of an artificial RNA ligase enzyme representing 32% of all possible deletions. The quality of the library was assessed by next-generation sequencing and detailed bioinformatics analysis. Finally, we subjected this library to in vitro selection and obtained fully functional variants with deletions of up to 18 amino acids of the parental enzyme that had been 95 amino acids in length.

Pursuing DNA catalysts for protein modification

Catalysis is a fundamental chemical concept, and many kinds of catalysts have considerable practical value. Developing entirely new catalysts is an exciting challenge. Rational design and screening have provided many new small-molecule catalysts, and directed evolution has been used to optimize or redefine the function of many protein enzymes. However, these approaches have inherent limitations that prompt the pursuit of different kinds of catalysts using other experimental methods. Nature evolved RNA enzymes, or ribozymes, for key catalytic roles that in modern biology are limited to phosphodiester cleavage/ligation and amide bond formation. Artificial DNA enzymes, or deoxyribozymes, have great promise for a broad range of catalytic activities. They can be identified from unbiased (random) sequence populations as long as the appropriate in vitro selection strategies can be implemented for their identification. Notably, in vitro selection is different in key conceptual and practical ways from rational design, screening, and directed evolution. This Account describes the development by in vitro selection of DNA catalysts for many different kinds of covalent modification reactions of peptide and protein substrates, inspired in part by our earlier work with DNA-catalyzed RNA ligation reactions. In one set of studies, we have sought DNA-catalyzed peptide backbone cleavage, with the long-term goal of artificial DNA-based proteases. We originally anticipated that amide hydrolysis should be readily achieved, but in vitro selection instead surprisingly led to deoxyribozymes for DNA phosphodiester hydrolysis; this was unexpected because uncatalyzed amide bond hydrolysis is 10(5)-fold faster. After developing a suitable selection approach that actively avoids DNA hydrolysis, we were able to identify deoxyribozymes for hydrolysis of esters and aromatic amides (anilides). Aliphatic amide cleavage remains an ongoing focus, including via inclusion of chemically modified DNA nucleotides in the catalyst, which we have recently found to enable this cleavage reaction. In numerous other efforts, we have investigated DNA-catalyzed peptide side chain modification reactions. Key successes include nucleopeptide formation (attachment of oligonucleotides to peptide side chains) and phosphatase and kinase activities (removal and attachment of phosphoryl groups to side chains). Through all of these efforts, we have learned the importance of careful selection design, including the frequent need to develop specific "capture" reactions that enable the selection process to provide only those DNA sequences that have the desired catalytic functions. We have established strategies for identifying deoxyribozymes that accept discrete peptide and protein substrates, and we have obtained data to inform the key choice of random region length at the outset of selection experiments. Finally, we have demonstrated the viability of modular deoxyribozymes that include a small-molecule-binding aptamer domain, although the value of such modularity is found to be minimal, with implications for many selection endeavors. Advances such as those summarized in this Account reveal that DNA has considerable catalytic abilities for biochemically relevant reactions, specifically including covalent protein modifications. Moreover, DNA has substantially different, and in many ways better, characteristics than do small molecules or proteins for a catalyst that is obtained "from scratch" without demanding any existing information on catalyst structure or mechanism. Therefore, prospects are very strong for continued development and eventual practical applications of deoxyribozymes for peptide and protein modification.

Enzymatic combinatorial nucleoside deletion scanning mutagenesis of functional RNA

We describe a general and simple method to identify catalytically and structurally important nucleotides in functional RNAs. Our approach is based on statistical replacement of each nucleoside with a non-nucleosidic spacer (C3 linker, Δ), followed by separation of active library variants and readout of interference effects by analysis of enzymatic primer extension reactions.

Optochemical control of deoxyoligonucleotide function via a nucleobase-caging approach

Synthetic oligonucleotides have been extensively applied tocontrol a wide range of biological processes such as gene expression, gene repair, DNA replication, and protein activity. Based on well-established sequence design rules that typically rely on Watson-Crick base pairing interactions researchers can readily program the function of these oligonucleotides. Therefore oligonucleotides provide a flexible platform for targeting a wide range of biological molecules, including DNA, RNA, and proteins. In addition, oligonucleotides are commonly used research tools in cell biology and developmental biology. However, a lack of conditional control methods has hampered the precise spatial and temporal regulation of oligonucleotide activity, which limits the application of these reagents to investigate complex biological questions. Nature controls biological function with a high level of spatial and temporal resolution and in order to elucidate the molecular mechanisms of biological processes, researchers need tools that allow for the perturbation of these processes with Nature's precision. Light represents an excellent external regulatory element since irradiation can be easily controlled spatially and temporally. Thus, researchers have developed several different methods to conditionally control oligonucleotide activity with light. One of the most versatile strategies is optochemical regulation through the installation and removal of photolabile caging groups on oligonucleotides. To produce switches that can control nucleic acid function with light, chemists introduce caging groups into the oligomer backbone or on specific nucleobases to block oligonucleotide function until the caging groups are removed by light exposure. In this Account, we focus on the application of caged nucleobases to the photoregulation of DNA function. Using this approach, we have both activated and deactivated gene expression optochemically at the transcriptional and translational level with spatial and temporal control. Specifically, we have used caged triplex-forming oligomers and DNA decoys to regulate transcription, and we have regulated translation with light-activated antisense agents. Moreover, we also discuss strategies that can trigger DNA enzymatic activity, DNA amplification, and DNA mutagenesis by light illumination. More recently, we have developed light-activated DNA logic operations, an advance that may lay the foundation for the optochemical control of complex DNA calculations.