Snapshots of an evolved DNA polymerase pre- and post-incorporation of an unnatural nucleotide.

The next challenge in synthetic biology is to be able to replicate synthetic nucleic acid sequences efficiently. The synthetic pair, 2-amino-8-(1-beta-d-2'- deoxyribofuranosyl) imidazo [1,2-a]-1,3,5-triazin-[8H]-4-one (trivially designated P) with 6-amino-3-(2'-deoxyribofuranosyl)-5-nitro-1H-pyridin-2-one (trivially designated Z), is replicated by certain Family A polymerases, albeit with lower efficiency. Through directed evolution, we identified a variant KlenTaq polymerase (M444V, P527A, D551E, E832V) that incorporates dZTP opposite P more efficiently than the wild-type enzyme. Here, we report two crystal structures of this variant KlenTaq, a post-incorporation complex that includes a template-primer with P:Z trapped in the active site (binary complex) and a pre-incorporation complex with dZTP paired to template P in the active site (ternary complex). In forming the ternary complex, the fingers domain exhibits a larger closure angle than in natural complexes but engages the template-primer and incoming dNTP through similar interactions. In the binary complex, although many of the interactions found in the natural complexes are retained, there is increased relative motion of the thumb domain. Collectively, our analyses suggest that it is the post-incorporation complex for unnatural substrates that presents a challenge to the natural enzyme and that more efficient replication of P:Z pairs requires a more flexible polymerase.

Enzyme-assisted high throughput sequencing of an expanded genetic alphabet at single base resolution.

With just four building blocks, low sequence information density, few functional groups, poor control over folding, and difficulties in forming compact folds, natural DNA and RNA have been disappointing platforms from which to evolve receptors, ligands, and catalysts. Accordingly, synthetic biology has created "artificially expanded genetic information systems" (AEGIS) to add nucleotides, functionality, and information density. With the expected improvements seen in AegisBodies and AegisZymes, the task for synthetic biologists shifts to developing for expanded DNA the same analytical tools available to natural DNA. Here we report one of these, an enzyme-assisted sequencing of expanded genetic alphabet (ESEGA) method to sequence six-letter AEGIS DNA. We show how ESEGA analyses this DNA at single base resolution, and applies it to optimized conditions for six-nucleotide PCR, assessing the fidelity of various DNA polymerases, and extending this to AEGIS components with functional groups. This supports the renewed exploitation of expanded DNA alphabets in biotechnology.

Directed evolution of polymerases to accept nucleotides with nonstandard hydrogen bond patterns.

Artificial genetic systems have been developed by synthetic biologists over the past two decades to include additional nucleotides that form additional nucleobase pairs independent of the standard T:A and C:G pairs. Their use in various tools to detect and analyze DNA and RNA requires polymerases that synthesize duplex DNA containing unnatural base pairs. This is especially true for nested polymerase chain reaction (PCR), which has been shown to dramatically lower noise in multiplexed nested PCR if nonstandard nucleotides are used in their external primers. We report here the results of a directed evolution experiment seeking variants of Taq DNA polymerase that can support the nested PCR amplification with external primers containing two particular nonstandard nucleotides, 2-amino-8-(1'-ß-d-2'-deoxyribofuranosyl)imidazo[1,2-a]-1,3,5-triazin-4(8H)-one (trivially called P) that pairs with 6-amino-5-nitro-3-(1'-ß-d-2'-deoxyribofuranosyl)-2(1H)-pyridone (trivially called Z). Variants emerging from the directed evolution experiments were shown to pause less when challenged in vitro to incorporate dZTP opposite P in a template. Interestingly, several sites involved in the adaptation of Taq polymerases in the laboratory were also found to have displayed "heterotachy" (different rates of change) in their natural history, suggesting that these sites were involved in an adaptive change in natural polymerase evolution. Also remarkably, the polymerases evolved to be less able to incorporate dPTP opposite Z in the template, something that was not selected. In addition to being useful in certain assay architectures, this result underscores the general rule in directed evolution that "you get what you select for".

Enzyme-Assisted High Throughput Sequencing of an Expanded Genetic Alphabet at Single Base Resolution.

Many efforts have sought to apply laboratory in vitro evolution (LIVE) to natural nucleic acid (NA) scaffolds to directly evolve functional molecules. However, synthetic biology can move beyond natural NA scaffolds to create molecular systems whose libraries are far richer reservoirs of functionality than natural NAs. For example, "artificially expanded genetic information systems" (AEGIS) add up to eight nucleotides to the four found in standard NA. Even in its simplest 6-letter versions, AEGIS adds functional groups, information density, and folding motifs that natural NA libraries lack. To complete this vision, however, tools are needed to sequence molecules that are created by AEGIS LIVE. Previous sequencing approaches, including approaches from our laboratories, exhibited limited performance and lost many sequences in diverse library mixtures. Here, we present a new approach that enzymatically transforms the target AEGIS DNA. With higher transliteration efficiency and fidelity, this Enzyme-Assisted Sequencing of Expanded Genetic Alphabet (ESEGA) approach produces substantially better sequences of 6-letter (AGCTZP) DNA than previous transliteration approaches. Therefore, ESEGA facilitates precise analysis of libraries, allowing 'next-generation deep sequencing' to accurately quantify the sequences of 6-letter DNA molecules at single base resolution. We then applied ESEGA to three tasks: (a) defining optimal conditions to perform 6-nucleotide PCR (b) evaluating the fidelity of 6-nucleotide PCR with various DNA polymerases, and (c) extending that evaluation to AEGIS components functionalized with alkynyl and aromatic groups. No other approach at present has this scope, allowing this work to be the next step towards exploiting the potential of expanded DNA alphabets in biotechnology.

Fluorinated oil-surfactant mixtures with the density of water: Artificial cells for synthetic biology.

There is a rising interest in biotechnology for the compartmentalization of biochemical reactions in water droplets. Several applications, such as the widely used digital PCR, seek to encapsulate a single molecule in a droplet to be amplified. Directed evolution, another technology with growing popularity, seeks to replicate what happens in nature by encapsulating a single gene and the protein encoded by this gene, linking genotype with phenotype. Compartmentalizing reactions in droplets also allows the experimentalist to run millions of different reactions in parallel. Compartmentalization requires a fluid that is immiscible with water and a surfactant to stabilize the droplets. While there are fluids and surfactants on the market that have been used to accomplish encapsulation, there are reported concerns with these. Span® 80, for example, a commonly used surfactant, has contaminants that interfere with various biochemical reactions. Similarly, synthetic fluids distributed by the cosmetic industry allow some researchers to produce experimental results that can be published, but then other researchers fail to reproduce some of these protocols due to the unreliable nature of these products, which are not manufactured with the intent of being used in biotechnology. The most reliable fluids, immiscible with water and suitable for biochemical reactions, are fluorinated fluids. Fluorinated compounds have the peculiar characteristic of being immiscible with water while at the same time not mixing with hydrophobic molecules. This peculiar characteristic has made fluorinated fluids attractive because it seems to be the basis of their being biologically inert. However, commercially available fluorinated fluids have densities between 1.4 to 1.6 g/mL. The higher-than-water density of fluorinated oils complicates handling of the droplets since these would float on the fluid since the water droplets would be less dense. This can cause aggregation and coalescence of the droplets. Here, we report the synthesis, characterization, and use of fluorinated polysiloxane oils that have densities similar to the one of water at room temperature, and when mixed with non-ionic fluorinated surfactants, can produce droplets encapsulating biochemical reactions. We show how droplets in these emulsions can host many biological processes, including PCR, DNA origami, rolling circle amplification (RCA), and Taqman® assays. Some of these use unnatural DNA built from an Artificially Expanded Genetic Information System (AEGIS) with six nucleotide "letters".

The surprising pairing of 2-aminoimidazo[1,2-a][1,3,5]triazin-4-one, a component of an expanded DNA alphabet.

Synthetic biologists demonstrate their command over natural biology by reproducing the behaviors of natural living systems on synthetic biomolecular platforms. For nucleic acids, this is being done stepwise, first by adding replicable nucleotides to DNA, and then removing its standard nucleotides. This challenge has been met in vitro with `six-letter' DNA and RNA, where the Watson-Crick pairing `concept' is recruited to increase the number of independently replicable nucleotides from four to six. The two nucleobases most successfully added so far are Z and P, which present a donor-donor-acceptor and an acceptor-acceptor-donor pattern, respectively. This pair of nucleobases are part of an `artificially expanded genetic information system' (AEGIS). The Z nucleobase has been already crystallized, characterized, and published in this journal [Matsuura et al. (2016). Acta Cryst. C72, 952-959]. More recently, variants of Taq polymerase have been crystallized with the pair P:Z trapped in the active site. Here we report the crystal structure of the nucleobase 2-aminoimidazo[1,2-a][1,3,5]triazin-4-one (trivially named P) as the monohydrate, C5H5N5O·H2O. The nucleobase P was crystallized from water and characterized by X-ray diffraction. Interestingly, the crystal structure shows two tautomers of P packed in a Watson-Crick fashion that cocrystallized in a 1:1 ratio.

Alternative Watson-Crick Synthetic Genetic Systems.

In its "grand challenge" format in chemistry, "synthesis" as an activity sets out a goal that is substantially beyond current theoretical and technological capabilities. In pursuit of this goal, scientists are forced across uncharted territory, where they must answer unscripted questions and solve unscripted problems, creating new theories and new technologies in ways that would not be created by hypothesis-directed research. Thus, synthesis drives discovery and paradigm changes in ways that analysis cannot. Described here are the products that have arisen so far through the pursuit of one grand challenge in synthetic biology: Recreate the genetics, catalysis, evolution, and adaptation that we value in life, but using genetic and catalytic biopolymers different from those that have been delivered to us by natural history on Earth. The outcomes in technology include new diagnostic tools that have helped personalize the care of hundreds of thousands of patients worldwide. In science, the effort has generated a fundamentally different view of DNA, RNA, and how they work.

DNA polymerases engineered by directed evolution to incorporate non-standard nucleotides.

DNA polymerases have evolved for billions of years to accept natural nucleoside triphosphate substrates with high fidelity and to exclude closely related structures, such as the analogous ribonucleoside triphosphates. However, polymerases that can accept unnatural nucleoside triphosphates are desired for many applications in biotechnology. The focus of this review is on non-standard nucleotides that expand the genetic "alphabet." This review focuses on experiments that, by directed evolution, have created variants of DNA polymerases that are better able to accept unnatural nucleotides. In many cases, an analysis of past evolution of these polymerases (as inferred by examining multiple sequence alignments) can help explain some of the mutations delivered by directed evolution.

Solution 1H NMR confirmation of folding in short o-phenylene ethynylene oligomers.

Oligomers based on an o-phenylene ethynylene (oPE) backbone with polar substituents have been synthesized using Sonogashira methods. Folding of these extremely short oligomers was confirmed via 1D and 2D (NOESY) NMR methods. Utilizing electron-rich and electron-poor phenylene building blocks, variations of these oPE oligomers have been synthesized to determine the folded stability of pi-rich vs pi-poor vs pi-rich-pi-poor systems. Slight variations in temperature offer a route, aside from solvent denaturation, to probe the stability of the folded structure. This is the first report of an NMR solution characterization of folding for a PE backbone without hydrogen bonds.