"Skinny" and "Fat" DNA: Two New Double Helices.

According to the iconic model, the Watson-Crick double helix exploits nucleobase pairs that are both size complementary (big purines pair with small pyrimidines) and hydrogen bond complementary (hydrogen bond donors pair with hydrogen bond acceptors). Using a synthetic biology strategy, we report here the discovery of two new DNA-like systems that appear to support molecular recognition with the same proficiency as standard Watson-Crick DNA. However, these both violate size complementarity (big pairs with small), retaining hydrogen bond complementarity (donors pair with acceptors) as their only specificity principle. They exclude mismatches as well as standard Watson-Crick DNA excludes mismatches. In crystal structures, these "skinny" and "fat" systems form the expected hydrogen bonds, while conferring novel minor groove properties to the resultant duplex regions of the DNA oligonucleotides. Further, computational tools, previously tested primarily on natural DNA, appear to work well for these two new molecular recognition systems, offering a validation of the power of modern computational biology. These new molecular recognition systems may have application in materials science and synthetic biology, and in developing our understanding of alternative ways that genetic information might be stored and transmitted.

Polymerase Interactions with Wobble Mismatches in Synthetic Genetic Systems and Their Evolutionary Implications.

In addition to completing the Watson-Crick nucleobase matching "concept" (big pairs with small, hydrogen bond donors pair with hydrogen bond acceptors), artificially expanded genetic information systems (AEGIS) also challenge DNA polymerases with a complete set of mismatches, including wobble mismatches. Here, we explore wobble mismatches with AEGIS with DNA polymerase 1 from Escherichia coli. Remarkably, we find that the polymerase tolerates an AEGIS:standard wobble that has the same geometry as the G:T wobble that polymerases have evolved to exclude but excludes a wobble geometry that polymerases have never encountered in natural history. These results suggest certain limits to "structural analogy" and "evolutionary guidance" as tools to help synthetic biologists expand DNA alphabets.

Evolution of functional six-nucleotide DNA.

Axiomatically, the density of information stored in DNA, with just four nucleotides (GACT), is higher than in a binary code, but less than it might be if synthetic biologists succeed in adding independently replicating nucleotides to genetic systems. Such addition could also add functional groups not found in natural DNA, but useful for molecular performance. Here, we consider two new nucleotides (Z and P, 6-amino-5-nitro-3-(1'-ß-D-2'-deoxyribo-furanosyl)-2(1H)-pyridone and 2-amino-8-(1'-ß-D-2'-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one). These are designed to pair via complete Watson-Crick geometry. These were added to a library of oligonucleotides used in a laboratory in vitro evolution (LIVE) experiment; the GACTZP library was challenged to deliver molecules that bind selectively to liver cancer cells, but not to untransformed liver cells. Unlike in classical in vitro selection, low levels of mutation allow this system to evolve to create binding molecules not necessarily present in the original library. Over a dozen binding species were recovered. The best had Z and/or P in their sequences. Several had multiple, nearby, and adjacent Zs and Ps. Only the weaker binders contained no Z or P at all. This suggests that this system explored much of the sequence space available to this genetic system and that GACTZP libraries are richer reservoirs of functionality than standard libraries.

A Crystal Structure of a Functional RNA Molecule Containing an Artificial Nucleobase Pair.

As one of its goals, synthetic biology seeks to increase the number of building blocks in nucleic acids. While efforts towards this goal are well advanced for DNA, they have hardly begun for RNA. Herein, we present a crystal structure for an RNA riboswitch where a stem C:G pair has been replaced by a pair between two components of an artificially expanded genetic-information system (AEGIS), Z and P, (6-amino-5-nitro-2(1H)-pyridone and 2-amino-imidazo[1,2-a]-1,3,5-triazin-4-(8H)-one). The structure shows that the Z:P pair does not greatly change the conformation of the RNA molecule nor the details of its interaction with a hypoxanthine ligand. This was confirmed in solution by in-line probing, which also measured a 3.7 nM affinity of the riboswitch for guanine. These data show that the Z:P pair mimics the natural Watson-Crick geometry in RNA in the first example of a crystal structure of an RNA molecule that contains an orthogonal added nucleobase pair.

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.

Synthetic Biology Pathway to Nucleoside Triphosphates for Expanded Genetic Alphabets.

One horizon in synthetic biology seeks alternative forms of DNA that store, transcribe, and support the evolution of biological information. Here, hydrogen bond donor and acceptor groups are rearranged within a Watson-Crick geometry to get 12 nucleotides that form 6 independently replicating pairs. Such artificially expanded genetic information systems (AEGIS) support Darwinian evolution in vitro. To move AEGIS into living cells, metabolic pathways are next required to make AEGIS triphosphates economically from their nucleosides, eliminating the need to feed these expensive compounds in growth media. We report that "polyphosphate kinases" can be recruited for such pathways, working with natural diphosphate kinases and engineered nucleoside kinases. This pathway in vitro makes AEGIS triphosphates, including third-generation triphosphates having improved ability to survive in living bacterial cells. In α-32P-labeled forms, produced here for the first time, they were used to study DNA polymerases, finding cases where third-generation AEGIS triphosphates perform better with natural enzymes than second-generation AEGIS triphosphates.

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.

Enzymatic synthesis and nanopore sequencing of 12-letter supernumerary DNA.

The 4-letter DNA alphabet (A, T, G, C) as found in Nature is an elegant, yet non-exhaustive solution to the problem of storage, transfer, and evolution of biological information. Here, we report on strategies for both writing and reading DNA with expanded alphabets composed of up to 12 letters (A, T, G, C, B, S, P, Z, X, K, J, V). For writing, we devise an enzymatic strategy for inserting a singular, orthogonal xenonucleic acid (XNA) base pair into standard DNA sequences using 2'-deoxy-xenonucleoside triphosphates as substrates. Integrating this strategy with combinatorial oligos generated on a chip, we construct libraries containing single XNA bases for parameterizing kmer basecalling models for commercially available nanopore sequencing. These elementary steps are combined to synthesize and sequence DNA containing 12 letters - the upper limit of what is accessible within the electroneutral, canonical base pairing framework. By introducing low-barrier synthesis and sequencing strategies, this work overcomes previous obstacles paving the way for making expanded alphabets widely accessible.

Synthesis of carbohydrates in mineral-guided prebiotic cycles.

One present obstacle to the "RNA-first" model for the origin of life is an inability to generate reasonable "hands off" scenarios for the formation of carbohydrates under conditions where they might have survived for reasonable times once formed. Such scenarios would be especially compelling if they deliver pent(ul)oses, five-carbon sugars found in terran genetics, and exclude other carbohydrates (e.g., aldotetroses) that may also be able to function in genetic systems. Here, we provide detailed chemical analyses of carbohydrate premetabolism, showing how borate, molybdate, and calcium minerals guide the formation of tetroses (C(4)H(8)O(4)), heptoses (C(7)H(14)O(7)), and pentoses (C(5)H(10)O(5)), including the ribose found in RNA, in "hands off" experiments, starting with formaldehyde and glycolaldehyde. These results show that pent(ul)oses would almost certainly have formed as stable borate complexes on the surface of an early Earth beneath a humid CO(2) atmosphere suffering electrical discharge. While aldotetroses form extremely stable complexes with borate, they are not accessible by pathways plausible under the most likely early Earth scenarios. The stabilization by borate is not, however, absolute. Over longer times, material is expected to have passed from borate-bound pent(ul)oses to a branched heptulose, which is susceptible to Cannizzaro reduction to give dead end products. We show how this fate might be avoided using molybdate-catalyzed rearrangement of a branched pentose that is central to borate-moderated cycles that fix carbon from formaldehyde. Our emerging understanding of the nature of the early Earth, including the presence of hydrated rocks undergoing subduction to form felsic magmas in the early Hadean eon, may have made borate and molydate species available to prebiotic chemistry, despite the overall "reduced" state of the planet.

Hachimoji DNA and RNA: A genetic system with eight building blocks.

We report DNA- and RNA-like systems built from eight nucleotide "letters" (hence the name "hachimoji") that form four orthogonal pairs. These synthetic systems meet the structural requirements needed to support Darwinian evolution, including a polyelectrolyte backbone, predictable thermodynamic stability, and stereoregular building blocks that fit a Schrödinger aperiodic crystal. Measured thermodynamic parameters predict the stability of hachimoji duplexes, allowing hachimoji DNA to increase the information density of natural terran DNA. Three crystal structures show that the synthetic building blocks do not perturb the aperiodic crystal seen in the DNA double helix. Hachimoji DNA was then transcribed to give hachimoji RNA in the form of a functioning fluorescent hachimoji aptamer. These results expand the scope of molecular structures that might support life, including life throughout the cosmos.

Synthesis of pyrophosphates for in vitro selection of catalytic RNA molecules.

Reported here are synthetic routes to pyrophosphates linking riboflavin with various nucleosides. The focus is on a flavin-uracil dinucleotide having a biotin tag on the uracil, a molecule that has potential value in the selection of RNA enzymes that catalyze the template-directed polymerization of RNA in the 3'-to-5' direction, which is the direction opposite that catalyzed by standard protein polymerases. Two detailed procedures are presented to prepare this new compound, as well as one procedure to prepare the new flavin-2,6-diaminopurine dinucleotide.

Structure and Biophysics for a Six Letter DNA Alphabet that Includes Imidazo[1,2-a]-1,3,5-triazine-2(8H)-4(3H)-dione (X) and 2,4-Diaminopyrimidine (K).

A goal of synthetic biology is to develop new nucleobases that retain the desirable properties of natural nucleobases at the same time as expanding the genetic alphabet. The nonstandard Watson-Crick pair between imidazo[1,2-a]-1,3,5-triazine-2(8H)-4(3H)-dione (X) and 2,4-diaminopyrimidine (K) does exactly this, pairing via complementary arrangements of hydrogen bonding in these two nucleobases, which do not complement any natural nucleobase. Here, we report the crystal structure of a duplex DNA oligonucleotide in B-form including two consecutive X:K pairs in GATCXK DNA determined as a host-guest complex at 1.75 Å resolution. X:K pairs have significant propeller twist angles, similar to those observed for A:T pairs, and a calculated hydrogen bonding pairing energy that is weaker than that of A:T. Thus, although inclusion of X:K pairs results in a duplex DNA structure that is globally similar to that of an analogous G:C structure, the X:K pairs locally and energetically more closely resemble A:T pairs.

Expanded Genetic Alphabets: Managing Nucleotides That Lack Tautomeric, Protonated, or Deprotonated Versions Complementary to Natural Nucleotides.

2,4-Diaminopyrimidine (trivially K) and imidazo[1,2-a]-1,3,5-triazine-2(8H)-4(3H)-dione (trivially X) form a nucleobase pair with Watson-Crick geometry as part of an artificially expanded genetic information system (AEGIS). Neither K nor X can form a Watson-Crick pair with any natural nucleobase. Further, neither K nor X has an accessible tautomeric form or a protonated/deprotonated state that can form a Watson-Crick pair with any natural nucleobase. In vitro experiments show how DNA polymerase I from E. coli manages replication of DNA templates with one K:X pair, but fails with templates containing two adjacent K:X pairs. In analogous in vivo experiments, E. coli lacking dKTP/dXTP cannot rescue chloramphenicol resistance from a plasmid containing two adjacent K:X pairs. These studies identify bacteria able to serve as selection environments for engineering cells that replicate AEGIS pairs that lack forms that are Watson-Crick complementary to any natural nucleobase.

Biophysics of Artificially Expanded Genetic Information Systems. Thermodynamics of DNA Duplexes Containing Matches and Mismatches Involving 2-Amino-3-nitropyridin-6-one (Z) and Imidazo[1,2-a]-1,3,5-triazin-4(8H)one (P).

Synthetic nucleobases presenting non-Watson-Crick arrangements of hydrogen bond donor and acceptor groups can form additional nucleotide pairs that stabilize duplex DNA independent of the standard A:T and G:C pairs. The pair between 2-amino-3-nitropyridin-6-one 2'-deoxyriboside (presenting a {donor-donor-acceptor} hydrogen bonding pattern on the Watson-Crick face of the small component, trivially designated Z) and imidazo[1,2-a]-1,3,5-triazin-4(8H)one 2'-deoxyriboside (presenting an {acceptor-acceptor-donor} hydrogen bonding pattern on the large component, trivially designated P) is one of these extra pairs for which a substantial amount of molecular biology has been developed. Here, we report the results of UV absorbance melting measurements and determine the energetics of binding of DNA strands containing Z and P to give short duplexes containing Z:P pairs as well as various mismatches comprising Z and P. All measurements were done at 1 M NaCl in buffer (10 mM Na cacodylate, 0.5 mM EDTA, pH 7.0). Thermodynamic parameters (ΔH°, ΔS°, and ΔG°37) for oligonucleotide hybridization were extracted. Consistent with the Watson-Crick model that considers both geometric and hydrogen bonding complementarity, the Z:P pair was found to contribute more to duplex stability than any mismatches involving either nonstandard nucleotide. Further, the Z:P pair is more stable than a C:G pair. The Z:G pair was found to be the most stable mismatch, forming either a deprotonated mismatched pair or a wobble base pair analogous to the stable T:G mismatch. The C:P pair is less stable, perhaps analogous to the wobble pair observed for C:O6-methyl-G, in which the pyrimidine is displaced into the minor groove. The Z:A and T:P mismatches are much less stable. Parameters for predicting the thermodynamics of oligonucleotides containing Z and P bases are provided. This represents the first case where this has been done for a synthetic genetic system.

A Single Deoxynucleoside Kinase Variant from Drosophila melanogaster Synthesizes Monophosphates of Nucleosides That Are Components of an Expanded Genetic System.

Deoxynucleoside kinase from D. melanogaster (DmdNK) has broad specificity; although it catalyzes the phosphorylation of natural pyrimidine more efficiently than natural purine nucleosides, it accepts all four 2'-deoxynucleosides and many analogues, using ATP as a phosphate donor to give the corresponding deoxynucleoside monophosphates. Here, we show that replacing a single amino acid (glutamine 81 by glutamate) in DmdNK creates a variant that also catalyzes the phosphorylation of nucleosides that form part of an artificially expanded genetic information system (AEGIS). By shuffling hydrogen bonding groups on the nucleobases, AEGIS adds potentially as many as four additional nucleobase pairs to the genetic "alphabet". Specifically, we show that DmdNK Q81E creates the monophosphates from the AEGIS nucleosides dP, dZ, dX, and dK (respectively 2-amino-8-(1'-ß-d-2'-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one, dP; 6-amino-3-(1'-ß-d-2'-deoxyribofuranosyl)-5-nitro-1H-pyridin-2-one, dZ; 8-(1'ß-d-2'-deoxy-ribofuranosyl)imidazo[1,2-a]-1,3,5-triazine-2(8H)-4(3H)-dione, dX; and 2,4-diamino-5-(1'-ß-d-2'-deoxyribofuranosyl)-pyrimidine, dK). Using a coupled enzyme assay, in vitro kinetic parameters were obtained for three of these nucleosides (dP, dX, and dK; the UV absorbance of dZ made it impossible to get its precise kinetic parameters). Thus, DmdNK Q81E appears to be a suitable enzyme to catalyze the first step in the biosynthesis of AEGIS 2'-deoxynucleoside triphosphates in vitro and, perhaps, in vivo, in a cell able to manage plasmids containing AEGIS DNA.

Orthogonal activation of the reengineered A3 adenosine receptor (neoceptor) using tailored nucleoside agonists.

An alternative approach to overcome the inherent lack of specificity of conventional agonist therapy can be the reengineering of the GPCRs and their agonists. A reengineered receptor (neoceptor) could be selectively activated by a modified agonist, but not by the endogenous agonist. Assisted by rhodopsin-based molecular modeling, we pinpointed mutations of the A(3) adenosine receptor (AR) for selective affinity enhancement following complementary modifications of adenosine. Ribose modifications examined included, at 3': amino, aminomethyl, azido, guanidino, ureido; and at 5': uronamido, azidodeoxy. N(6)-Variations included 3-iodobenzyl, 5-chloro-2-methyloxybenzyl, and methyl. An N(6)-3-iodobenzyl-3'-ureido adenosine derivative 10 activated phospholipase C in COS-7 cells (EC(50) = 0.18 microM) or phospholipase D in chick primary cardiomyocytes, both mediated by a mutant (H272E), but not the wild-type, A(3)AR. The affinity enhancements for 10 and the corresponding 3'-acetamidomethyl analogue 6 were >100-fold and >20-fold, respectively. 10 concentration-dependently protected cardiomyocytes transfected with the neoceptor against hypoxia. Unlike 10, adenosine activated the wild-type A(3)AR (EC(50) of 1.0 microM), but had no effect on the H272E mutant A(3)AR (100 microM). Compound 10 was inactive at human A(1), A(2A), and A(2B)ARs. The orthogonal pair comprising an engineered receptor and a modified agonist should be useful for elucidating signaling pathways and could be therapeutically applied to diseases following organ-targeted delivery of the neoceptor gene.

Assays To Detect the Formation of Triphosphates of Unnatural Nucleotides: Application to Escherichia coli Nucleoside Diphosphate Kinase.

One frontier in synthetic biology seeks to move artificially expanded genetic information systems (AEGIS) into natural living cells and to arrange the metabolism of those cells to allow them to replicate plasmids built from these unnatural genetic systems. In addition to requiring polymerases that replicate AEGIS oligonucleotides, such cells require metabolic pathways that biosynthesize the triphosphates of AEGIS nucleosides, the substrates for those polymerases. Such pathways generally require nucleoside and nucleotide kinases to phosphorylate AEGIS nucleosides and nucleotides on the path to these triphosphates. Thus, constructing such pathways focuses on engineering natural nucleoside and nucleotide kinases, which often do not accept the unnatural AEGIS biosynthetic intermediates. This, in turn, requires assays that allow the enzyme engineer to follow the kinase reaction, assays that are easily confused by ATPase and other spurious activities that might arise through "site-directed damage" of the natural kinases being engineered. This article introduces three assays that can detect the formation of both natural and unnatural deoxyribonucleoside triphosphates, assessing their value as polymerase substrates at the same time as monitoring the progress of kinase engineering. Here, we focus on two complementary AEGIS nucleoside diphosphates, 6-amino-5-nitro-3-(1'-ß-D-2'-deoxyribofuranosyl)-2(1H)-pyridone and 2-amino-8-(1'-ß-D-2'-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one. These assays provide new ways to detect the formation of unnatural deoxyribonucleoside triphosphates in vitro and to confirm their incorporation into DNA. Thus, these assays can be used with other unnatural nucleotides.

Synthesis of 3'-deoxy-3'-C-hydroxymethyl analogues of tiazofurin and ribavirin.

On the basis of potent biological activity of 3'-branched-3'-deoxynucleoside analogues, novel ribavirin and tiazofurin derivatives with 3'-C-hydroxymethyl substituent were synthesized, starting from D-xylose.

Synthesis of homo-N-nucleoside with 1,2,4-triazole-3-carboxamide.

On the basis of potent biological activities of ribavirin and homo-N-nucleosides, a novel homo-N-1,2,4-triazole-3-carboxamide derivative 1 was synthesized starting from 2,3,5-tri-O-benzoylribofuranosyl-1-acetate as a potential antiviral agent.

Synthesis of 3'-ureidoadenosine analogues and their binding affinity to the A3 adenosine receptor.

Novel 3'-ureidoadenosine analogues were synthesized from 1,2:5, 6-di-O-isopropylidene-D-glucose in order to lead to stronger hydrogen bonding at the A3 adenosine receptor than the corresponding 3'-aminoadenosine derivatives. However, all synthesized 3'-ureidoadenosine analogues have lost their binding affinities to the all subtypes of adenosine receptors, indicating that bulky 3'-urea moiety led to conformational distortion.