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The Watson−Crick-Franklin (WCF) rules describing nucleobase pairing in antiparallel strands of DNA and RNA can be exploited to create artificially expanded genetic information systems (AEGIS) with as many as 12 independently replicable nucleotides joined by six hydrogen bond pairing schemes. One of these additional pairs joins two nucleotides trivially designated as Z (6-amino-5-nitro-(1H)-pyridin-2-one) and P (2-amino-imidazo-[1,2-a]-1,3,5-triazin-(8H)-4-one). The Z:P pair has supported 6- nucleotide PCR to give diagnostics products, in environmental surveillance kits, and for laboratory in vitro evolution (LIVE) that has generated, inter alia, molecules that inactivate toxins, antibody analogs that bind cancer cells, therapeutic candidates that deliver drugs to those cells, reagents to identify targets on those cells’ surfaces, reagents to move cargoes across the blood−brain barrier, and catalysts with ribonuclease activity. However, the Z nucleoside is acidic, with a pKa of ∼7.8. In its deprotonated form, Z− forms a WCF pair with G. This leads to the slow replacement of Z:P pairs by C:G pairs during PCR or, in the reverse process, their introduction. Here, we examine analogs of Z that retain the same donor:donor:acceptor hydrogen bonding pattern as earlier generations of the Z heterocycle, still form a WCF pair with P, but have a higher pKa. Experiments with Taq polymerase show that the rate of loss of Z:P pairs decreases markedly as the pKa of the Z heterocycle increases. This provides direct support for the hypothesis that Z:P pairs are in fact lost via deprotonated Z−:G mismatches. Further, it provides a Z:P system that can be replicated with very high fidelity, with >97% retention of the Z:P pairs over 10,000-fold amplification. 

Not AvailabRibose is the defining sugar in ribonucleic acid (RNA), which is often proposed to have carried the genetic information and catalyzed the biological reactions of the first life on Earth. Thus, abiological processes that yield ribose under prebiotic conditions have been studied for decades. However, aqueous environments required for the formation of ribose from materials available in quantity under geologically reasonable models, where the ribose formed is not immediately destroyed, remain unclear. This is due in large part to the challenge of analysis of carbohydrates formed under a wide range of aqueous conditions. Thus, the formation of ribose on prebiotic Earth has sometimes been questioned. We investigated the quantitative effects of pH, temperature, cation, and the concentrations of formaldehyde and glycolaldehyde on the synthesis of diverse sugars, including ribose. The results suggest a range of conditions that produce ribose and that ribose could have formed in constrained aquifers on prebiotic Earth.le 

Chemists have now synthesized new kinds of DNA that add nucleotides to the four standard nucleotides (guanine, adenine, cytosine, and thymine) found in standard Terran DNA. Such “artificially expanded genetic information systems” are today used in molecular diagnostics; to support directed evolution to create medically useful receptors, ligands, and catalysts; and to explore issues related to the early evolution of life. Further applications are limited by the inability to directly sequence DNA containing nonstandard nucleotides. Nanopore sequencing is well-suited for this purpose, as it does not require enzymatic synthesis, amplification, or nucleotide modification. Here, we take the first steps to realize nanopore sequencing of an 8-letter “hachimoji” expanded DNA alphabet by assessing its nanopore signal range using the MspA (Mycobacterium smegmatis porin A) nanopore. We find that hachimoji DNA exhibits a broader signal range in nanopore sequencing than standard DNA alone and that hachimoji single-base substitutions are distinguishable with high confidence. Because nanopore sequencing relies on a molecular motor to control the motion of DNA, we then assessed the compatibility of the Hel308 motor enzyme with nonstandard nucleotides by tracking the translocation of single Hel308 molecules along hachimoji DNA, monitoring the enzyme kinetics and premature enzyme dissociation from the DNA. We find that Hel308 is compatible with hachimoji DNA but dissociates more frequently when walking over C-glycoside nucleosides, compared to N-glycosides. C-glycocide nucleosides passing a particular site within Hel308 induce a higher likelihood of dissociation. This highlights the need to optimize nanopore sequencing motors to handle different glycosidic bonds. It may also inform designs of future alternative DNA systems that can be sequenced with existing motors and pores.