I am a postdoctoral researcher working on the chemical origin of life at the MRC-LMB (Cambridge, UK)
Non-enzymatic recycling for the degradation and repair of unnatural RNA on the early Earth
Researchers from all over the world are currently involved in finding an answer to the big question of the origin of life. From a chemistry perspective, the best clues can be found in the intrinsic reactivity of the molecules that were present on the early Earth and current investigations are focused on understanding how these molecules combined to form the building blocks of modern biology.
RNA is a central molecule in life, able to both store genetic information and catalyze biological reactions. Understanding how RNA arose and evolved in a prebiotic context is one of the major challenges of the origin of life research.
In biology, RNA comprises building blocks joined by 3′,5′-linkages, but unnatural 2′,5′-linkages are also chemically possible and ribonucleic acid containing both types of linkages is the expected product of prebiotic chemistry on the early Earth. How backbone heterogeneous ribonucleic acid became modern RNA is thus central to the origin of life question. We designed and experimentally demonstrate a model in which unnatural linkages can be converted to the natural linkages of RNA by a non-enzymatic process of degradation and repair, defining a plausible recycling scenario for the prebiotic evolution of ribonucleic acid.
Abstract: Non-enzymatic oligomerization of activated ribonucleotides leads to ribonucleic acids that contain a mixture of 2',5'- and 3',5'-linkages, and overcoming this backbone heterogeneity has long been considered a major limitation to the prebiotic emergence of RNA. Herein, we demonstrate non-enzymatic chemistry that progressively converts 2',5'-linkages into 3',5'-linkages through iterative degradation and repair. The energetic costs of this proofreading are met by the hydrolytic turnover of a phosphate activating agent and an acylating agent. With multiple rounds of this energy-dissipative recycling, we show that all-3',5'-linked duplex RNA can emerge from a backbone heterogeneous mixture, thereby delineating a route that could have driven RNA evolution on the early earth.
Pub.: 04 May '17, Pinned: 22 Jun '17
Abstract: The recent synthesis of pyrimidine ribonucleoside-2',3'-cyclic phosphates under prebiotically plausible conditions has strengthened the case for the involvement of ribonucleic acid (RNA) at an early stage in the origin of life. However, a prebiotic conversion of these weakly activated monomers, and their purine counterparts, to the 3',5'-linked RNA polymers of extant biochemistry has been lacking (previous attempts led only to short oligomers with mixed linkages). Here we show that the 2'-hydroxyl group of oligoribonucleotide-3'-phosphates can be chemoselectively acetylated in water under prebiotically credible conditions, which allows rapid and efficient template-directed ligation. The 2'-O-acetyl group at the ligation junction of the product RNA strand can be removed under conditions that leave the internucleotide bonds intact. Remarkably, acetylation of mixed oligomers that possess either 2'- or 3'-terminal phosphates is selective for the 2'-hydroxyl group of the latter. This newly discovered chemistry thus suggests a prebiotic route from ribonucleoside-2',3'-cyclic phosphates to predominantly 3',5'-linked RNA via partially 2'-O-acetylated RNA.
Pub.: 24 Apr '13, Pinned: 22 Jun '17
Abstract: A minimal cell can be thought of as comprising informational, compartment-forming and metabolic subsystems. To imagine the abiotic assembly of such an overall system, however, places great demands on hypothetical prebiotic chemistry. The perceived differences and incompatibilities between these subsystems have led to the widely held assumption that one or other subsystem must have preceded the others. Here we experimentally investigate the validity of this assumption by examining the assembly of various biomolecular building blocks from prebiotically plausible intermediates and one-carbon feedstock molecules. We show that precursors of ribonucleotides, amino acids and lipids can all be derived by the reductive homologation of hydrogen cyanide and some of its derivatives, and thus that all the cellular subsystems could have arisen simultaneously through common chemistry. The key reaction steps are driven by ultraviolet light, use hydrogen sulfide as the reductant and can be accelerated by Cu(I)-Cu(II) photoredox cycling.
Pub.: 25 Mar '15, Pinned: 22 Jun '17
Abstract: A plausible process for non-enzymatic RNA replication would greatly simplify models of the transition from prebiotic chemistry to simple biology. However, all known conditions for the chemical copying of an RNA template result in the synthesis of a complementary strand that contains a mixture of 2'-5' and 3'-5' linkages, rather than the selective synthesis of only 3'-5' linkages as found in contemporary RNA. Here we show that such backbone heterogeneity is compatible with RNA folding into defined three-dimensional structures that retain molecular recognition and catalytic properties and, therefore, would not prevent the evolution of functional RNAs such as ribozymes. Moreover, the same backbone heterogeneity lowers the melting temperature of RNA duplexes that would otherwise be too stable for thermal strand separation. By allowing copied strands to dissociate, this heterogeneity may have been one of the essential features that allowed RNA to emerge as the first biopolymer.
Pub.: 24 Apr '13, Pinned: 30 Jun '17
Abstract: Multiple lines of evidence support the hypothesis that the early evolution of life was dominated by RNA, which can both transfer information from generation to generation through replication directed by base-pairing, and carry out biochemical activities by folding into functional structures. To understand how life emerged from prebiotic chemistry we must therefore explain the steps that led to the emergence of the RNA world, and in particular, the synthesis of RNA. The generation of pools of highly pure ribonucleotides on the early Earth seems unlikely, but the presence of alternative nucleotides would support the assembly of nucleic acid polymers containing nonheritable backbone heterogeneity. We suggest that homogeneous monomers might not have been necessary if populations of heterogeneous nucleic acid molecules could evolve reproducible function. For such evolution to be possible, function would have to be maintained despite the repeated scrambling of backbone chemistry from generation to generation. We have tested this possibility in a simplified model system, by using a T7 RNA polymerase variant capable of transcribing nucleic acids that contain an approximately 11 mixture of deoxy- and ribonucleotides. We readily isolated nucleotide-binding aptamers by utilizing an in vitro selection process that shuffles the order of deoxy- and ribonucleotides in each round. We describe two such RNA/DNA mosaic nucleic acid aptamers that specifically bind ATP and GTP, respectively. We conclude that nonheritable variations in nucleic acid backbone structure may not have posed an insurmountable barrier to the emergence of functionality in early nucleic acids.
Pub.: 10 Aug '11, Pinned: 30 Jun '17
Abstract: Either to sustain autotrophy, or as a prelude to heterotrophy, organic synthesis from an environmentally available C1 feedstock molecule is crucial to the origin of life. Recent findings augment key literature results and suggest that hydrogen cyanide—“Blausäure”—was that feedstock.
Pub.: 29 Oct '15, Pinned: 25 Jun '17
Abstract: We have recently shown that 2-aminoimidazole is a superior nucleotide activating group for nonenzymatic RNA copying. Here we describe a prebiotic synthesis of 2-aminoimidazole that shares a common mechanistic pathway with that of 2-aminooxazole, a previously described key intermediate in prebiotic nucleotide synthesis. In the presence of glycolaldehyde, cyanamide, phosphate and ammonium ion, both 2-aminoimidazole and 2-aminooxazole are produced, with higher concentrations of ammonium ion and acidic pH favoring the former. Given a 1:1 mixture of 2-aminoimidazole and 2-aminooxazole, glyceraldehyde preferentially reacts and cyclizes with the latter, forming a mixture of pentose aminooxazolines, and leaving free 2-aminoimidazole available for nucleotide activation. The common synthetic origin of 2-aminoimidazole and 2-aminooxazole and their distinct reactivities are suggestive of a reaction network that could lead to both the synthesis of RNA monomers and to their subsequent chemical activation.
Pub.: 24 Jun '17, Pinned: 30 Jun '17
Abstract: At some stage in the origin of life, an informational polymer must have arisen by purely chemical means. According to one version of the 'RNA world' hypothesis this polymer was RNA, but attempts to provide experimental support for this have failed. In particular, although there has been some success demonstrating that 'activated' ribonucleotides can polymerize to form RNA, it is far from obvious how such ribonucleotides could have formed from their constituent parts (ribose and nucleobases). Ribose is difficult to form selectively, and the addition of nucleobases to ribose is inefficient in the case of purines and does not occur at all in the case of the canonical pyrimidines. Here we show that activated pyrimidine ribonucleotides can be formed in a short sequence that bypasses free ribose and the nucleobases, and instead proceeds through arabinose amino-oxazoline and anhydronucleoside intermediates. The starting materials for the synthesis-cyanamide, cyanoacetylene, glycolaldehyde, glyceraldehyde and inorganic phosphate-are plausible prebiotic feedstock molecules, and the conditions of the synthesis are consistent with potential early-Earth geochemical models. Although inorganic phosphate is only incorporated into the nucleotides at a late stage of the sequence, its presence from the start is essential as it controls three reactions in the earlier stages by acting as a general acid/base catalyst, a nucleophilic catalyst, a pH buffer and a chemical buffer. For prebiotic reaction sequences, our results highlight the importance of working with mixed chemical systems in which reactants for a particular reaction step can also control other steps.
Pub.: 16 May '09, Pinned: 22 Jun '17
Abstract: A sudden transition in a system from an inanimate state to the living state-defined on the basis of present day living organisms-would constitute a highly unlikely event hardly predictable from physical laws. From this uncontroversial idea, a self-consistent representation of the origin of life process is built up, which is based on the possibility of a series of intermediate stages. This approach requires a particular kind of stability for these stages-dynamic kinetic stability (DKS)-which is not usually observed in regular chemistry, and which is reflected in the persistence of entities capable of self-reproduction. The necessary connection of this kinetic behaviour with far-from-equilibrium thermodynamic conditions is emphasized and this leads to an evolutionary view for the origin of life in which multiplying entities must be associated with the dissipation of free energy. Any kind of entity involved in this process has to pay the energetic cost of irreversibility, but, by doing so, the contingent emergence of new functions is made feasible. The consequences of these views on the studies of processes by which life can emerge are inferred.
Pub.: 08 Nov '13, Pinned: 22 Jun '17
Abstract: Understanding prebiotic nucleotide synthesis is a long standing challenge thought to be essential to elucidating the origins of life on Earth. Recently, remarkable progress has been made, but to date all proposed syntheses account separately for the pyrimidine and purine ribonucleotides; no divergent synthesis from common precursors has been proposed. Moreover, the prebiotic syntheses of pyrimidine and purine nucleotides that have been demonstrated operate under mutually incompatible conditions. Here, we tackle this mutual incompatibility by recognizing that the 8-oxo-purines share an underlying generational parity with the pyrimidine nucleotides. We present a divergent synthesis of pyrimidine and 8-oxo-purine nucleotides starting from a common prebiotic precursor that yields the β-ribo-stereochemistry found in the sugar phosphate backbone of biological nucleic acids. The generational relationship between pyrimidine and 8-oxo-purine nucleotides suggests that 8-oxo-purine ribonucleotides may have played a key role in primordial nucleic acids prior to the emergence of the canonical nucleotides of biology.
Pub.: 20 May '17, Pinned: 30 Jun '17
Abstract: The replication of nucleic acids is central to the origin of life. On the early Earth, suitable non-equilibrium boundary conditions would have been required to surmount the effects of thermodynamic equilibrium such as the dilution and degradation of oligonucleotides. One particularly intractable experimental finding is that short genetic polymers replicate faster and outcompete longer ones, which leads to ever shorter sequences and the loss of genetic information. Here we show that a heat flux across an open pore in submerged rock concentrates replicating oligonucleotides from a constant feeding flow and selects for longer strands. Our experiments utilize the interplay of molecular thermophoresis and laminar convection, the latter driving strand separation and exponential replication. Strands of 75 nucleotides survive whereas strands half as long die out, which inverts the above dilemma of the survival of the shortest. The combined feeding, thermal cycling and positive length selection opens the door for a stable molecular evolution in the long-term microhabitat of heated porous rock.
Pub.: 24 Feb '15, Pinned: 22 Jun '17
Abstract: Fuel-driven self-assembly of actin filaments and microtubules is a key component of cellular organization. Continuous energy supply maintains these transient biomolecular assemblies far from thermodynamic equilibrium, unlike typical synthetic systems that spontaneously assemble at thermodynamic equilibrium. Here, we report the transient self-assembly of synthetic molecules into active materials, driven by the consumption of a chemical fuel. In these materials, reaction rates and fuel levels, instead of equilibrium composition, determine properties such as lifetime, stiffness, and self-regeneration capability. Fibers exhibit strongly nonlinear behavior including stochastic collapse and simultaneous growth and shrinkage, reminiscent of microtubule dynamics.
Pub.: 05 Sep '15, Pinned: 22 Jun '17
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