A Mechanistic Explanation for the Regioselectivity of Nonenzymatic RNA Primer Extension

A working model of nonenzymatic RNA primer extension could illuminate how prebiotic chemistry transitioned to biology. All currently known experimental reconstructions of nonenzymatic RNA primer extension yield a mixture of 2′-5′ and 3′-5′ internucleotide linkages. Although long seen as a major problem, the causes of the poor regioselectivity of the reaction are unknown. We used a combination of different leaving groups, nucleobases, and templating sequences to uncover the factors that yield selective formation of 3′-5′ internucleotide linkages. We found that fast and high yielding reactions selectively form 3′-5′ linkages. Additionally, in all cases with high 3′-5′ regioselectivity, Watson–Crick base pairing between the RNA monomers and the template is observed at the extension site and at the adjacent downstream position. Mismatched base-pairs and other factors that would perturb the geometry of the imidazolium bridged intermediate lower both the rate and regioselectivity of the reaction

Common and Potentially Prebiotic Origin for Precursors of Nucleotide Synthesis and Activation

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