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The mechanism of nonenzymatic template copying with imidazole-activated nucleotides.

The emergence of a chemical pathway for the replication of RNA oligonucleotides was a critical step in the origin of life. An important laboratory model for the study of nonenzymatic template copying, which would be a key part of any such pathway, involves the reaction of ribonucleoside-5'-phosphorimidazolides with an RNA primer/template complex. The chemical mechanism of the reaction by which the primer becomes extended by one nucleotide was long assumed to be a classical in-line nucleophilic substitution reaction in which the 3'-hydroxyl of the primer attacks the phosphate of the incoming activated monomer with displacement of the imidazole leaving group. Surprisingly, this simple model has turned out to be incorrect, and the dominant pathway has now been shown to involve the reaction of two activated nucleotides with each other to form a 5'-5'-imidazolium bridged dinucleotide intermediate. Here we review the discovery of this unexpected intermediate, and the chemical, kinetic and structural evidence for its role in template copying chemistry.