Ice as a medium for RNA-catalysed RNA synthesis and evolution
A critical event in the origin of life is thought to have been the emergence of a molecule capable of self-replication and evolution. According to the RNA World hypothesis, this could have been an RNA polymerase ribozyme capable of generating copies of itself from simple nucleotide precursors. In vitro evolution experiments have provided modern examples of such ribozymes, such as the R18 RNA polymerase ribozyme, exhibiting basic levels of this crucial catalytic activity; R18’s activity, however, falls far short of that required of an RNA replicase, leaving unanswered the question of whether RNA can catalyse its self-replication. This thesis describes the development and use of a novel in vitro selection system, Compartmentalised Bead-Tagging (CBT), to isolate variants of the R18 ribozyme with improved sequence generality and extension capabilities. CBT evolution and engineering of polymerase ribozymes, together with RNA template evolution, allowed the synthesis of RNA molecules over 100 nucleotides long, as well as the RNA-catalysed transcription of a catalytic hammerhead ribozyme. This demonstrates the catalytic capabilities of ribozyme polymerases. The R18 ribozyme was also exploited as an analogue of a primordial replicase, to determine replicase behaviour in different reaction environments. Substantial ribozyme polymerisation occurred at −7˚C in the liquid eutectic phase of water-ice; increased ribozyme stability at these low temperatures allowed longer extension products to be generated than at ambient temperatures. The concentration effect of eutectic phase formation could also yield RNA synthesis from dilute solutions of substrates, and provide quasicellular compartmentalisation of ribozymes. These beneficial physicochemical features of ice make it a potential protocellular medium for the emergence of primordial replicases. Ice also could serve as a medium for CBT, allowing the isolation of a polymerase ribozyme adapted to the low temperatures in the ice phase, demonstrating the primordial potential and modern feasibility of ribozyme evolution in ice.
RNA, Ribozyme, Origin of Life, RNA world, Ice
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