4 Towards non-enzymatic RNA replication

Abstract

The direct path from prebiotic chemistry to the RNA World requires a plausible route for the synthesis of activated ribonucleotides and RNA templates, along with a means for the complete replication of potentially useful RNA sequences. However, many apparent roadblocks make non-enzymatic RNA replication look quite difficult, if not impossible. These problems include the slow rate, low accuracy and poor regioselectivity of non-enzymatic template copying, the hydrolysis of activated monomers and absence of good re-activation chemistry, the difficulty of strand separation after template copying, the rapidity of strand reannealing, the absence of primers in any realistic replication scenario, and the apparent incompatibly of RNA copying chemistry (which requires a high Mg2+ concentration) with fatty acid-based protocell membranes, which are destroyed by low Mg2+ concentrations. I will discuss recent progress from my laboratory on four of these issues. We have found that functional RNAs such as aptamers and ribozymes can tolerate moderate levels of 2′–5′ linkages without great loss of activity. It therefore appears that the presence of 10–25% of such linkages in the products of non-enzymatic copying would not prevent the evolution of functional RNAs. Furthermore, 2′–5′ linkages can be helpful, as they decrease the melting temperature of RNA duplexes enough to allow strand separation to occur under geophysically plausible conditions. Recently, we have found that small chemical changes to the nucleobases can greatly increase the fidelity of non-enzymatic template copying, and we have found conditions that render RNA copying chemistry compatible with vesicle integrity, thereby allowing RNA copying to occur inside fatty acid-based model protocell membranes. I will discuss potential approaches to solving the remaining issues that stand in the way of complete RNA replication. If all of the problems with RNA replication can be overcome, it should be possible to construct functioning protocells in the laboratory.