5 Re-creating an RNA Replicase

The idea that certain RNA sequences can use the information from one RNA strand to accurately and efficiently produce another RNA is central to the RNA World hypothesis (Pace and Marsh 1985; Sharp 1985; Cech 1986; Orgel 1986). Such RNA-dependent RNA polymerase ribozymes would have been responsible for replicating the ribozymes of the RNA World, including themselves (via their complement sequences). The most extreme versions of the RNA World hypothesis suggest that life began with a self-replicating system seeded by a pair of replicase molecules, one serving as the template, the other as the enzyme. Less extreme versions of the RNA World hypothesis cite difficulties with prebiotic synthesis and stability of RNA on the early Earth and instead postulate that life originated in a “pre-RNA world”—an era dominated by an RNA-like polymer that could catalyze reactions and code for its own replication (Chapter 2). Because the pre-RNA polymer presumably facilitated production of RNA, issues regarding prebiotic availability of RNA are less problematic in this conservative RNA World view. However, the scenario calls for eventual transition to the RNA World, and thus still relies on some radical assumptions regarding the intrinsic catalytic capability of RNA—including the idea that there exist, somewhere among all RNA sequence possibilities, sequences that can promote the replicase reaction. The RNA replicase is one of the few assumptions common to all versions of the RNA World hypothesis. Because nearly four billion years of evolution have obscured any vestiges of the presumed RNA replicases, our only...