RNA-Catalyzed Polymerization and Replication of RNA
Gerald F. Joyce
The Scripps Research Institute
According to the RNA world hypothesis, ancestors of extant life stored and expressed genetic information in RNA molecules that were replicated by an RNA polymerase ribozyme. In an effort to reconstruct RNA-based life, directed evolution was used to improve the activity and generality of an RNA polymerase ribozyme by selecting for variants that can synthesize functional RNA molecules from an RNA template. The resulting polymerase can synthesize a variety of complex structured RNAs, including aptamers, ribozymes, and tRNA. Furthermore, the polymerase can be used to replicate and amplify short RNA templates in an RNA-catalyzed form of polymerase chain reaction (riboPCR). Directed evolution also was used to obtain a cross-chiral ribozyme that catalyzes the RNA-templated ligation and polymerization of RNAs of the opposite handedness. This ribozyme is able to synthesize its own enantiomer by joining multiple component oligonucleotides. Recently, an optimized form of the cross-chiral ribozyme was obtained that can operate on a broad range of template sequences, a key attribute on the path toward achieving the cross-chiral replication of RNA. The homochiral and heterochiral polymerization systems offer two approaches for the synthesis and replication of functional RNAs, prerequisites for the realization of RNA-based life.