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The RNA world hypothesis regarding the early evolution of life relies on the premise that some RNA sequences can catalyze RNA replication. In support of this conjecture, we describe here an RNA molecule that catalyzes the type of polymerization needed for RNA replication. The ribozyme uses nucleoside triphosphates and the coding information of an RNA template to extend an RNA primer by the successive addition of up to 14 nucleotides-more than a complete turn of an RNA helix. Its polymerization activity is general in terms of the sequence and the length of the primer and template RNAs, provided that the 3' terminus of the primer pairs with the template. Its polymerization is also quite accurate: when primers extended by 11 nucleotides were cloned and sequenced, 1088 of 1100 sequenced nucleotides matched the template.
The RNA world hypothesis states that early life forms lacked protein enzymes and depended instead on enzymes composed of RNA (1). Much of the appeal of this hypothesis comes from the realization that ribozymes would have been far easier to duplicate than proteinaceous enzymes (2-5). Whereas coded protein replication requires numerous macromolecular components [including mRNA, transfer RNAs (tRNAs), aminoacyl-tRNA synthetases, and the ribosome], replication of a ribozyme requires only a single macromolecular activity: an RNA-dependent RNA polymerase that synthesizes first a complement, and then a copy of the ribozyme. If this RNA polymerase were itself a ribozyme, then a simple ensemble of molecules might be capable of self-replication and eventually, in the course of evolution, give rise to the protein-nucleic acid world of contemporary biology. Finding a ribozyme that can efficiently catalyze general RNA polymerization would support the idea of the RNA world (1, 6, 7) and would provide a key component for the laboratory synthesis of minimal life forms based on RNA (8, 9).
Although progress has been made in finding ribozymes capable of template-directed RNA synthesis, none of these ribozymes has the sophisticated substrate-binding properties needed for general polymerization (7). Derivatives of self-splicing introns are able to join oligonucleotides assembled on a template (10-12). However, the templates that can be copied are limited to those that match the oligonucleotide substrates, and it has not been possible to include sufficient concentrations of all the oligonucleotide substrates needed for a general...