3 Speculations on the Origin of Ribosomal Translocation

Abstract

The discovery of catalytic RNA suggested a way out of a troublesome problem concerning the origin of life (Gilbert 1986). Given the known roles of the various biological informational molecules, it had been unclear how a genetic system could ever have gotten started. The replication of a nucleic acid would require a proteinaceous polymerase, but without replication, nothing, protein synthesis included, could evolve. Put another way, there was no molecule known that could both serve as a template for replication and express its information without some sort of translation, in the loose sense of the word. The discovery of modern catalytic RNA demonstrated in dramatic fashion that RNA could be such a molecule. The first nucleic acid polymerase, it is hypothesized, was an RNA molecule that catalyzed its own replication. Such self-replicating RNAs were the first genetic systems and might well be called the first living things. Once there were replicators, there was selection. Whereas the first selection-driven changes probably were “improvements” to polymerase activity, eventually metabolic innovations arose, including template-directed protein synthesis. The precursor of the modern ribosome, then, was a purely, or mostly, RNA-based system, confirming early suspicions about the origins of rRNA, mRNA, and tRNA (Woese 1967; Crick 1968; Orgel 1968). The fact that the modern translational apparatus contains so much RNA is explained by such an origin of translation. In fact, it is tempting to think of ribosomal RNA as catalytic RNA, and experimental evidence indicates that at least some of the ribosome’s activities are associated...