The Evolutionary History of the Translation Machinery

Translation and the Origin of Life Current theories on the origin of life envision an RNA World as the culmination of chemical evolution. The extent of this RNA World, and the biochemical complexity of the progenotes that populated it, is subject to much debate. It, nevertheless, is likely a point of agreement among workers in the field that the discovery of machinery for the chiral synthesis of defined sequence peptides would have paved the way for transition to the modern protein world. With the discovery of an RNA replicase, which might initially have been a catalytic RNA or an early peptide product, the stage would be set for the development of populations of progenotes that had both of these features in one enclosure. Such advanced progenotes would be the first entities capable of having the genetic couple between replication, transcription and translation that is the hallmark of life, as we know it. The modern day tmRNA at one stage is recognized as a tRNA by the ribosome while it subsequently serves as a mRNA during translation. This unusual RNA might be representative of the types of entities present in the late RNA World. The addition of DNA as a better storage medium for genetic information would finalize the transition from the progenotic world to the living systems that exist in the modern world. The origins view described above suggests that the cellular protein synthesis machinery began to congeal to its modern form before the emergence of the last common ancestor of modern life. Is this consistent with what we know? Defining ancestors is in principal straightforward; genes that are shared by essentially all descendants are likely to have been genes of the ancestor, if the possibility of polyphyletic gene losses or gains can be reasonably excluded. Several attempts to outline the properties of the last common ancestor in this way were undertaken and the list of shared properties is rather extensive with many relating to the translation machinery. The approach has, however, been challenged by Doolittle because of the possibility that extensive lateral gene transfer could confuse the results but defended by Glansdorff who has argued that the significance of lateral gene transfer has been overemphasized. A somewhat different approach has been to examine the information processing systems in toto. Thus, when complete genome data from all three Domains first became available the information processes in the three major lineages were compared. It was concluded that the (eu)bacterial machinery of DNA replication is unique to that lineage. Subsequently, this issue was reassessed by examining each essential protein and the conclusion reaffirmed. In the case of transcription, core components were found in all three lineages with the archaeal and eukaryal versions appearing more similar. Despite this affinity, it was observed that transcriptional regulation may be more similar between the Archaea and the Bacteria as both have an