RELICS FROM THE RNA WORLD

An RNA world is widely accepted as a probable stage in the early evolu tion of life. Two implications are that proteins have gradually replac ed RNA as the main biological catalysts and that RNA has not taken on any major de novo catalytic function after the evolution of protein sy nthesis, that is, there is an essentially irreversible series of steps RNA --> RNP --> protein. This transition, as expected from a consider ation of catalytic perfection, is essentially complete for reactions w hen the substrates are small molecules. Based on these principles we d erive criteria for identifying RNAs in modern organisms that are relie s from the RNA world and then examine the function and phylogenetic di stribution of RNA for such remnants of the RNA world. This allows an e stimate of the minimum complexity of the last ribo-organism-the stage just preceding the advent of genetically encoded protein synthesis. De spite the constraints placed on its size by a low fidelity of replicat ion (the Eigen limit), we conclude that the genome of this organism re ached a considerable level of complexity that included several RNA-pro cessing steps. It would include a large protoribosome with many smalle r RNAs involved in its assembly, pre-tRNAs and tRNA processing, an abi lity for recombination of RNA, some RNA editing, an ability to copy to the end of each RNA strand, and some transport functions. It is harde r to recognize specific metabolic reactions that must have existed but synthetic and bio-energetic functions would be necessary. Overall, th is requires that such an organism maintained a multiple copy, double-s tranded linear RNA genome capable of recombination and splicing. The g enome was most likely fragmented, allowing each ''chromosome'' to be r eplicated with minimum error, that is, within the Eigen limit. The mod el as developed serves as an outgroup to root the tree of life and is an alternative to using sequence data for inferring properties of the earliest cells.