Schematic depiction of the progression from an RNA world to the modern biological world. We use the RNA and protein subunits of RNase P as well as tRNA synthetase to illustrate this progression. RNA's tendency to form long-lived misfolded states is depicted on the far left. (i) Short peptide sequences that nonspecifically bind RNA can act as chaperones and provide a selective advantage for an organism that can escape these traps to yield a higher fraction of RNA molecules in functional conformations. (ii) Once peptides gained a foothold there would be a selective advantage for longer and more accurate peptide production that could lead to specific binding and to structured proteins. (iii) As protein catalytic function emerged and competed with ribozymes, RNA association rates may have limited catalytic efficiency and contributed to the current widespread use of proteins for catalysis. The blue circle corresponds to the amino acid covalently attached to tRNA by tRNA synthetase.
