Peptide-specific Ribosomes, Genomic Tags, and the Origin of the Genetic Code

Contemporary protein synthesis requires more than 100 components. The ribosome itself consists of two large RNAs, one or two small RNAs, and over 50 polypeptides. In addition, active translation requires an mRNA template, numerous initiation, elongation, and termination factors, several dozen specifically charged tRNAs, the cognate tRNA synthetases, and a continu-ing source of ATP and GTP. To reconstruct the origin of protein synthesis, it is necessary to conceive of a scenario in which one of these interdependent compo-nents had a role in the absence of the others. Most previous models have focused on the emergence of the ribosome and the genetic code, and have simply as-sumed the prior existence of tRNAs, tRNA synthe-tases, and suitable mRNAs.