The origin of the genetic code and protein synthesis.

A model for a parallel evolution of the genetic code and protein synthesis is presented. The main tenet of this model is that the genetic code, that is, a correspondence between nucleotide and amino-acid coding units, originated from sequence-specific interaction between abiotically synthesized polynucleotides and polypeptides. A sequence-specific binding between oligonucleotides and oligopeptides is supported by experimental findings. Moreover, it is parsimonious enough to be consistent with the relatively simple chemistry of a primordial environment. Proximity between peptides and RNA increased the rate of formation of ester bonds between them. This lead to the accumulation of sequence-specific polypeptide-polynucleotide pairs, that is, of primordial-loaded tRNA. Condensation of short polypeptides into longer products could be catalyzed by a sequence-specific juxtaposition of loaded tRNA over complementary RNA, originating the core of protein synthesis. The accumulation of useful encoded products, for example, catalysts for tRNA loading (primordial aminoacyl-tRNA synthetases) or stabilizers of tRNA-mRNA interactions (primordial ribosomes), permitted the subsequent evolution of protein synthesis and of the genetic code to their mature form. This occurred via a parallel reduction in length of the interacting polynucleotides and polypeptides. Thus, it maintained the correct reading frame of mRNA from the preceding stages of evolution.