On the Origin of Genetically Coded Protein Synthesis

How proteins with a specific amino acid sequence are synthesized in living organisms is no longer a mystery after the deciphering of the genetic code six decades ago. However, the origin of the system of genetically coded protein synthesis is still the subject of hypotheses that are only partially experimentally justified. Based on the review of works mainly of the last decade, the author formulated a hypothesis that largely concretizes certain stages of the formation of the translation system and the genetic code. The hypothesis is based on the concept of evolutionary reduction of the ambiguity of the primordial code. The distinctive positions of the hypothesis are as follows: (i) the emergence of aptamers, enriching the protocell by the rare catalytically active amino acids, and their evolutionary transformation into aaRS-like ribozymes which resulted in the operational code, (ii) simultaneous participation of a large number of amino acids (including noncanonical) in the formation of operational and genetic codes, (iii) the recognition of the whole codon of mRNA by the anticodon loop of proto-tRNA from the very beginning of coded protein synthesis, (iv) the coevolution of the operational and genetic codes that eliminated their initial ambiguity.