The key role of the elongation factors in the origin of the organization of the genetic code.

Abstract

I suggest a model based on primordial elongation factors able to explain a relevant aspect of the organization of the code itself: why the biosynthetic relationships between amino acids are mainly allocated on the rows of the genetic code. I hypothesize the existence of specific primordial elongation factors able to recognize two aspects of the ancestral aminoacylated-tRNA: the third base of the anticodon and a distinctive recognition sequence, i.e., specific for pre-tRNAs of early amino acids. If, according to the coevolution theory, it is assumed that the biosynthetic transformations of amino acids occurred on pre-tRNAs, then a duplication of the pre-tRNA of the biosynthetic progenitor followed both by a point mutation in the third base of the anticodon and evolution of the specific elongation factor so as to make it capable of recognizing this new third anticodon base. All these events were sufficient to allow the progenitor pre-tRNA to acquire other codons in a new row of the genetic code. The conquest of further codons in the same row might occur very easily, requiring only point mutations in the first two bases of the anticodon. Since the biosynthetic transformations occurred on pre-tRNAs loaded by amino acids, new amino acids produced by biosynthetic pathways were able to easily acquire the codons of their amino acid precursors. The consequence of all this is that the evolution of biosynthetic families of amino acids would have occurred almost exclusively along the rows of the genetic code. Experimental evidence in favor of the model is represented by specific elongation factors such as that of selenocysteine, which specifically recognizes Sec-tRNASec and brings it to the ribosome. Such elongation factors and their mechanism of recognition would represent molecular fossils of the mechanism hypothesized by the model and these would be a very strong corroboration for it.