Stereochemical origins of the genetic code.

The origin of the genetic code may be attributed to a postulated prebiological stereochemistry in which amino acid dimers, the trans -R,R'-diketopiperazines, interacted with prototype codon and anticodon nucleotide sequences. An intricately coupled stereochemistry is formulated which displays a binary logic for amino acid-codon recognition. It is shown that the diketopiperazine ring system can be inserted between any terminal pair of base paired nucleotides in a codon-anticodon structure with exact registration of complementary hydrogen bonding functional groups. This yields a codon-dimer-anticodon structure in which each amino acid residue is projected towards and interacts with a particular sequence of vicinal nucleotides on either codon or anticodon. The projection direction and the sequence of nucleotides encountered is a strongly coupled function of the choice of codon terminal nucleotide and the handedness of the amino acid. The reciprocal chemical nature of the complementary base pairs drives the selection of dimers containing quite dissimilar and chirally opposed amino acids. Application of the stereochemical model to the in vivo system leads to a general correlation for amino acid-codon assignments. The genetic code is restated in terms of the dimers selected. The profound symmetry of the code is elucidated and this proves useful for correlative and predictive purposes.