Origin and Evolution of the Triplet Code
A complete chronology of amino acids is derived, by exploiting all known criteria of evolutionary age of the amino acids. 40 such different criteria and hypotheses about chronological order of appearance of amino acids in the early evolution are summarized in consensus ranking, earliest first (1, 2 and this work):
A, G, V, D, P, S, E, L, T, N, R, K, Q, C, I, H, F, M, Y, W
The established order allows to reconstruct the chronology of codons as well. Three striking features are revealed: (i) the codons providing the most stable codon-anticodon interactions appear first, (ii) the new codons appear simultaneously with their complementary counterparts, and (iii) the new codons are lowest cost derivatives of the codons acquired earlier - by mutations in the third position, and by complementary copying. Two essentially independent amino-acid alphabets are suggested by the above evolutionary scheme, for two complementary coding strands of the earliest small genes. The Glycine family includes amino acids encoded by triplets with purines in central position - G, D, S, E, Q, N, R, K, C, H, Y and W. The Alanine family consists of amino acids A, V, P, S, L, T, I, M and F, with pyrimidines in the central positions of their codons. After the earliest genes were fused to form longer molecules, the encoded protein sequences, presumably, contained a mosaic of short patches of residues from two different alphabets (3). This sequence organization may still be recognized in extant proteins.
1. E. N. Trifonov, Annals NY Acad. Sci. 870, 330 (1999).
E. N. Trifonov
Department of Structural Biology,