Proteins that were present during the origin of life may have consisted of only seven primordial amino acids, a study suggests. The 20 amino acids commonly found in proteins are encoded by a total of 61 codons, or sequences of three DNA bases. Glycine is thought to be the most primitive amino acid, but it has not been clear how codons for other amino acids evolved. Masayori Inouye and colleagues analyzed 4,225 protein-coding genes in the Escherichia coli genome. The authors discovered a disproportionately greater useage of codons with the sequence AGY (wherein Y = U or C), compared with codons with the sequence UCX ( wherein X = U, C, A, or G). The result suggests that these two sets of serine codons emerged independently during evolution. Based on a comparison of all the codons, the authors propose a hypothesis for the multistage evolution of amino acids. At the time of the origin of life, glycine likely gave rise to serine encoded by AGY, in addition to valine, alanine, aspartic acid, glutamic acid, and arginine. Later in evolution, the seven primordial amino acids gave rise to serine encoded by UCX as well as the 13 remaining amino acids. This hypothesis contradicts previously reported results from prebiotic chemical experiments and meteorite analyses, which suggested that there were 10 primordial amino acids, according to the authors.
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Article #20-14567: "Evolution of the genetic code; Evidence from serine codon use disparity in Escherichia coli," by Masayori Inouye, Risa Takino, Yojiro Ishida and Keiko Inouye.
MEDIA CONTACT: Masayori Inouye, Rutgers University, Piscataway, NJ; e-mail: < inouye@cabm.rutgers.edu >
Proceedings of the National Academy of Sciences