We have recently developed “simplified genetic codes” in which only 19 amino acids are assigned to one sense codon each. In one simplified code, for example, the UGG codon for tryptophan is reassigned to alanine (Ala) by a tRNA^Ala variant. We have previously generated such codes each of which was constructed using only one kind of tRNA variant. In this study, we describe a novel simplified genetic code in which the codons of Arg were reassigned to Ala using 3 kinds of tRNA^Ala variants. The usage of multiple tRNA variants will be important to construct more simplified genetic codes containing less than 19 amino acids. A Simplified genetic code will provide not only new insights into primordial genetic codes, but also an engineering tool for the assessment of protein evolution.

  Although abiotic polymerization of amino acids is an important step in origin of life study, it has often eluded that their formation could be limited in the hydrothermal systems. To show this we heated an aqueous solution of several amino acids in a simulated hydrothermal environment to assess the possible formation of peptides among the heated products. Peptide concentrations estimated by the Lowry method were significantly higher when the mixture was heated at 300℃ than those at lower temperatures, despite having more than 80% of the initial amino acids decomposed. We also revealed that the peptides measured here were only part of the bonds in the heated products. The major heat products were non-peptide amino acid condensates (NPACs) that only possess partial peptide bonds. The role of NPACs should be examined though they were often ignored in the classical chemical evolution scenario so far.