title: Genetic code evolution started with the incorporation of glycine, followed by other small hydrophilic amino acids.
abstract: We propose that glycine was the first amino acid to be incorporated into the genetic code, followed by serine, aspartic and/or glutamic acid-small hydrophilic amino acids that all have codons in the bottom right-hand corner of the standard genetic code table. Because primordial ribosomal synthesis is presumed to have been rudimentary, this stage would have been characterized by the synthesis of short, water-soluble peptides, the first of which would have comprised polyglycine. Evolution of the code is proposed to have occurred by the duplication and mutation of tRNA sequences, which produced a radiation of codon assignment outwards from the bottom right-hand corner. As a result of this expansion, we propose a trend from small hydrophilic to hydrophobic amino acids, with selection for longer polypeptides requiring a hydrophobic core for folding and stability driving the incorporation of hydrophobic amino acids into the code.
authors.affiliation: Department of Biochemistry, University of Otago, PO Box 56, Dunedin, 9054, New Zealand, harold.bernhardt@otago.ac.nz.
authors.forename: Harold S
authors.initials: HS
authors.lastname: Bernhardt
authors.forename: Wayne M
authors.initials: WM
authors.lastname: Patrick
mesh_terms: D000596:Amino Acids / Q000235:genetics*; D019143:Evolution, Molecular; D005815:Genetic Code; D005998:Glycine / Q000235:genetics*; D057927:Hydrophobic and Hydrophilic Interactions
chemical_list: D000596:Amino Acids; D005998:Glycine