title: A model of proto-anti-codon RNA enzymes requiring L-amino acid homochirality.
abstract: All living organisms encode the 20 natural amino acid units of polypeptides using a universal scheme of triplet nucleotide "codons". Disparate features of this codon scheme are potentially informative of early molecular evolution: (i) the absence of any codons for D-amino acids; (ii) the odd combination of alternate codon patterns for some amino acids; (iii) the confinement of synonymous positions to a codon's third nucleotide; (iv) the use of 20 specific amino acids rather than a number closer to the full coding potential of 64; and (v) the evolutionary relationship of patterns in stop codons to amino acid codons. Here I propose a model for an ancestral proto-anti-codon RNA (pacRNA) auto-aminoacylation system and show that pacRNAs would naturally manifest features of the codon table. I show that pacRNAs could implement all the steps for auto-aminoacylation: amino acid coordination, intermediate activation of the amino acid by the 5'-end of the pacRNA, and 3'-aminoacylation of the pacRNA. The anti-codon cradles of pacRNAs would have been able to recognize and coordinate only a small number of L-amino acids via hydrogen bonding. A need for proper spatial coordination would have limited the number of chargeable amino acids for all anti-codon sequences, in addition to making some anti-codon sequences unsuitable. Thus, the pacRNA model implies that the idiosyncrasies of the anti-codon table and L-amino acid homochirality co-evolved during a single evolutionary period. These results further imply that early life consisted of an aminoacylated RNA world with a richer enzymatic potential than ribonucleotides alone.
authors.affiliation: Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA. Albert.J.Erives@Dartmouth.edu
authors.forename: Albert
authors.initials: A
authors.lastname: Erives
mesh_terms: D000596:Amino Acids / Q000737:chemistry* / Q000378:metabolism; D049148:Aminoacylation; D000926:Anticodon; D002384:Catalysis; D019143:Evolution, Molecular; D008957:Models, Genetic; D016337:RNA, Catalytic / Q000737:chemistry* / Q000378:metabolism
chemical_list: D000596:Amino Acids; D000926:Anticodon; D016337:RNA, Catalytic