Amino acid biogenesis, evolution of the genetic code and aminoacyl-tRNA synthetases

The aminoacyl-tRNA synthetases (aaRSs) ensure the fidelity of the translation of the genetic code, covalently attaching appropriate amino acids to the corresponding nucleic acid adaptor molecules-tRNA. The fundamental role of aminoacylation reaction catalysed by aaRSs implies that representatives of the family are thought to be among the earliest proteins to appear. Based on sequence analysis and catalytic domain structure, aaRSs have been partitioned into two classes of 10 enzymes each. However, based on the structural and sequence data only, it will not be easily understood that the present partitioning is not governed by chance. Our findings suggest that organization of amino acid biosynthetic pathways and clustering of aaRSs into different classes are intimately related to one another. A plausible explanation for such a relationship is dictated by early link between aaRSs and amino acids biosynthetic proteins. The aaRSs catalytic cores are highly relevant to the ancient metabolic reactions, namely, amino acids and cofactors biosynthesis. In particular we show that class II aaRSs mostly associated with the primordial amino acids, while class I aaRSs are usually related to amino acids evolved lately. Reasoning from this we propose a possible chronology of genetic code evolution.