Evolution of the major domains of aminoacyl-tRNA synthetase (aaRS) enzymes. A. The leucyl-tRNA synthetase (LeuRS) enzyme in complex with tRNALeu (PDB entry 1WZ2) with its three domains (catalytic, editing and anticodon-binding) colored according to their age of origin. Domain ages were derived from a ToD at FF level of structural complexity . Note how the variable arm of tRNA makes crucial contact with the anticodon-binding domain, which is evolutionarily derived, while the acceptor arm contacts the ancient catalytic domain in pre-editing conformation. B. Occurrence (box plots) and abundance (pie charts) of 28 fold family (FF) domains of aaRS enzymes with known structures in the total genomic dataset of 1,037 cellular organisms and viruses. The name and function of domains are described in Table S2 of the Additional file. C. Phylogenomic tree of protein domain structures describing the evolution of the 28 FFs of aaRSs in the 1,037 proteomes (982 parsimony informative characters; 26,638 steps; CI = 0.8479; RI = 0.8742; g1 = −0.1.401). Taxa are aaRS FF domains and characters are proteomes. FFs are labeled with SCOP concise classification strings. Numbers on the branches indicate bootstrap values. FF domains present in viruses are highlighted in red. Note that d.104.1.1 has been identified in megavirus (not included in this study).