Common evolutionary origins of the bacterial glycyl tRNA synthetaseand alanyl tRNA synthetase

Glycyl tRNA aminoacylation is catalyzed by different enzymes in different lineages of the Tree of Life. The archaeal (arcGlyRS) and bacterial (bacGlyRS) versions of glycyl tRNA synthetase are globally different. arcGlyRS is a relatively small protein with two domains: a catalytic domain and a tRNA recognition domain. bacGlyRS is a large, multi-domain protein. We have investigated the complex evolutionary histories of these enzymes. Component domains of bacGlyRS show common ancestry with 1) the catalytic domain of class II tRNA synthetases; 2) the HD domain of the bacterial RNase Y; 3) the body and tail domains of the archaeal CCA-adding enzyme; 5) the anti-codon binding domain of the arginyl tRNA synthetase; and a previously unrecognized domain that we call ATL (Ancient tRNA linkup). The emergence of bacGlyRS in its extant form would have required the recruitment of domains from a pool of existing homologs, possibly at different times, by a process of molecular bricolage. A domain-by-domain analysis reveals that the catalytic and ATL domains are closely related to domains in the universal alanyl tRNA synthetase. Homology of the HD domain to bacteria-specific proteins and homology of the body and tail domains to the archaeal CCA-adding enzyme suggest a later recruitment. The data suggest that bacGlyRS was present at the last bacterial common ancestor and that bacGlyRS and uniAlaRS emerged from a shared core before the last common universal ancestor, suggesting that bacGlyRS and uniAlaRS evolved by a stepwise accumulation of protein domains during the dawn of biology.