Recognition of tRNA Backbone for Aminoacylation with Cysteine: Evolution from Escherichia coli to Human

The underlying basis of the genetic code is specific aminoacylation of tRNAs by aminoacyl-tRNA synthetases. Although the code is conserved, bases in tRNA that establish aminoacylation are not necessarily conserved. Even when the bases are conserved, positions of backbone groups that contribute to aminoacylation may vary. We show here that, although the Escherichia coli and human cysteinyl-tRNA synthetases both recognize the same bases (U73 and the GCA anticodon) of tRNA for aminoacylation, they have different emphasis on the tRNA backbone. The E. coli enzyme recognizes two clusters of phosphate groups. One is at A36 in the anticodon and the other is in the core of the tRNA structure and includes phosphate groups at positions 9, 12, 14, and 60. Metal-ion rescue experiments show that those at positions 9, 12, and 60 are involved with binding divalent metal ions that are important for aminoacylation. The E. coli enzyme also recognizes 2′-hydroxyl groups within the same two clusters: at positions 33, 35, and 36 in the anticodon loop, and at positions 49, 55, and 61 in the core. The human enzyme, by contrast, recognizes few phosphate or 2′-hydroxy groups for aminoacylation. The evolution from the backbone-dependent recognition by the E. coli enzyme to the backbone-independent recognition by the human enzyme demonstrates a previously unrecognized shift that nonetheless has preserved the specificity for aminoacylation with cysteine.