Post-Transfer Editing in vitro and in vivo by the Beta-subunit of Phenylalanyl-tRNA Synthetase

Translation of the genetic code requires attachment of tRNAs to their cognate amino acids. Errors during amino‐acid activation and tRNA esterification are corrected by aminoacyl‐tRNA synthetase‐catalyzed editing reactions, as extensively described for aliphatic amino acids. The contribution of editing to aromatic amino‐acid discrimination is less well understood. We show that phenylalanyl‐tRNA synthetase misactivates tyrosine and that it subsequently corrects such errors through hydrolysis of tyrosyl‐adenylate and Tyr‐tRNAPhe. Structural modeling combined with an in vivo genetic screen identified the editing site in the B3/B4 domain of the β subunit, 40 Å from the active site in the α subunit. Replacements of residues within the editing site had no effect on Phe‐tRNAPhe synthesis, but abolished hydrolysis of Tyr‐tRNAPhe in vitro. Expression of the corresponding mutants in Escherichia coli significantly slowed growth, and changed the activity of a recoded β‐galactosidase variant by misincorporating tyrosine in place of phenylalanine. This loss in aromatic amino‐acid discrimination in vivo revealed that editing by phenylalanyl‐tRNA synthetase is essential for faithful translation of the genetic code