| Abstract
| - The genetic code depends on amino acid fine structure discrimination by aminoacyl-tRNAsynthetases. For isoleucyl- (IleRS) and valyl-tRNA synthetases (ValRS), reactions that hydrolyze misactivated noncognate amino acids help to achieve high accuracy in aminoacylation. Two editing pathwayscontribute to aminoacylation fidelity: pretransfer and post-transfer. In pretransfer editing, the misactivatedamino acid is hydrolyzed as an aminoacyl adenylate, while in post-transfer editing a misacylated tRNAis deacylated. Both reactions are dependent on a tRNA cofactor and require translocation to a site located∼30 Å from the site of amino acid activation. Using a series of 3‘-end modified tRNAs that are deficientin either aminoacylation, deacylation, or both, total editing (the sum of pre- and post-transfer editing)was shown to require both aminoacylation and deacylation activities. These and additional results withIleRS are consistent with a post-transfer deacylation event initiating formation of an editing-active enzyme/tRNA complex. In this state, the primed complex processively edits misactivated valyl-adenylate via thepretransfer route. Thus, misacylated tRNA is an obligatory intermediate for editing by either pathway.
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