Nucleotide determinants for tRNA-dependent amino acid discrimination by a class I tRNA synthetase

The high accuracy of the genetic code relies on the ability of tRNA synthet ases to discriminate rigorously between closely similar amino acids. While the enzymes can detect differences between closely similar amino acids at a n accuracy of about 1 part in 100-200, a finer discrimination requires the presence of the cognate tRNA. The role of the tRNA is to direct the misacti vated amino acid to a distinct catalytic site for editing where hydrolysis occurs. Previous work showed that three nucleotides at the corner of the L- shaped tRNA were collectively required. Here we show that each of these nuc leotides individually contributes to the efficiency of editing. However, al l are dispensable for the chemical step of hydrolysis. instead, these nucle otides are required for translocation of a misactivated amino acid from the active site to the center for editing.