Mutational unmasking of a tRNA-dependent pathway for preventing genetic code ambiguity.

Aminoacyl-tRNA synthetases establish the genetic code by matching each amino acid with its cognate tRNA. Aminoacylation errors lead to genetic code ambiguity and statistical proteins. Some synthetases have editing activities that clear the wrong amino acid (aa) by hydrolysis of either of two substrates: misactivated aminoacyl-adenylates ("pretransfer" of aa to tRNA) or misacylated aa-tRNA ("posttransfer"). Whereas posttransfer editing can be directly measured, pretransfer editing is difficult to demonstrate, because adenylates are inherently labile and transient, and activity occurs against a background of posttransfer editing. Herein, different mutations in Escherichia coli leucyl-tRNA synthetase are combined to unmask the pretransfer pathway. The mutant enzymes completely lack posttransfer editing but prevent misacylations by clearing misactivated adenylates. We hypothesize that these mutations isolate a pretransfer translocation step that moves misactivated adenylates from the activation site for editing. The results highlight how evolution redundantly created two distinct pathways to prevent genetic code ambiguity.