RNA determinants for translational editing. Mischarging a minihelix substrate by a tRNA synthetase.

The fidelity of protein synthesis requires efficient discrimination of amino acid substrates by aminoacyl-tRNA synthetases. Accurate discrimination of the structurally similar amino acids, valine and isoleucine, by isoleucyl-tRNA synthetase (IleRS) results, in part, from a hydrolytic editing reaction, which prevents misactivated valine from being stably joined to tRNAIle. The editing reaction is dependent on the presence of tRNAIle, which contains discrete D-loop nucleotides that are necessary to promote editing of misactivated valine. RNA minihelices comprised of just the acceptor-TPsiC helix of tRNAIle are substrates for specific aminoacylation by IleRS. These substrates lack the aforementioned D-loop nucleotides. Because minihelices contain determinants for aminoacylation, we thought that they might also play a role in editing that has not previously been recognized. Here we show that, in contrast to tRNAIle, minihelixIle is unable to trigger the hydrolysis of misactivated valine and, in fact, is mischarged with valine. In addition, mutations in minihelixIle that enhance or suppress charging with isoleucine do the same with valine. Thus, minihelixIle contains signals for charging (by IleRS) that are independent of the amino acid and, by itself, minihelixIle provides no determinants for editing. An RNA hairpin that mimics the D-stem/loop of tRNAIle is also unable to induce the hydrolysis of misactivated valine, both by itself and in combination with minihelixIle. Thus, the native tertiary fold of tRNAIle is required to promote efficient editing. Considering that the minihelix is thought to be the more ancestral part of the tRNA structure, these results are consistent with the idea that, during the development of the genetic code, RNA determinants for editing were added after the establishment of an aminoacylation system.