Role of the CCA terminal sequence of tRNA(Val) in aminoacylation with valyl-tRNA synthetase.

All known tRNAs have a universal CCA sequence at the 3'-terminal. To study the role of this terminal sequence in the aminoacylation process, base substitutions were introduced into a transcript of Escherichia coli valine tRNA and the effects on the aminoacylation activity with valyl-tRNA synthetase were evaluated. Substitution of the terminal adenosine residue at position 76 by C or U caused a 5-7-fold decrease of valine charging activity in Vmax/Km, while substitution by G resulted in about a 300-fold decrease. In addition, these mutations gave rise to an appreciable level of misaminoacylation with threonine. ATP hydrolysis activity during threonylation was lower in the terminal adenosine mutants than in the wild-type. Mutations introduced at positions 75 and 74 also caused threonylation instead of reducing valylation, albeit to a much smaller extent. These results indicate that the CCA sequence, especially the base portion of the terminal adenosine residue, plays an important role not only in amino-acylation efficiency with valine but also in preventing misaminoacylation by hydrolyzing misactivated threonyl-tRNA(Val).