Functional importance of the 3'-terminal adenosine of tRNA in ribosomal translation.

The universally conserved 3'-terminal CCA sequence of tRNA interacts with large ribosomal subunit RNA during translation. The functional importance of the interaction between the 3'-terminal nucleotide of tRNA and the ribosome was studied in vitro using mutant in vitro transcribed tRNA(Val) A76G. Val-tRNA(CCG) does not support polypeptide synthesis on poly(GUA) as a message. However, in a co-translation system, where Val-tRNA(CCG) represented only a small fraction of total Val-tRNA, the mutant tRNA is able to transfer valine into a polypeptide chain, albeit at a reduced level. The A76G mutation does not affect binding of Val- or NAcVal-tRNA(CCG) to the A- or P-sites as shown by efficient peptide bond formation, although the donor activity of the mutant NAcVal-tRNA(CCG) in the peptidyl transfer reaction is slightly reduced compared with wild-type NAcVal-tRNA. Translocation of 3'-CCG-tRNA from the P- to the E-site is not significantly influenced. However, the A76G mutation drastically inhibits translocation of peptidyl-tRNA G(76) from the ribosomal A-site to the P-site, which apparently explains its failure to support cell-free protein synthesis. Our results indicate that the identity of the 3'-terminal nucleotide of tRNA is critical for tRNA movement in the ribosome.