Thermal Stability of Poly(U) · tRNA: Ribosome Complexes with Phe‐tRNA^Phe and Peptidyl‐tRNA^Phe

The stability of ribosomal complexes, carrying [¹⁴C]Phe‐tRNA^Phe, Gly‐Gly‐[¹⁴C]Phe‐tRNA^Phe or Ac[¹⁴C]Phe‐tRNA^Phe at the A site (aminoacyl site) or at the P site (peptidyl site) has been studied with thermal dissociation techniques. Gly‐Gly‐Phe‐tRNA^Phe forms a more stable P‐site complex but a less stable A‐site complex than Phe‐tRNA^Phe. The P‐site complex with Gry‐Gly‐Phe‐tRNA^Phe is more stable than the A‐site complex with the same substrate. Phe‐tRNA^Phe, bound enzymatically at the A site, forms a more stable complex than Phe‐tRNA^Phe bound non‐enzymatically at the P site. The complex with Phe‐tRNA^Phe, formed enzymatically in the presence of GTP, is considerably more thermostable than the complex formed in the presence of Guo‐5′‐P₂‐CH₂‐P. These results provide additional evidence that aminoacyl‐tRNA and N‐blocked aminoacyl‐tRNA interact with different stabilities with the ribosomal binding sites. It is suggested that the observed differences in thermal stabilities reflect the substrate specificity of the peptidyl transferase.