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

The stability of ribosomal complexes, carrying [ 14 C]Phe‐tRNA Phe , Gly‐Gly‐[ 14 C]Phe‐tRNA Phe or Ac[ 14 C]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′‐ P 2 ‐CH 2 ‐ 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.