Interaction of 70 S Ribosomes from Escherichia Coli with Spin-labeled N-Cbz-Phe-tRNAPhe. An Electron Paramagnetic Resonance Study

Two selectively spin-labeled Cbz-Phe-tRNAsPhe, one at position s4U8 and the other at position U33, have been used to study the dynamics of tRNA-ribosome interaction in the presence of poly(U) and factors washable from ribosomes. Upon binding to the ribosome, the correlation time of the spin label at position s4U8 decreases markedly while the same parameter for the label in the anticodon increases. The presence of poly(U) is not a prerequisite condition for the EPR spectral changes observed but larger variation occurs in the presence of factors washable from ribosomes. No variation in the correlation time is observed if uncharged spin-labeled tRNAPhe (on the s4U8 residue) is used in these experiments. Most of the ribosome-bound spin-labeled Cbz-Phe-tRNAPhe are puromycin-reactive, and consequently, the observed effect is manifested mainly at the ribosomal P site. These observations seem to suggest that the interaction between the N-blocked aminoacyl residue on the tRNA and the ribosome results in a conformational change on the tRNA, possibly involving tertiary interactions in a region close to s4U8. The role that the amino acid at the 3'-end can possibly play on this structural change is discussed.