Fluorescence studies of the location of proteins L7 and L12 on the Escherichia coli ribosome

Abstract

The location of ribosomal protein L7/L12 has been examined by fluorescence spectroscopy. Protein L7 modified mainly at the COOH terminus by 5-(iodoacetamidoethyl)-aminonaphthalene-1-sulfonic acid, and yeast tRNAPhe-containing fluorescent dyes located specifically near the anticodon or at the 3'-end were used in these studies. Fluorescence-quenching measurements indicate that the COOH terminus of protein L7/L12 is very accessible to the solvent. Steady state anisotropy measurements indicate that this region has a high degree of rotational mobility. Singlet-singlet energy transfer measurements show that the COOH-terminal region of L7/L12 is far away from the peptidyltransferase center, and the main body of the ribosome in general. These results are most easily explained if all four copies of L7/L12 are in a parallel arrangement. Further energy transfer experiments show that the COOH terminus of L7/L12 must be greater than 70 A away from both the 3'-end and the anticodon region of ribosome-bound tRNAs. This indicates that all four copies of L7/L12 must be oriented so that their COOH termini are far away from the tRNAs. However, changes in both the spectrum and anisotropy of labeled L7/L12 accompany tRNA binding. The most simple explanation for these results is a structural change in the 70 S ribosome which somehow alters the environment or location of L7/L12 when tRNA is present. One simple possibility is an alteration in the quaternary arrangement of the multiple copies of L7/L12.