Fluorescence studies on the location of L7/L12 relative to L10 in the 50S ribosome of Escherichia coli

The localization of the protein L7/L12 relative to protein L10 in the Escherichia coli ribosome was studied by fluorescence energy transfer. N-[7-(Dimethylamino)-4-methylcoumarinyl]maleimide, coupled to Cys-70 of L10, served as a donor for fluorescein which was attached to Lys-51 or to the N terminus of L7/L12. The binding of the fluorescein-L7/L12 dimers to a strong and a weak binding site in 50S ribosomes could be distinguished. Therefore, it was possible to measure the distances between Cys-70 of L10 and Lys-51 and the N terminus of each L7/L12 dimer. For L7/L12 in the strong binding site, these two distances are both about 43 A, and for L7/L12 in the weak binding site, both are about 56 A.