Fluorescence studies of the accessibility of the 3' ends of the ribosomal RNAs in Escherichia coli ribosomes and subunits.

The accessibility of the 3'-ends of E. coli in various states has been probed by reaction, after periodate oxidation, with the fluorescent dye proflavine semicarbazide. Free oxidized 16S and 23S rRNAs each react with 2 equivalents of dye. The 23S rRNA is equally reactive in the 50S subunit and the 70S ribosome. The 16S RRNA 3'-end is accessible in the 30S subunit. In the intact 70S particle, periodate can reach the 3'-end of the 16S rRNA but the dye cannot. The 5S rRNA is relatively inaccessible to periodate oxidation or dye reaction in the 70S particle. Dye-labelled 16S rRNA will reconstitute into 30S particles but they are inactive in polypeptide synthesis. This is apparently due to the inability of the 30S particles to form tight complexes with 50S subunits. Iodide quenching studies indicate that the environment of the 3'-end of 16S rRNA in the 30S particle is different from that of the free rRNA.