Electron microscopic mapping of secondary structures in bacterial 16S and 23S ribosomal ribonucleic acid and 30S precursor ribosomal ribonucleic acid

Electron microscopy revealed reproducible secondary structure patterns within partially denatured 16S and 23S ribosomal ribonucleic acid (rRNA) from Escherichia coli. When prepared with 50% formamide-100 mM ammonium acetate, 16S rRNA included two small hairpins that appeared in over 50% of all molecules. Three open loops were observed with frequencies of less than 25%. In contrast, 23S rRNA included a terminal open loop and two additional large structures in over 75% of all molecules. These secondary structure patterns were conserved in the 16S and 23S rRNA from Pseudomonas aeruginosa. The secondary structure of the 30S precursor rRNA from the ribonclease III-deficient E. coli mutant AB105 was mapped after partial denaturation in 70% formamide-100 mM ammonium acetate. Two large open loops were superimposed on the 16S and 23S rRNA secondary structure patterns. These loops were the most frequent structures found on the precursor, and their stems coincided with ribonuclease III cleavage sites. A tentative 5'-3 orientation was determined for the secondary structure patterns of 16S and 23S rRNA from their relative locations within 30S precursor rRNA. The relation of secondary structure to ribosomal protein binding and ribonuclease III cleavage is discussed.