Electron microscopy of the secondary structure in partially denatured rRNAs of Escherichia coli and Bacillus stearothermophilus.

Partially denatured 16S and 23S rRNAs from the thermophile Bacillus stearothermophilus show characteristic loop patterns when observed by electron microscopy. The patterns are very similar to those seen in rRNAs from Escherichia coli. At least 2 of 4 most stable interactions in 16S rRNA and 8 of 12 interactions in 23S rRNA are in common for the two species. These interactions correspond well to features of secondary structure in models inferred for rRNA from phylogenetic sequence comparisons and chemical modification studies. However, two additional large loops, enclosing large portions of the 23S rRNA, have been detected in B. stearothermophilus for the first time, and even though other loops are similar, their relative frequencies vary in the two species. Much of the variation is consistent with relative delta G degree values for putative base-paired stems at the base of different loops; but the 5'-terminal loops in 23S rRNA, for example, are unaccountably far less stable in B. stearothermophilus. Also, in general, structural features are not differentially stabilized in B. stearothermophilus; the relative stability of secondary structure in its ribosomes at elevated growth temperatures must involve interactions with ribosomal proteins or other cellular components.