Higher-order structure in the 3'-terminal domain VI of the 23 S ribosomal RNAs from Escherichia coli and Bacillus stearothermophilus.

An experimental approach was used to determine, and compare, the higher-order structure within domain VI of the 23 S ribosomal RNAs from Escherichia coli and Bacillus stearothermophilus. This domain, which encompasses approximately 300 nucleotides at the 3' end of the RNAs, consists of two large subdomains. The 5' subdomain has been conserved during evolution and appears to be functionally important for the binding of the EF-1 X GTP X aminoacyl-tRNA complex in eukaryotes. The 3' subdomain has diverged widely between eubacteria and eukaryotes, and has produced the 4.5 S RNA in the chloroplast ribosomes of flowering plants. The structure of domain VI within the eubacterial RNAs was probed with chemical reagents in order to establish the degree of stacking and/or accessibility of each adenosine, cytidine and guanosine residue; the double-helical segments were localized with the cobra venom ribonuclease from Naja naja oxiana, and the relatively unstructured and accessible sequences were detected with the single-strand-specific ribonucleases A, T1 and T2. The data enabled the three secondary structural models, proposed for the E. coli 23 S RNAs, to be examined critically and it was concluded that many of their structural features are correct. Various differences between the models were considered and evidence is provided for additional structuring in the RNA including the stacking of juxtaposed purines into double helices. The 5' subdomain constitutes a compact and resistant structure whereas the 3' subdomain is relatively accessible and contains most of the potential protein binding sites. Moreover, comparison of our results with the published results on 4.5 S RNA suggests that the latter forms essentially the same structure as the 3' subdomain, in contrast to earlier conclusions. A high level of structural conservation has occurred throughout the RNA domain during the evolution of the Gram negative and Gram positive bacteria although the thermophile was generally more stable at base-pairs adjacent to the terminal loops.