Electron microscopy of secondary structure in partially denatured Escherichia coli 16S rRNA and 30S subunits.

Loops observed in partially denatured 16S rRNA lie within three domains, each about 500 nucleotides long. The loops observed in the 5' and central domains agree well with features of the model proposed by Woese et al. [Woese, C. R., Gutell, R., Gupta, R., & Noller, H. F. (1983) Microbiol. Rev. 47, 621-669]. The structure in the 3' domain is more complex and variable but is still consistent with the model. Published psoralen cross-linking studies have reported one of the observed loops but have also identified loops other than those observed here or predicted by any secondary structure model. These loops are stabilized by increasing concentrations of Mg2+ ions and by bound ribosomal proteins. For example, protein S4 in LiCl core particles stabilizes a loop of 370 nucleotides which forms part of its putative binding site on rRNA. The loop structures are characteristic enough to permit an overall comparison of the most stable of the predicted and observed loops in 16S and 23S rRNAs. Both rRNAs show a stable 5'-terminal loop and a set of subterminal nested loops near the 3' end.