Structure of the archaebacterial 7S RNA molecule.

The genes encoding the 7S RNAs of the archaebacteria Archaeoglobus fulgidus, Methanosarcina acetivorans, Sulfolobus, solfataricus, and Thermococcus celer have been isolated. All four genes occur as single genomic copies and are flanked by sequences containing potential signals for transcriptional promotion and termination. The genes encode RNA molecules approximately 300 nucleotides in length which conform strictly to a model of secondary structure common to all described archaebacterial 7S RNAs. Archaebacterial 7S RNAs exhibit a strong similarity to eukaryotic 7S RNAs in terms of overall secondary structure, while primary sequence conservation is limited to a specific structural domain of the molecule. This domain displays strong primary and secondary structural similarity to features of small eubacterial RNAs, including the small cytoplasmic (sc) RNA of Bacillus subtilis and the 4.5S RNA of Escherichia coli. Conservation of this structural domain among divergent RNA molecules across three kingdoms suggests that these RNAs are the descendants of a unique subcellular structure present before the divergence of the archaebacterial, eubacterial and eukaryotic kingdoms.