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Summary The nucleotide sequences of the 5 S ribosomal RNAs of Metbanosarcina barkeri and Methanobacterium thermoautotropbicum were determined. The latter species contains two 5 S RNA variants distinguishable by internal differences in their nucleotide sequences. In addition, each variant shows length heterogeneity at the 3'-terminus. Construction of secondary structure models for eleven archaebacterial 5 S RNA sequences presently known to us, and comparison with models for eubacterial and eukaryotic 5 S RNAs, allowed us to identify features specific for each of the primary kingdoms. Among the archaebacteria, there seems to be more variability in secondary structure than among eukaryotes or among eubacteria. It is possible to distinguish a base pairing scheme valid for methanogens and halophiles from one specific for Sulfolobus and one for Thermoplasma. Hence five variants of the 5 S RNA secondary structure model can be considered: one for eukaryotes, one for eubacteria, and three for archaebacteria. The distinction among these models rests in part on the observation of sites for potential equilibrium structures. The evolutionary position of archaebacterial taxa with respect to each other, and with respect to the other primary kingdoms, was studied by reconstruction of an evolutionary tree from an alignment of 251 5 S RNA sequences, by means of a clustering algorithm. |
