THE PRIMARY AND SECONDARY STRUCTURES OF THE 23S RIBOSOMAL-RNA FROM THERMOPLASMA-ACIDOPHILUM DEFINE AN ANCIENT ARCHAEAL DIVERGENCE

The nucleotide sequence of the 23S rRNA gene from the archaeon Thermop lasma acidophilum has been determined and a secondary-structure model of the rRNA molecule proposed. Both the primary and secondary structur es of T. acidophilum 23S rRNA exhibit a number of characteristic archa eal features. Pairwise similarities were calculated for 14 archaeal 23 S rRNA sequences and a phylogenetic tree derived from the correspondin g distance measurements. The results show that T. acidophilum stems fr om an ancient, and possibly unique, divergence within the archaeal dom ain. This conclusion is supported by the observation that T. acidophil um 23S rRNA shares secondary-structure characteristics with both of th e main archaeal lineages, the euryarchaeota (methanogens and halophile s) and the crenarchaeota (sulfur-dependent thermoacidophiles), in roug hly equal measure and that it possesses some unique features as well.