Phylogenetic analysis of the archaeal order of Sulfolobales based on sequences of 23S rRNA genes and 16S/23S rDNA spacers

23S rRNA genes from six members of the order of Sulfolobales, within the crenarchaeotal branch of the archaea, were cloned and sequenced. For each of the organisms Acidianus brierleyi, Acidianus infernus, Sulfolobus acidocaldarius, Sulfolobus solfataricus, Sulfolobus shibatae and Stygiolobus azoricus the 23S rRNA gene is linked directly to that of 16S rRNA by a short (similar to 200 bp) spacer with no intervening tRNA gene and no closely linked 5S rRNA gene which is typical for the Crenarchaeota. A phylogenetic tree, derived from the 23S rRNA gene sequences, shows that the six organisms are closely related, compatible with their belonging to one Order, and cluster in three pairs: the two Acidianus species, S. shibatae and S. solfataricus, and S. acidocaldarius and St. azoricus.