An intron in the 23S ribosomal gene of the archaebacterium Desulfurococcus mobilis

The archaebacteria have been defined, at a molecular level, as constituting a third primary kingdom consisting of the methanogens, the extreme halophiles, and the sulphur-dependent extreme thermophiles. In reaching this conclusion, Woese and colleagues used the 16S ribosomal RNA as an approximate chronometer for evolutionary time and demonstrated that, at a nucleotide sequence level, the archaebacteria are as different from the eubacteria and eukaryotes as the latter kingdoms are from one another. Current research on archaebacteria is yielding valuable insights into the evolutionary relationships between archaebacteria, eubacteria and eukaryotes, and into the early forms of cellular life. Here, we extend this knowledge by providing the first evidence for the occurrence of an intron within any prokaryotic ribosomal RNA. The intron was found within the 23S rRNA gene of the sulphur-dependent and anaerobic Thermoproteale Desulfurococcus mobilis, which was isolated from hot acidic springs in Iceland at temperatures up to 97°C. The intron contains 622 base pairs (bp); it is very A+T-rich (65%) compared with the 23S rRNA gene (34%), and it exhibits a large open reading frame. The splicing site occurs in domain IV of the 23S RNA at a position close to that of an intron of the lower eukaryote Physarum polycephalum; the intron does not readily fall into one of the three classes of eukaryotic nuclear introns because it has features in common with those of classes I (rRNA) and III (transfer RNA).