Novel expression of the ribosomal RNA genes in the extreme thermophile and archaebacterium Desulfurococcus mobilis.

The single 16S/23S rRNA operon of the sulfur-dependent extreme thermophile Desulfurococcus mobilis is transcriptionally uncoupled from the single 5S RNA gene. Transcription of the former operon initiates at five sites, P5-P1. RNA polymerases intiating at sites P5-P2 terminate and then reinitiate at the next downstream site, thus providing a queue of polymerases at the primary initiation site P1. Putative promoter sequences that are AT-rich lie upstream from each site. Termination occurs at two main sites within long pyrimidine sequences downstream from the 23S RNA gene. A processing site for both large RNAs occurs at a secondary structural feature common to other archaebacterial rRNAs. Transcription of 5S RNA starts mainly at P1/5S at the 5' end of the gene with a low level of initiation from two upstream sites; a similar mechanism of initiation-termination-reinitiation appears to operate and the sites are all preceded by AT-rich promoter-like sequences. Termination occurs a few nucleotides beyound the 3' end of the 5S RNA. The mature RNA contains 133 nucleotides, has a 78% GC bp content and exhibits an unusually extended version of helix I with heterogeneous ends. The expression of the rRNA genes shows characteristics of eukaryotes, eubacteria and archaebacteria.