Complex promoter pattern of the single ribosomal RNA operon of an archaebacteriumHalobacterium halobium

The transcription of the single ribosomal RNA gene cluster is initiated at an assemblage of promoters five of which were previously mapped in the 910 base pair region in front of the 16S rRNA gene. The nucleotide sequence between positions -1680 and -910 was determined and S1 nuclease mapping revealed therein two sites corresponding to the 5' ends of the primary transcripts. These two presumably represent the most distal promoters, P1 and P2, of the operon, P1 being separated from the 16S rRNA gene by more than 1250 bp. An additional putative "internal" promoter located within the operon body in the 16S/23S intergenic spacer was revealed by S1 nuclease mapping. A 528 bp long open reading frame is co-transcribed with the rRNA genes from the P1 promoter. Its possible functional implication is discussed.