An unusual rRNA operon constellation: in Thermus thermophilus HB8 the 23S/5S rRNA operon is a separate entity from the 16S rRNA operon.

We succeeded in identifying a promoter element within 200 base pairs upstream a transcriptional unit comprising only a 23S rRNA, 5S rRNA and a tRNA(gly) gene in Thermus thermophilus HB8 [1, 2]. This element shows a high degree of homology to the -35 and -10 consensus sequences for promoters described for Escherichia coli [3, 4]. The promoter activity was measured by the induction of the synthesis of functional chloramphenicol acetyltransferase in Escherichia coli. A region located at the transcriptional start, rich in guanosines and cytidines, is very similar in sequence to the one believed to be under stringent control in stable RNA and ribosomal protein genes of Escherichia coli [5]. Employing nuclease S1 protection we were able to determine the in vivo start of transcription, which was identical with the in vitro start using Escherichia coli RNA-polymerase. Furthermore we identified sequences in the region following the origin of transcription, which are homologous to sections in Escherichia coli rrn promoter-leader regions responsible for antitermination. Our finding of a promoter immediately preceding a 23S/5S rRNA operon proves a transcriptional decoupling of the 16S rRNA genes, a situation so far unprecedented among prokaryotes.