The single ribosomal RNA () operons of slow-growing mycobacteria comprise the genes for 16S, 23S and 5S rRNA, in that order. PCR methodology was used to amplify parts of the operons, namely the spacer-1 region separating the 16S rRNA and 23S rRNA genes and the spacer-2 region separating the 23S rRNA and 5S rRNA genes of The amplified DNA was sequenced. The spacer-2 region, the 5S rRNA gene, the trailer region and the downstream region of the operon of were cloned and sequenced. These data, together with those obtained previously for , were used to identify putative antitermination signals and RNase III processing sites within the spacer-1 region. Notable features include two adjacent potential Box B elements and a Box A element. The latter is located within a sequence of 46 nucleotides which is very highly conserved among the slow-growers which were examined. The conserved sequence has the capacity to interact through base-pairing with part of the spacer-2 region. Secondary structures for mycobacterial precursor 23S rRNA and for precursor 5S rRNA were devised, based on sequence homologies and homologous nucleotide substitutions. All the slow-growers, including , conform to the same scheme of secondary structure. A putative motif for the intrinsic termination of transcription was identified approximately 33 bp downstream from the 3'-end of the 5S rRNA gene. The spacer-1 and spacer-2 sequences may prove a useful supplement to 16S rRNA sequences in establishing phylogenetic relationships between very closely related species.


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