In order to study the organization of the ribosomal RNA genes of some spirochetes the rRNA genes were cloned in phage lambda EMBL3. By subcloning the re striction fragments into pUC18 plasmid DNA and analyzing the resulting clones by Sourthern blot hybridization, a restriction map of the rRNA genes was generated and the organization of the rRNA genes was determined. The results obtained from pathogenic and nonpathogenic treponemes show that the genes for three types of rRNA (rrs, rrl, rrf) are closely linked and occur twice in their genome. Fukuna-ga et al., Microbiol. Immunol., 36, 161-167, 1992.
Correlation of genomic hybridization data with physical charactedrization of recombinant plasmid DNAs of the rRNA genes in Borrelia burgdorferi, the etiological agent of Lyme disease, showed that, in contrast to the general organization of rDNA operons ineubacteria, the rrs gene is separated from the closely linked and tandemly repeated rrl/rrf genes. Direct repeat of rRNA genes in eubacterial
genome is the first discovery and this is a remarkable feature in the organization of the eubacterial rRNA genes. Fukunaga et al., J. Gen. Microbiol., 138, in press and Fukunaga and Sohnaka, Biochem. Bioshys. Res. Commun., in press.
To elucidate the organization of the transcription units encoding the rRNAs in the Leptospira interrogans, the nucleotide sequences of the genes and their flanking regions were determined, and the 5' termini of the transcripts were mapped by primer extention experiments. The results show that each of the rRNAs is transcribed separately, and the fact is consistent with the lack of physical linkage among them. This was the only organism known in the eubacteria, at that time, in which all the rRNA genes are present and expressed separately. We also found that there is no linkage among those in the organisms belonging to the genera Leptospira and Leptonema. Furthermore, the number of rrf gene in leptospires was correlated with their parasitism. In the strains of nonparasitic leptospires, there are two genes for rrf, whereas only one gene is carried in parasitic leptospires. Fukunaga et al., J. Bacteriol., 172, 32643268, 1990., Fukunaga et al., J. Gen. Microbiol., 137, 1523-1528, 1991.