CONSERVATION OF GENE ARRANGEMENT AND AN UNUSUAL ORGANIZATION OF RIBOSOMAL-RNA GENES IN THE LINEAR CHROMOSOMES OF THE LYME-DISEASE SPIROCHETES BORRELIA-BURGDORFERI, B-GARINII AND B-AFZELII

Physical maps of the chromosomes of the Lyme disease spirochaetes Borrelia garinii and Borrelia afzelii have been elucidated for the enzymes CspI, SgrAI, I-CeuI, SmaI. EagI, BssHII, MluI and ApaI by two-dimensional pulsed-field gel electrophoresis techniques. The maps contain 42 sites for B garinii and 32 for B. afzelii. The mapping studies showed that the two chromosomes are linear DNA molecules of 953 and 948 kbp, respectively. A comparison of the physical maps of B. garinii and B. afzelii and the published map of the other Lyme disease spirochaete, Borrelia burgdorferi [Davidson, B. E., MacDougall, J. and Saint Girons, I. (1992) J Bacteriol 174, 3766-3774] revealed that the three chromosomes have few endonuclease sites in common, apart from a cluster in rrl (encoding 235 rRNA) and rrs (encoding 16S rRNA). Cloned borrelial genes were used as specific hybridization probes to construct genetic maps, using the physical maps as a basis. The resulting maps contain 41 genetic loci for B. burgdorferi, 39 for B. garinii, and 33 for B. afzelii. In contrast to the physical maps, the three genetic maps are closely related, with no detectable differences in gene order along the entire length of the chromosome. It is concluded that the chromosomes of these three borrelial species have undergone no major rearrangements, deletions or insertions during their evolution from a common ancestor. Detailed mapping of the region of the B. garinii and B. afzelii chromosomes that encodes rRNA revealed that each chromosome contains one copy of rrs separated by 5 kbp from two copies each of rrl and rrf(encoding 5S rRNA). The gene order is rrs rrIA rrfA rrIB rrfB. B. burgdorferi is the only other member of the eubacteria for which this particular rRNA gene arrangement has been observed. A DNA length polymorphism in the region of the borrelial rRNA genes was shown to be due to the presence of 2.2 kbp more DNA between rrs and rrIA in B. garinii and B. afzelii than in B. burgdorferi.