Physical maps of the chromosomes of the Lyme disease spirochaetes and have been elucidated for the enzymes l, AI, I-I, I, I, HII, I and l by two-dimensional pulsed-field gel electrophoresis techniques. The maps contain 42 sites for and 32 for 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 and and the published map of the other Lyme disease spirochaete, [Davidson, B. E., MacDougall, J. & Saint Girons, I. (1992) 174, 3766-3774] revealed that the three chromosomes have few endonuclease sites in common, apart from a cluster in (encoding 23S rRNA) and (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 , 39 for , and 33 for 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 and chromosomes that encodes rRNA revealed that each chromosome contains one copy of separated by 5 kbp from two copies each of and (encoding 5S rRNA). The gene order is 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 and in and than in


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