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The spirochete Borrelia burgdorferi, which causes
Lyme disease, and other members of the Borrelia genus are unique among characterized bacteria in having a linear chromosome. A restriction map of the chromosome of B. burgdorferi 212 was constructed by making extensive use of digestions in agarose blocks of restriction endonuclease fragments or chromosomal DNA that had been purified by pulsed-field gel electrophoresis. A total of 47 digestion sites for the enzymes SgrAI, SacII, MluI, BssHII, EagI, SmaI, NaeI, and ApaI were located. In most regions of the map, the gap between sites is 50 kbp or less, and 122 kbp is the largest distance between adjacent sites. The mapping data were consistent with previous conclusions that the B. burgdorferi chromosome is linear. The total size of the B. burgdorferi 212 chromosome was determined to be 946 kbp from the sums of the sizes of SacII, MluI, BssHII, and SmaI fragments, making it one of the smallest known bacterial chromosomes. The rRNA genes were found to be located near the center of the chromosome. One copy of the 16S rRNA gene (rrs) and two copies of the 23S rRNA gene (designated rrlA and rrlB), the latter pair in a tandem repeat, were detected. This particular complement of these two genes has not been reported for another bacterium.