Genetic exchange among linear replicons of the lyme disease spirochete, Borrelia burgdorferi

Spirochetes of the genus Borrelia are unusual among members of the prokaryotic kingdom as they have a linear chromosome of 950±20 kb. Initial physical mapping studies of Borrelia species indicate that this linear chromosome is stable with the exception of some observed variability at the right end of the linear chromosome. Among the original seven B. burgdorferi isolates analyzed the terminal SgrAI restriction fragment was found to have three possible sizes, thus implying that some isolates carry extra DNA at the right end of the chromosome. A panel of 30 B. burgdorferi isolates was obtained to allow further investigation of this phenomenon. Isolates were first cleaved with SgrAI to determine the length of the terminal restriction fragment. These experiments demonstrated that there are four possible lengths for the B. burgdorferi chromosome. Long length chromosome isolates carry 14 kb of extra DNA that is not present on short chromosome isolates; longer and intermediate length isolates carry 12 kb and 6 kb of DNA respectively. DNA found on intermediate, longer and long length chromosome isolates but not on short chromosome isolates was termed right-end extension sequence. Cloning and sequencing of a 1.6 kb DNA fragment (from isolate 1352), which begins in sequence conserved in all chromosome lengths and extends into right-end extension sequence, revealed that chromosomal right-end extension sequence is carried on plasmids in 13 out of 30 isolates. The 30 members of the panel can carry zero, one or two copies of this cloned right-end extension sequence. When only one copy is present, it may be either on the chromosome or a linear plasmid. Further analysis of the right end revealed that some isolates carry up to 6 kb of chromosomal DNA on linear plasmids. The prevalence and conservation of these 6-14 kb right-end extension sequences imply that they must be biologically significant. Isolates representative of the other eight species do not have right-end extension sequence. B. burgdorferi must have acquired this extra DNA since diverging from a common Borrelia ancestor. The presence of B. burgdorferi chromosomal DNA on linear plasmids suggest that chromosome telomeres may be exchanging with plasmid telomeres through a single recombination event. These dynamic events may contribute to the organism's pathogenicity, and ability to avoid the host immune response or allow the organism to alternate between life in an aartropod vector and a mammalian host.