The Borrelia burgdorferi Genome

What has attracted molecular biologists to the study of B. burgdorferi is the unusual nature of its genome. Its most outstanding feature is its highly segmented or fragmented nature; the genome of the prototype B31 MI strain is composed of a small (911 kb) linear chromosome, as well as at least 12 linear and nine circular extrachromosomal elements that, collectively, account for 40% of the coding potential of the genome (Casjens et al. 2000; Fraser et al. 1997). This is the greatest number of genetic elements ever found to coexist within a bacterial cell.

In general terms, there appears to be a division of labour between the chromosome and the extrachromosomal elements or "plasmids". Genes encoded on the chromosome tend to be housekeeping genes present in single copy while the genes on the plasmids tend to be Borrelia-specific, of unknown function or involved in the infective cycle, and present in multiple, related copies. Many of these paralogous gene families (PFs) show evidence of undergoing mutational decay. The linear plasmids, especially near the telomeres, appear to be undergoing recent and ongoing genetic exchange (Casjens et al. 2000; Huang et al. 2004b).

The linear replicons are terminated by covalently closed DNA hairpins, referred to as telomeres. B. burgdorferi was the first bacterial species to be found with a linear chromosome with hp telomeres (Barbour and Garon 1987; Baril et al. 1989; Ferdows and Barbour 1989). The unique nature of the genome raises several intriguing questions. What is the mode of replication of the linear genetic elements? What is the evolutionary origin of the linearity? Does genome linearity contribute to genome evolution and function? How are so many genetic elements faithfully segregated?

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