Megabase-sized linear DNA in the bacterium Borrelia burgdorferi, the Lyme disease agent

Abstract

Using pulsed-field gel electrophoresis we examined the genome of Borrelia burgdorferi, a eubacterium of the spirochete phylum and the agent of Lyme disease. A population of this species' cells was lysed in situ in agarose blocks. An abundant DNA form that behaved as a linear duplex molecule under different electrophoretic conditions was found. The estimated size of the molecule was 950 kilobases. DNA from two other genera of spirochetes did not enter the gel under these conditions. These studies indicate that Borrelia spirochetes, perhaps uniquely among prokaryotic organisms, have linear chromosomes. Spirochetes have been assigned their own phylum among eubacteria on the basis of their unique architecture and ribosomal RNA sequences (1, 2). Members of the spirochete genus Borrelia are transmitted by arthropods from one vertebrate host to another. Included in this genus are species that cause relapsing fever and Borrelia burgdorferi, the cause of Lyme disease (reviewed in ref. 3). In Borrelia hermsii, a relapsing fever species, the genes for major outer membrane proteins are arrayed on linear plasmids (4). Investigations of similar plasmids of B. burgdorferi revealed that the linear molecules had covalently closed ends (5). Although replicons with this structure are unusual in prokaryotic organisms (5, 6) and despite evidence to date that bacteria have circular chromosomes (7-9), we wondered whether the chromosome of a borrelia might be linear. This possibility was studied with pulsed-field gel electrophoresis. The experiments described herein are first steps in the structural characterization of a borrelia's genome. The results indicate that B. burgdorferi contains novel forms of linear duplex DNA of a size consistent with a bacterial chromosome.