Linear DNA of Borrelia species and antigenic variation.

Members of the genus Borrelia may be unique among prokaryotic organisms in having a polyploid genome that is mostly linear. The smaller linear duplex replicons in these organisms have been called plasmids, but there is justification for designating them minichromosomes instead. The antigenic identities of the agents of Lyme disease and relapsing fever are largely determined by these extrachromosomal genes.