Linear plasmids of the bacterium Borrelia burgdorferi have covalently closed ends.

The genetics of spirochetes, a division of eubacteria, has been little studied. Double-stranded linear plasmids were found in Borrelia burgdorferi, the agent of Lyme disease. A 49-kilobase linear plasmid contained the ospA and ospB genes, which encode the major outer membrane proteins of strain B31. Molecules of the 49-kilobase plasmid rapidly reannealed after alkaline denaturation; rapid renaturation was prevented if the 49-kilobase plasmids were first treated with S1 nuclease. When denatured plasmid molecules were examined directly, single-stranded circles of approximately 100-kilobase circumference were seen. These studies provide direct visual evidence that the linear plasmids have covalently closed ends. This form of DNA occurs in some animal viruses, but it has not heretofore been described in prokaryotic organisms.