Juxtaposition of expressed variable antigen genes with a conserved telomere in the bacterium Borrelia hermsii

Borrelia hermsii, an agent of relapsing fever, survives in mammals through antigenic variation. Change in serotype-specific variable outer membrane proteins (Vmps) occurs when a Vmp gene at an expression site is replaced with a previously silent gene for another Vmp. Silent and active genes are on separate linear plasmids. The upstream site for a nonreciprocal recombination between two linear plasmids is near the 5' ends of the expressed and silent genes. In the present study we sought the downstream recombination sites in two serotypes, 7 and 21. Restriction fragments containing plasmid telomeres were identified by susceptibility to digestion with BAL-31 and rapid reannealment following denaturation. Whereas both silent genes and a minority population of both expression-linked genes were several kilobases from the telomeres, the predominant population of both expressed genes had 3' ends near plasmid telomeres. Sequence analysis of the predominant expression plasmids revealed that the telomeric sequences were the same in serotypes 7 and 21. Identical sequence was also downstream of silent Vmp genes. Switching of Vmp genes appears to occur by recombination that involves both upstream and downstream sites. The expression plasmid's telomere is preserved in the recombination event.