Extensive interplasmidic duplications change the virulence phenotype of the relapsing fever agent Borrelia turicatae

The relapsing fever agent Borrelia turicatae has two antigenically distinct serotypes, A and B, which differ in their variable small proteins (Vsps) a nd in their degree of virulence and neurotropism in mice. Each Vsp gene (vs pA or vspB) had rapt expression-linked copy that was unique to the serotype expressing it. This was located on one linear plasmid, which was defined b y the upstream sequence. The archived copies of vspA and vspB were each loc ated on different linear plasmids that were the same in both serotypes. In this feature, the mechanism of antigenic variation is similar to that of an other relapsing fever agent, B. hermsii. However, in of her features, the m echanisms of the two organisms differ, The expressed and archived loci for vspA and vspB of B. turicatae were near the centre of linear plasmids inste ad of near the telomeres. The vspA and vspB expression loci were duplicate copies of their respective silent loci: from the vsp itself to at least 13- 14 kb downstream. Despite the extensive interplasmidic duplications and the internal position of the expression focus, the only delectable difference between serotypes A and B was in whether they expressed VspA or VspB.