GENETIC-VARIATION OF THE BORRELIA-BURGDORFERI GENE VLSE INVOLVES CASSETTE-SPECIFIC, SEGMENTAL GENE CONVERSION

The Lyme disease spirochete Borrelia burgdorferi possesses 15 silent v is cassettes and a vis expression site (vlsE) encoding a surface-expos ed lipoprotein. Segments of the silent vis cassettes have been shown t o recombine with the vlsE cassette region in the mammalian host, resul ting in combinatorial antigenic variation. Despite promiscuous recombi nation within the vlsE cassette region, the 5' and 3' coding sequences of vlsE that flank the cassette region are not subject to sequence va riation during these recombination events. The segments of the silent vis cassettes recombine in the vlsE cassette region through a unidirec tional process such that the sequence and organization of the silent v is loci are not affected. As a result of recombination, the previously expressed segments are replaced by incoming segments and apparently d egraded. These results provide evidence for a gene conversion mechanis m in VlsE antigenic variation.