SEQUENCE VARIATION IN THE OUTER-SURFACE-PROTEIN GENES OF BORRELIA-BURGDORFERI

Borrelia burgdorferi is a spirochete pathogen transmitted among warm-b looded hosts by ixodid ticks. Frequency-dependent selection for varian t outer-surface proteins might be expected to arise in this species, s ince rare variants are more likely to avoid immune surveillance in pre viously infected hosts. We sequenced the OspA and OspB genes of nine N orth American strains and compared them with nine strains previously d escribed. For each gene, the mean number of synonymous substitutions p er synonymous site and the mean number of nonsynonymous substitutions per nonsynonymous site show only a twofold excess of silent mutations. Synonymous rates vary widely along the OspB protein. Some regions sho w a significant excess of silent substitutions, while divergence in ot her regions is constrained by biased base composition or selection. Th e presence, in antigenically important regions of the protein, of sign ificant variation among strains, as well as evidence for recombination among strains, should be considered in attempts to develop vaccines a gainst this disease.