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OspA and OspB are major outer membrane proteins and antigens of the
Lyme disease spirochete, Borrelia burgdorferi. We examined the organization of ospA and ospB, the genes encoding these proteins. The location and direction of transcription of each osp gene was determined by subcloning, deletion analysis, and transposon Tn5 mutagenesis. Transposon Tn5 insertions within the ospA gene abrogated expression of ospB, suggesting that these genes are transcribed from a common promoter. Northern blot analysis of mRNA from B. burgdorferi with two DNA probes individually specific for ospA or ospB identified a 2.2-kilobase transcript that hybridized with each probe. These studies indicate that the two osp genes of B. burgdorferi constitute a single transcriptional unit.