bdrF2 of Lyme disease spirochetes is coexpressed with a series of cytoplasmic proteins and is produced specifically early infection

The Bdr proteins are polymorphic inner membrane proteins produced by most Borrelia species. In Borrelia burgdorferi B31MI, the 18 bdr genes form three subfamilies, bdrD, bdrE, and bdrF. The production of at least one of the Bdr paralogs, BdrF(2), is up-regulated in host-adapted spirochetes, suggesting a role for the protein in the mammalian environment. Here, we demonstrate using reverse transcriptase (RT) PCR that BBG29, BBG30, BBG31, and BBG32, which reside upstream of bdrF(2), are cotranscribed with bdrF(2) as a five-gene operon. While the functions of most of these proteins are unknown, BBG32 encodes a putative DNA helicase. Real-time RT-PCR analyses demonstrated higher levels of bdrF(2) transcript relative to other genes of the operon, suggesting that bdrF(2) may also be transcribed independently from an internal promoter. Internal promoters were detected using the 5' rapid amplification of cDNA ends system. The putative promoter associated with bdrF(2) was found to be highly similar in sequence to the multiple promoters associated with the ospC gene. Real-time RT-PCR analyses, performed to assess the expression of these genes in infected mice, revealed that genes of the bdrF, locus are expressed only during early infection, suggesting a role in the establishment of infection. To further characterize the proteins encoded by the bdrF(2) locus, which have unknown functions, the cellular localizations of these proteins were determined by Triton X-114 extraction and phase partitioning. BBG29 and BBG31 were found to be cytoplasmic. To determine if these proteins elicit an antibody (Ab) response during infection, immunoblot analyses were performed. Abs to these proteins were not detected. Based on the analyses presented here, we offer the hypothesis that BdrF(2) and other proteins encoded by the operon form an inner-membrane-associated protein complex that may interact with DNA and which carries out its functional role during transmission or the early stages of infection.