Two independent transcriptional units control the complex and simultaneousexpression of the bmp paralogous chromosomal gene family in Borrelia burgdorferi

The chromosomal paralogous gene family 36 encodes for four lipoproteins wit h high amino acid homology that are expressed in vivo in humans and animals and are immunogenic. Transcriptional analysis of the bmp gene cluster indi cated that all four genes of this cluster are expressed in vitro and consti tute two transcriptional units with a complex pattern of transcription, inc luding alternative monocistronic and polycistronic messages. One unit consi sts of bmpD, whose transcription is coupled to the transcription of the rib osomal protein genes, rpsG and rpsL. The second unit includes bmpC, bmpA an d bmpB. The simultaneous expression of the four bmp genes in Borrelia burgd orferi suggests that their gene products may have either different or compl ementary functions. Primer extension experiments identified promoters for b mpD, bmpC and bmpA, but not for bmpB. The concentration of gene-specific mR NA paralleled its promoter homology to the Escherichia coli sigma (70) prom oter. The linkage of bmpD expression to rpsL and rpsG suggests that the exp ression of this gene may be controlled by growth-related global regulation mechanisms in B. burgdorferi. These results indicate that the bmp family co nstitutes a good model for the investigation of complex regulation of chrom osomal gene expression in this bacteria.