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

The chromosomal paralogous gene family 36 encodes for four lipoproteins with high amino acid homology that are expressed in vivo in humans and animals and are immunogenic. Transcriptional analysis of the bmp gene cluster indicated that all four genes of this cluster are expressed in vitro and constitute two transcriptional units with a complex pattern of transcription, including alternative monocistronic and polycistronic messages. One unit consists of bmpD, whose transcription is coupled to the transcription of the ribosomal protein genes, rpsG and rpsL. The second unit includes bmpC, bmpA and bmpB. The simultaneous expression of the four bmp genes in Borrelia burgdorferi suggests that their gene products may have either different or complementary functions. Primer extension experiments identified promoters for bmpD, bmpC and bmpA, but not for bmpB. The concentration of gene-specific mRNA paralleled its promoter homology to the Escherichia coli sigma (70) promoter. The linkage of bmpD expression to rpsL and rpsG suggests that the expression of this gene may be controlled by growth-related global regulation mechanisms in B. burgdorferi. These results indicate that the bmp family constitutes a good model for the investigation of complex regulation of chromosomal gene expression in this bacteria.