Functional analysis of the pertussis toxin promoter

The expression of the pertussis toxin ptx operon is positively regulated in cis by a promoter region of about 170 base pairs and in trans by the bvg locus, which codes for the transcriptional activator protein BvgA. The promoter contains two direct repeats which are essential for its activity. When the position of these direct repeats relative to the transcription start point was changed, the activity of the promoter was strongly impaired. The repeated sequences therefore do not represent enhancer-like elements similar to those which have been identified in other positively regulated promoters; instead, the integrity of the whole promoter region seems to be an important feature of ptx regulation. A transcription interference assay was carried out to analyze in vivo binding of regulatory proteins to the ptx promoter. The results suggest that the direct repeats are the recognition sequence of a protein, which binds to them only under conditions in which the promoter is activated. In vitro DNA binding experiments with BvgA protein purified from an overproducing Escherichia coli strain were performed. However, no binding of BvgA to the ptx promoter was observed under conditions where binding of BvgA to the fha and bvg promoters occurred. This suggests that factors in addition to the bvg system are involved in the regulation of the Bordetella virulence regulon.