Differential binding of BvgA to two classes of virulence genes of Bordetella pertussis directs promoter selectivity by RNA polymerase

Transcription of virulence genes of Bordetella pertussis is co- ordinately regulated by the BvgA and BvgS proteins, which are members of the two-component family of bacterial signal-transduction proteins. BvgS is the transmembrane sensor and BvgA the transcriptional regulator. By get mobility shift assays we demonstrate that phosphorylated BvgA (BvgA≃P) forms distinct complexes with the filamentous haemagglutinin (P(FHA)) promoter DNA at different BvgA≃P: DNA ratios. DNase I protection analyses show that phosphorylation of BvgA not only enhances affinity of the protein for the binding sites of the P(FHA) and bVgP1 promoters, but it extends significantly the bound region towards position -35 of these promoters. Conversely, a 10-fold higher amount of BvgA≃P is required for binding to a large DNA region, from 168 to -60, of the pertussis toxin (P(tox)) promoter sequence. These findings suggest that the molecular interaction of BvgA≃P with the P(tox) promoter is different from its interaction with the P(FHA) and bvgP1 promoters. The σ70 Escherichia coli RNA polymerase (RNP) does not bind to the bvg-regulated promoters. However, following the formation of a BvgA≃P-promoter complex, the E. coli RNP specifically recognizes and binds to the bvg-regulated promoters. Thus, BvgA≃P exerts its action at the level of promoter recognition by directing promoter selectivity by RNP.