Mutations which result in constitutive expression of the Bordetella pertussis filamentous haemagglutinin gene.

The expression of the filamentous haemagglutinin (FhaB) of Bordetella pertussis is positively regulated by the bvg locus which encodes a transcriptional activator, BvgA, and a transmembrane sensor protein, BvgS. The gene encoding FhaB, fhaB alone, is not expressed in Escherichia coli, but the introduction of the bvg locus in trans can restore fhaB expression. Using fhaB::lacZY fusions, we have isolated, in E. coli, partially bvg-independent constitutive mutants. The corresponding mutations have been localized to the upstream region of the fhaB promoter at position -15, -16 and -42 from the transcription initiation site. In the absence of the bvg locus, the strength of the mutated promoters was 15 and 200 times higher than the wild-type promoter in the absence of the bvg locus. The expression of these mutated promoters was still enhanced by the presence of the bvg locus.