Four single-basepair mutations in the ptx promoter of Bordetella bronchiseptica are sufficient to activate the expression of pertussis toxin

Secretion of pertussis toxin (PT) is the preeminent virulence trait of the human pathogen Bordetella pertussis, causing whooping cough. Bordetella bronchiseptica, although it harbors an intact 12-kb ptx–ptl operon, does not express PT due to an inactive ptx promoter (Pptx), which contains 18 SNPs (single nucleotide polymorphisms) relative to B. pertussis Pptx. A systematic analysis of these SNPs was undertaken to define the degree of mutational divergence necessary to activate B. bronchiseptica Pptx. A single change (C(−13)T), which created a better − 10 element, was capable of activating B. bronchiseptica Pptx sufficiently to allow secretion of low but measureable levels of active PT. Three additional changes in the BvgA-binding region, only in the context of C(−13)T mutant, raised the expression of PT to B. pertussis levels. These results illuminate a logical evolutionary pathway for acquisition of this key virulence trait in the evolution of B. pertussis from a B. bronchiseptica-like common ancestor.