BvgA and BvgS, which regulate virulence gene expression in Bordetella pertussis, are members of the two-component signal transduction family. The effects of growth conditions on the ability of BvgAS to activate transcription of fhaB (encoding filamentous hemagglutinin) and ptxA (encoding the S1 subunit of pertussis toxin) were assessed in Escherichia coli by using chromosomal fhaB-lacZYA and ptxA-lacZYA fusions. Although it had previously been reported that a ptxA-lacZYA transcriptional fusion was not activated by bvgAS in E. coli (J. F. Miller, C. R. Roy, and S. Falkow, J. Bacteriol. 171:6345-6348, 1989), we now present evidence that ptxA is activated by bvgAS in E. coli in a manner that is highly dependent on the growth conditions. Higher levels of -galactosidase were produced by ptxA-lacZYA in the presence of bvgAS during growth in Stainer-Scholte medium or M9 minimal salts medium with glucose than in Luria-Bertani medium. In contrast, the level of fhaB-lacZYA expression was high during growth in all media. Addition of modulating stimuli which inhibit BvgAS function eliminated expression of ptxA-lacZYA. Levels of -galactosidase expressed from the ptx-lacZYA fusion correlated with growth rate and with the final optical density at 600 nm, suggesting that the lower growth rate in M9-glucose and Stainer-Scholte media was responsible for greater accumulation of -galactosidase than was seen in Luria-Bertani medium. Overproduction of BvgA was not sufficient for activation of ptxA expression but was sufficient for fhaB expression. However, overproduction of a constitutive BvgA allele (bvgA-C1) or overproduction of BvgA in the presence of BvgS was able to activate ptxA. Our results demonstrate Bvg-dependent activation of a ptxA-lacZYA fusion in E. coli and indicate that bvg is the only Bordetella locus required for ptxA activation in this heterologous system. Bordetella pertussis, the causative agent of whooping cough, synthesizes toxins and adhesins which appear to aid the bacterium in colonization and multiplication (15,23,28,38,56,59). Expression of toxins (pertussis toxin, adenylate cyclasehemolysin, and dermonecrotic toxin) and adhesins (filamentous hemagglutinin, fimbriae, and pertactin) is positively regulated by the products of the bvg operon (11,57,59). In contrast to the multiple loci activated by bvg, vrg genes of B. pertussis (5,6,25), siderophore production in some Bordetella bronchiseptica strains (14), and motility genes of B. bronchiseptica (1-3) are repressed by bvg.Sequence analysis and in vitro studies have indicated that BvgA and BvgS are members of the two-component family of signal transduction proteins. BvgS is a 135-kDa sensor protein localized to the cytoplasmic membrane, and BvgA is a 23-kDa cytoplasmic response regulator (4, 7, 37, 49, 50-53). The signals sensed by BvgAS in vivo are not known, but BvgAS can be inactivated in vitro by growth at a low temperature or by addition of sulfate anion or nicotinic acid to the culture medium. This reversible inactivation of BvgAS is known as phenot...