Control of virulence factor expression in Bordetella pertussis is mediated by the products of the bvg operon. The BvgS membrane protein responds to certain environmental cues by activating the BvgA protein, which in turn modulates the expression of the target virulence factor genes. The BvgA and BvgS proteins are members of a large family of sensory transduction proteins called the two-component systems. We show that BvgA fusion proteins can activate transcription of a reporter gene containing the bvg promoter in Escherichia coli, and that this activity correlates with its ability to interact specifically with a recognition sequence in cognate promoters. Using homologies between BvgA and other bacterial response regulators as a guide, two BvgA truncation mutants were constructed and their transactivation and DNA-binding capacities were examined. We discovered that (1) DNA-binding activity is localized to the C-terminal half of BvgA, (2) sequence-specific DNA-binding is necessary, but not sufficient for transactivation, and (3) DNA-binding requires the last 20 amino acid residues at its carboxy terminus. A BvgA fusion protein lacking the receiver domain is inactive in transcriptional activation, but retains sequence-specific DNA-binding activity and forms multimeric complexes. We show that BvgA is able to utilize acetyl phosphate as a phosphoryl group donor and the instability of the covalent linkage at extremes of pH is consistent with an acyl phosphate group. Furthermore, the in vitro phosphorylated form of BvgA exhibits an enhanced capacity for binding DNA target sites, while a dephosphorylated form exhibits a limited capacity to bind these sites. We discuss the implications that these observations have on the mechanism by which BvgA is activated to a transcriptionally competent state.