Effects of TcpP-binding site mutations on toxT-lacZ and CT reporter activities in a ΔtoxR ΔtcpP background with TcpP-HSV overexpression. The positions of single- and multiple-base-pair transversions in the parental promoter are indicated relative to the toxT transcription start site. Black bars, values for ΔtoxR ΔtcpP/pMMB207 strains; white bars, values for ΔtoxR ΔtcpP/pEK41 strains overexpressing TcpP-HSV. (A) β-Galactosidase activities for 3.5-h cultures of strains carrying either pTG24 alone (empty vector) or toxT-lacZ fusions in which the parental promoter or mutant derivatives drive the expression of the lacZ reporter. Values represent the averages from at least three independent cultures. (B) CtxB levels secreted by 20-h cultures from strains carrying various chromosomal toxT promoter derivatives. toxTΔpro is a chromosomal deletion of the toxT promoter region from position −112 to +1. Values represent the averages from at least six independent cultures. ND, not determined. Strains were grown under ToxR-inducing conditions in the presence of 1 mM IPTG. Error bars represent the standard deviation for each data set. *, P < 0.05; **, P < 0.005 (relative to the parental toxT promoter).