Classical and El Tor biotypes of Vibrio cholerae differ in timing of transcription of tcpPH during growth in inducing conditions

Two protein pairs in Vibrio cholerae, ToxRS and TcpPH, are necessary for tr anscription from the toxT promoter and subsequent expression of cholera vir ulence genes. We have previously shown that transcription of tcpPH in class ical strains of V. cholerae is activated at mid-log-phase growth in ToxR-in ducing conditions, while transcription of tcpPH in El Tor strains is not. I n this study, we showed that while transcription of tcpPH differs at mid-lo g-phase growth in ToxR-inducing conditions between the biotypes, transcript ion is equivalently high during growth in AKI conditions. We used tcpPH::gu sA transcriptional fusions to quantitate expression of tcpPH in each biotyp e throughout growth in ToxR-inducing conditions and shelved that although t ranscription of tcpPH. is reduced at mid-log-phase growth in an El Tor stra in, transcription is turned on later in growth to levels in excess of those in the classical strain (although cholera toxin is not produced). This sug gests that the difference in expression of cholera virulence factors in res ponse to ToxR-inducing conditions between the El Tor and classical biotypes of V. cholerae mag. be related to the timing of transcription of tcpPH rat her than the absolute Levels of transcription.