A BRANCH IN THE TOXR REGULATORY CASCADE OF VIBRIO-CHOLERAE REVEALED BY CHARACTERIZATION OF TOXT MUTANT STRAINS

Co-ordinate expression of genes associated with pathogenicity in Vibri o cholerae requires two transcription activators, ToxR and ToxT. Work carried out to date suggests that ToxR activates transcription of the toxT gene and that ToxT directly activates transcription of several ge nes whose products play a role in colonization or CT production by V. cholerae. Previous work also suggests that ToxR can directly activate transcription of the CT operon (ctxAB) independently of ToxT, thereby implying a degree of complexity in control of the ctxAB operon not fou nd with other genes of the ToxR regulon. We tested the regulatory casc ade model of virulence gene expression by constructing strains of clas sical and El Tor V. cholerae deleted for the coding sequence for the p utative DNA-binding domain of toxT. Phenotypic analysis of these strai ns suggests that V. cholerae has ToxT-dependent and ToxT-independent b ranches of its virulence regulon. The results also raise questions abo ut the precise role for ToxR in activation of ctxAB transcription.