Fig. 1.

(A). The ToxR regulon. The ToxR regulon is responsible for activating transcription of toxT, which facilitates virulence gene expression. Environmental factors such as temperature, bile and pH modulate the ToxR regulon. ToxT is the direct transcriptional activator of the ctxAB (located in the CTXΦ), tcp, acfA, acfD, tcpI, aldA and tag genes (all located in the VPI), and it also responds to environmental signals to modulate expression of these genes. (B). ToxT is an AraC-like protein composed of two domains. The N-terminal domain, comprised of amino acids (aa) 1 to 164, is involved in dimerization and environmental sensing, while the C-terminal domain (aa 165 to 276) is responsible for DNA binding.