Schematic mechanism of the toggle switch. Schematic mechanism of the toggle switch . The constitutive P
promoter drives the expression of the lacI gene, which produces the lac repressor tetramer. The lac repressor tetramer binds the lac operator sites adjacent to the Ptrc - 2 promoter, thereby blocking transcription of cI. The constitutive Ptrc - 2 promoter drives the expression of the cI gene, which produces the λ-repressor dimer. The λ-repressor dimer cooperatively binds to the operator sites native to the P
promoter, which prevents transcription of lacI.