The repressor is a regulatory gene . However, when allolactose is present it binds to the repressor this prevents repressor from binding to lacO and permits RNA polymerase to bind to lacP and to initiate transcription. The lacI gene is transcribed constitutively from its own promoter. Brief Introduction to the Genetic Analysis of the Lactose Operon Regulatory mutants affecting the control of the lactose operon could be identified. There were two classes of constitutive mutants.
Normal repressors can form tetramers and can thus bind to the normal operator site.
Inducer-bound repressor does not bind to operator sites . This allows RNA polymerase to bind to the promoter and start transcribing the lac operon. Footprinting Regulatory Proteins Figure AE shows footprinting of CRP and the lac repressor. When CRP binds to its site, repressor binding at O3 is shifted upsteam by 6 bp. Figure 26.19 shows the sequences that bind CRP and repressor.