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Figure 4-D-3. Regulation of the lac operon transcription. (a) by CAP. (b) by lac repressor.
Transcriptional activation by CAP In the absence of any transcriptional activator, binding of the polymerase to the promoter of the lac operon is weak. The binding site of the catabolite activator protein (CAP) is located just upstream of the promoter. In such case, the activator can enhance transcription by direct contact with the polymerase, making it bind more efficiently to the promoter. Transcriptional inhibition by lac repressor The silencer (also known as operator) of the lac operon is located at the transcriptional start site. When the lac repressor binds to this region, it will block the movement of the polymerase, thereby inhibiting transcription.
Figure 4-D-4. Structure of the lac repressor/operator complex. The repressor is a homodimer. Each subunit is represented by a different color. PDB ID = 1LBG.
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