|MoBio||Regulation of lac Operon Transcription||Chapter 4|
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.