Figure 2 .

Overview of lac operon regulation. See main text for details about the regulation of the lac operon. The lac operon is under the control of two regulatory molecules, the lac repressor and the CAP. These molecules are responsible for switching the lac operon ON or OFF, depending on sugar availability. In the absence of extracellular glucose (depicted with degradation circles), the hunger molecule cAMP (blue node) binds to CAP (green node) and stabilizes a conformation with a high affinity for the regulator CAP site of DNA (depicted with a green box with a cross). The cAMP-CAP binding favors the binding of the RNA polymerase (orange node) to the promoter (thick black arrow) site. This also happens because, as extracellular lactose (depicted with pale orange nodes) is imported into the cell, a fraction of it is converted to allolactose (green nodes). Allolactose binds to the lac repressor (big purple node), stabilizing a conformation unable to bind the operator (orange box with a black minus). RNA polymerase is thus free to start transcribing the operon. When the operon is active, the three structural genes will be produced, namely lacZ, lacY and lacA. LacI, the gene encoding for the lac repressor, is not part of the operon and is under the control of its own promoter. lacI is continuously transcribed and the repressor protein is always present. The lac repressor binds to the operator, which is partially overlapping with the promoter region. This binding prevents the RNA polymerase from binding and starting the transcription process. Illustration was created with TinkerCell (http://www.tinkercell.com).