lac promoter

This lac promoter includes a CAP binding site and the lac operator sequence lacO. The lac promoter is not as strong as the tac or the trc promoter, but in high copy-number vectors it allows expression of foreign proteins at respectable levels. In the absence of the lac repressor (LacI) expression from the lac promoter is turned on. Maximum induction of the lac promoter requires the action of the cAMP activator protein (CAP, crp gene product), which is most active when cells are grown in medium lacking glucose. Media that contain glucose as carbon source should not be used to express genes from this promoter. For inducible gene expression from the lac promoter the lac repressor (lacI) is necessary. The repressor binds to the operator DNA with high specificity and inhibits gene expression until an inducer like allolactose or the analog IPTG (Isopropyl-?-D-thio-galactoside) is added to the medium. The inducer binds to the repressor, causing a change in its shape. Thus altered, the repressor is unable to bind to the operator, allowing RNA polymerase (RNAP) to transcribe the following genes and thereby leading to high levels of the encoded proteins.