The CAP binding site in the lac promoter is the location of positive regulation of gene expression o

The CAP binding site is part of the lac promoter and is located at approximately -60. The CAP-c AMP complex binds to this region and alters the structure of the promoter to allow efficient RNA polymerase binding at the lac promoter.


In the absence of arabinose, araC protein negatively regulates transcription by binding at araO1 and araO2. The araC proteins at araI and araO2 dimerize, inducing the formation of a DNA loop that prevents RNA polymerase from binding to Para thus blocking transcription of the ara operon genes. When arabinose is present, it binds to araC proteins at the ara operator site, which prevents dimerization of araC protein at araI and araO2. This prevents DNA loop formation and makes a CAP binding site available for binding. CAP-c AMP activates ara operon gene transcription.