CAP act as a transcriptional activator only in the absence of "E. coli"'s preferred sugar, glucose. When glucose is absent, high level of CAP protein/cAMP complex bind to CAP binding site, a site between "araI1" and "araO1". Binding of CAP/cAMP is responsible for opening up the DNA loop between "araI1" and "araO2", increasing the binding affinity of AraC protein for "araI2" and thereby promoting RNA polymerase to bind to "araBAD" promoter to switch on the expression of the "araBAD" required for metabolising L-arabinose.