When E. coli cells are exposed to both glucose and lactose as carbon source, the bacterium will metabolize glucose. Although lactose is present from the beginning of the bacterial growth phase, lac enzymes are not produced until glucose in the medium is exhausted. This repression of lac operon by glucose is termed as ‘catabolite repression’ and is mediated by cAMP receptor protein (CRP), which is also called catabolite gene activator protein (CAP) along with cAMP.
RNA polymerase has less affinity for the promoter of the lac operon unless helped by the regulatory protein, CRP. CRP becomes activated only if the concentration of cyclic AMP (cAMP) is high. Glucose inhibits the formation of cAMP. Therefore, if the concentration of glucose is high, the concentration of cAMP is low and hence RNA polymerase will not be able to bind well to the promoter and the operon will be operating at a very low level (i.e., almost switched off) (Figure 7.4b).
Figure 7.4 Regulation of Lac operon by cAMP (a) Positive regulation by cAMP (b) Catabolite repression
