Transcription catabolite gene activator protein

There is also an aspect of positive control in the lac operon. The catabolite activator protein (CAP), carrying bound cAMP, is required for the binding of RNA polymerase to the promoter i.e., it has a direct, positive effect on transcription. However, relief of repression (i.e., induction) will not occur in the presence of glucose, because glucose lowers the level of cAMP, so that CAP is unable to exert its effect. This reflects the preference of the cell to use glucose rather than lactose as a carbon source. Thus it can be seen that the cell stringently controls expression of the lac genes it expresses them only if it needs to metabolize lactose. 

All the DNA-binding proteins discussed thus far function by inhibiting transcription until some environmental condition, such as the presence of lactose, is met. There are also DNA-binding proteins that stimulate transcription. One particularly well studied example is the catabolite activator protein (CAP), which is also known as the cAMP response protein (CRP). When bound to cAMP, CAP, which also is a sequence-specific DNA-binding protein, stimulates the transcription of lactose- and arabinose-catabolizing genes. Within the lac operon, CAP binds to an inverted repeat that is centered near position -61 relative to the start site for transcription (Figure 31.10). CAP functions as a dimer of identical subunits. 

The catabolite activator protein is a transcription factor in E. coli that stimulates transcription of the lac operon structural genes. It responds to cAMP levels such that the lac operon is transcribed only when the cells must use lactose as a fuel source. 

Catabolite Activation In bacteria, a transcriptional control system that induces the synthesis of enzymes for the catabolism of energy substrates other than glucose when glucose levels are low. It involves an activator protein, CRP, that binds cyclic AMP under conditions of low glucose this complex then binds to DNA sites and promotes transcription of the appropriate genes. 

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