Catabolite activator protein

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Lac operon. (Catabolite Activator Protein is labeled "cAMP activator protein.")

Catabolite Activator Protein or CAP binds to DNA more tightly in the presence of cAMP (cyclic AMP). Cytosolic cAMP levels rise when the amount of glucose transported into the cell is low, but lactose is readily available.

CAP has a characteristic helix-turn-helix structure which allows it to bind to successive major grooves on DNA. The two helices are reinforcing each causing a 43° turn in the structure so overall causing a 94° degree turn in the DNA.^[1]

This opens the DNA molecule up, allowing RNA polymerase to bind and transcribe the genes involved in lactose catabolism. CAP thus enhances the expression of the lac operon when lactose is present but not glucose.

This requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose. The cell "prefers" glucose, and if it is available, the lac operon is not activated, even when lactose is present.

[edit] External links

• MeSH Catabolite+Activator+Protein
• Flash animation at McGraw-Hill genetics-Lac%20operon

[edit] References

1. ^ Schultz SC, Shields GC, Steitz TA (1991). Crystal structure of a CAP-DNA complex: the DNA is bent by 90 degrees. Science. 253(5023):1001-7. PMID 1653449

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