Catabolite repressor operon

Figure 3-28. Catabolite repression. The cAMP-CAP complex facilitates initiation of transcription by RNA polymerase. Thus, the operon is transcribed only when glucose is low, cAMP is elevated, and the inducer is bound to the repressor, inactivating it. The lac operon exhibits catabolite repression.

B. cAMP is involved in catabolite repression. Bacterial cells preferentially use glucose. When glucose is low, cAMP rises. cAMP binds to a protein and complexes near the lac promoter region, facilitating binding of RNA polymerase. Lactose must be present to inactivate the repressor, so that the operon may be expressed. 

Fig. 16.7. Catabolite repression of stimulatory proteins. The lac operon is used as an example. A. The inducer allolactose (a metabolite of lactose) inactivates the repressor. However, because of the absence of the required coactivator, cAMP-CRP, no transcription occurs unless glucose is absent. B. In the absence of glucose, cAMP levels rise. cAMP forms a complex with the cAMP receptor protein (CRP). The binding of the cAMP-CRP complex to a regulatory region of the operon permits the binding of RNA polymerase to the promoter. Now the operon is transcribed, and the proteins are produced.

A number of publications have appeared on the dynamics of enzyme synthesis in a variety of situations. Most of the models are based on more or less sophisticated versions of the operon model of Jacob and Monod. The role of m-RNA and its stability were modeled by Terui (1972). Repressor and inducer control was treated by Knorre (1968), Imanaka et al. (1972 1973), van Dedem and Moo-Young (1973), and Suga et al. (1975). Allowance for dual control and catabolite repression was made by Toda (1976). [See also the kinetic treatment by Yagil and Yagil (1971), Imanaka and Aiba (1977), and Bajpai and Ghose (1978)]. A simple structured model was developed by Roels (1978) showing a combination of the features of the models published. More recently Toda (1981) reviewed the effects of induction and repression of enzymes in microbial cultures and their modeling. 

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