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You are watching: What is the role of glucose in catabolite repression?

Griffiths AJF, müller JH, Suzuki DT, et al. An arrival to hereditary Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An extr control device is superimposed on the repressor– operator system. Thissystem exists because cells have specific enzymes the favor glucose uptake andmetabolism. If both lactose and also glucose space present, synthetic ofβ-galactosidase is no induced till all the glucose has actually been utilized. Thus, thecell conserves its metabolic machinery (that, for example, cause thelac enzymes) by utilizing any existing glucose prior to goingthrough the steps of creating new machinery come metabolize the lactose. The operonmodel outlined earlier in this chapter will certainly not account because that the suppression ofinduction by glucose, so we have to modify it.

Studies show that in fact some catabolic break down product of glucose (no exactidentity is however known) prevents activation the the lac operon bylactose, so this impact was originally dubbed catabolite repression. The effect of the glucose catabolite is exertedon vital cellular constituent dubbed cyclic adenosene monophosphate(cAMP).

When glucose is current in high concentrations, the cAMP concentration is low; as theglucose concentration decreases, the concentration the cAMP increasescorrespondingly. The high concentration the cAMP is important for activation the thelac operon. Mutants that cannot transform ATP right into cAMP can not beinduced to produce β-galactosidase, because the concentration that cAMP is no greatenough come activate the lac operon. In addition, there are othermutants that do make cAMP yet cannot activate the lac enzymes,because this mutants absence yet an additional protein, called catabolite activator protein(CAP), made by the crp gene. CAP creates a complicated with cAMP, and itis this complex that is may be to tie to the CAP website of the operon. The DNA-boundCAP is climate able to communicate physically v RNA polymerase and also essentially increasethe affinity the RNA polymerase because that the lac promoter. In this way,the catabolite repression mechanism contributes come the selective activation that thelac operon (Figure11-11).


Figure 11-11

Catabolite regulate of the lac operon. The operon isinducible by lactose come the maximal levels as soon as cAMP and also CAP kind acomplex. (a) Under problems of high glucose, a glucose breakdownproduct inhibits the enzyme adenylate cyclase, preventing the conversionof (more...)

Glucose control is achieved because a glucose break down product inhibitsformation the the CAP-cAMP facility required for RNA polymerase to attach at thelac promoter site. Also when there is a shortage the glucosecatabolites and also CAP-cAMP forms, the enzyme taking component in lactose move andmetabolism are created only if lactose is present. This level of control isaccomplished due to the fact that lac operon inducers need to bind to the repressor protein to eliminate it from the operator site and permit transcription of thelac operon. Thus, the cabinet conserves its energy and also resourcesby developing the lactose-metabolizing enzyme only as soon as they room both required anduseful. These ideas are summarized in Figure11-12, which likewise depicts the bending that the DNA result from CAP-cAMPbinding come the lid site, presumably enhancing the affinity the RNA polymerase come thepromoter.


Figure 11-12

Negative and also positive control of the lac operon through the Lacrepressor and the catabolite activator protein (CAP), respectively. (a) Inthe lack of lactose to serve as one inducer, the Lac repressor is able tobind the operator; regardless of the levels (more...)


The lac operon has an included level of control so the the operonremains inactive in the existence of glucose also if lactose likewise is present.High concentrations of glucose catabolites produce low concentration of cAMP,which must form a complicated with lid to allow the induction that thelac operon.

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