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Genetic Regulatory Mechanisms

Genetic Regulatory Mechanisms. Inducible Genes - Operon Model. Definition: Genes whose expression is turned on by the presence of some substance Lactose induces expression of the lac genes Catabolic pathways. Absence of lactose. z. y. a. i. p. o. Active. No lac mRNA.

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Genetic Regulatory Mechanisms

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  1. Genetic Regulatory Mechanisms

  2. Inducible Genes - Operon Model • Definition: Genes whose expression is turned on by the presence of some substance • Lactose induces expression of the lac genes • Catabolic pathways

  3. Absence of lactose z y a i p o Active No lac mRNA Presence of lactose z y a i p o Inactive -Galactosidase Permease Transacetylase Lactose Operon • Inducer -- lactose • Absence • Active repressor • No expression • Presence • Inactivation of repressor • Expression • Negative control

  4. Operator RNA polymerase can bindand transcribe Promoter DNA lacl lacZ CAP-binding site ActiveCAP cAMP Inactive lac repressor InactiveCAP (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway. Figure 18.23a Positive Gene Regulation- CAP • In E. coli, when glucose is always the preferred food source • When glucose is scarce, the lac operon is activated by the binding of the catabolite activator protein (CAP)

  5. Promoter Operator DNA lacl lacZ CAP-binding site RNA polymerase can’t bind InactiveCAP Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. (b) Figure 18.23b • When glucose is abundant, • CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoter

  6. 1. When lactose becomes available, a small amount of it is taken up and converted to allolactose by β-galactosidase. The allolactose binds to the repressor, causing it to fall off the operator site. Induction of Lac Operon Transacetylase Lac repressor Lac repressor Lactose b-galactosidase lac operon lac operon Lactose permease 2. lac operon proteins are synthesized. This promotes the efficient metabolism of lactose. 4. Most proteins involved with lactose utilization are degraded. lac operon lac operon 3. The lactose is depleted. Allolactose levels decrease. Allolactose is released from the repressor, allowing it to bind to the operator site. Lac repressor

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