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

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

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) Promoter 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. (a) CAP-binding site Operator RNA polymerase can bind and transcribe Inactive CAP Active CAP cAMP DNA Inactive lac repressor lacl lacZ Figure 18.23a

When glucose is abundant, CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoter Figure 18.23b (b) 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. Inactive lac repressor Inactive CAP DNA RNA polymerase can’t bind Operator lacl lacZ CAP-binding site Promoter

Lac - operon High level of glucose, not present lactose, negative regulation on and there is not a positive regulation High level glucose and lactose, negative regulation and positive regulation are off, lac operon is not active In cells is a low level of glucos and high level of lactose, there is not negative regulation and there is positive regulation, all enzymes are in very high amount Low level of glucose and lactose – not active

Induction of Lac Operon 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. Transacetylase Lac repressor b-galactosidase Lactose Lac repressor lac operon lac operon Lactose permease 4. Most proteins involved with lactose utilization are degraded. 2. lac operon proteins are synthesized. This promotes the efficient metabolism of lactose. 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