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Glucose-1-phosphate gal operon The lac operon This enzyme is not essential for lactose utilization.

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Presentazione sul tema: "Glucose-1-phosphate gal operon The lac operon This enzyme is not essential for lactose utilization."— Transcript della presentazione:

1 Glucose-1-phosphate gal operon The lac operon This enzyme is not essential for lactose utilization

2 La presenza di lattosio e/o glucosio controlla lespressione dei geni lac

3 lac operon is induced only when lactose is present - lacI gene is located outside the operon - It is constitutively expressed (10 molecules per cell) - LacI repressor is a tetrameric protein -The repressor protein has the greatest affinity for binding to O 1 and lesser affinity to O 2 and O 3 - The lac operon is occasionally transcribed also in the presence of LacI repressor

4 The mRNA transcribed from lac operon is very unstable

5 Loperatore è caratterizzato da una sequenza palindromica. Ogni meta del sito è legato dal DNA binding domain del repressore LacI. La struttura secondaria, denominata elica- giro-elica (helix-turn-helix) media il riconoscimento del repressore LacI con la sequenza target. Questo dominio costa di 2 α -eliche di cui una si inserisce nel solco maggiore del DNA. Il repressore LacI si lega come tetramero. O1O1 O2O2 O3O3

6 Allolactose is the inducer, a molecule that binds to the repressor causing it to leave the DNA. Mutations in the lac operon 2) Operator mutation In most cases, mutations in operators either (O 1 ) prevent repressor binding or reduce the binding strength, resulting in constitutive expression of lac operon. 3) Mutations in enzymes genes lacZ and lacY alleles cause a lac- phenotype, whereas lacA- does not. Mutations in lacZ are polar mutations. 1) Repressor gene mutations

7 Negative and positive regulation of lac operon The CAP (catabolite activator protein), called also CRP, is able to activate the expression of the lac operon. Activation occurs only in the absence of glucose. Cyclic AMP interacts directly with CAP. When the concentration of glucose is low or absent, the concentration of cAMP is high. With no cAMP bound to it, CAP separates from DNA and looses its capacity yo stimulate lac expression. (5-3)

8 high conc. cAMP No glucose, no lactose Glucose and lactose Only lactose very low conc. cAMP CAP is inactive in absence of cAMP CAP is active in presence of cAMP


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