glucose-1-phosphate gal operon the lac operon this enzyme is not essential for lactose utilization
TRANSCRIPT
Glucose-1-phosphate
gal operon
The lac operon
This enzyme is not essential for lactose utilization
La presenza di lattosio e/o glucosio controlla l’espressione dei geni lac
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 O1 and lesser affinity to O2 and O3
- The lac operon is occasionally transcribed also in the presence of LacI repressor
The mRNA transcribed from lac operon is very unstable
L’operatore 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.
O1 O2
O3
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 (O1) 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
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’)
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