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1
The lac operon
? This enzyme is not essential for lactose
utilization
gal operon
Glucose-1-phosphate
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 O1 and lesser affinity to O2 and
  O3
• 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 a-eliche di cui una si
inserisce nel solco maggiore del DNA. Il
repressore LacI si lega come tetramero.
O3
O2
O1
6
Allolactose is the inducer, a molecule that binds
to the repressor causing it to leave the DNA.
Mutations in the lac operon
1) Repressor gene mutations
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.
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
No glucose, no lactose
Glucose and lactose
CAP is inactive in absence of cAMP
very low conc. cAMP
Only lactose
high conc. cAMP
CAP is active in presence of cAMP