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

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. O3 O1 O2

Mutations in the lac operon 1) Repressor gene mutations 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.

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