LAC OPERON SYSTEM

1. LAC OPERON SYSTEM

2. INTRODUCTION
The lac operon is an operon required for the transport
and metabolism of lactose in E.coli.
When lactose is absent- the lac repressor binds tightly to
the operator;
When lactose is present- the lac repressor
loses its ability to bind DNA.
This change in the lac repressor is caused by
the small molecule allolactose, an isomer of lactose.
The first operon to be described was the lac
operon in E.coli. It was discovered by Francois
Jacob, Jacques Monod, Michel Lwoff on 1960s
and on 1965, Nobel Prize was awarded to them
for their discoveries concerning the operon and
virus synthesis.

3. STRUCTURE OF LAG OPERON
Lag operon contains three genes: LacZ, LacY and LacA.
LacZ encodes β-galactosidase
LacY encodes lactose permease
LacA encodes transacetylase
In addition of the three genes, the lac operon also contains a number of
regulatory DNA sequences:
 The promoter is the binding site for RNA polymerase.
 The operator is a negative regulatory site bound by the lac repressor protein.
 The CAP binding site is a positive regulatory site that is bound by catabolite
activator protein (CAP).

4. WHEN LACTOSE IS ABSENT
Repressor protein active
Bound to the operator
Blocks the promoter site
Prevent transcription

5. WHEN LACTOSE IS PRESENT
Binds to the
repressor protein
Change it’s shape
Operator site
becomes free
Transcription
occurs