The document discusses the lac operon, which was first described by Francois Jacob and Jacques Monod in 1961. The lac operon includes the lacI gene, which encodes the lac repressor protein. The lac repressor protein regulates the lac operon by binding to the operator region and preventing transcription of the structural genes under normal conditions. When lactose is present, it binds to the lac repressor and causes a conformational change that prevents it from binding to the operator, allowing transcription to occur. The lac repressor is a tetramer protein made up of four identical 360 amino acid subunits with a total molecular weight of around 154,520 Daltons.

1. Lac I

2. lac Operon

3. Jacob & Monod: lac Operon
• Francois Jacob & Jacques Monod
– first to describe operon system
– coined the phrase “operon”
1961 | 1965
Francois JacobJacques Monod

4. Lac I
Lactose inhibitor

5. Lac I
Lipoprotein-associated cogulation
inhibitor

6. lacI gene
• weak promoter
• constitutively expressed at low levels
• 1040bp
• The regulatory gene lacI produces an mRNA
• that produces a Lac repressor protein
• which can bind to the operator of the lac
operon.
• lacI regulatory gene is called the lacI regulator
gene

7. In an inducible operon, the
regulatory protein is an active
repressor and is made from a
separate gene with its own
promoter)
Repressor binds to the operator,
interferes with the binding of the
RNA polymerase and prevent
transcription of the structural genes.
This is the default state.

8. The lac repressor is a protein tetramer, where all four
identical components are 360 amino acids in length .
When associated into its active tetramer form,
the repressor has a molecular weight of 154, 520
Daltons.
The repressor protein binds to a palindromic sequence of
DNA on the lac promoter at the NH2 terminus.
In this molecule, the DNA is bound to a 21 base-pair
symmetric DNA duplex (GAATTGTGAGC-
GCTCACAATT)

9. the NH2 terminal domain of the lac repressor is
involved with the DNA binding
The NH2 terminal ends can actually be subdivided into two
separate functioning domains: the DNA-binding region
(residues 1-45) and the hinge region (residues 46-62
The DNA-binding region, also known as the repressor head-
piece, contains a helix-turn- helixmotif (HTH). This motif
occurs over the first two helixes of the molecule (residues 6-
13, 17-24)
The hinge region links the DNA-binding region to the core of
the protein.

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