Lac Operon

CHAPTER 6

EXPRESSION OF
BIOLOGICAL

INFORMATION

6.4 Gene Regulation And Expression – Lac Operon

LEARNING OUTCOMES :

At the end of this topic, students should be
able to:

(a) Explain the concept of operon and gene
regulation

(b) Describe the components of lac operon and their
functions in E. coli

(c) Describe the mechanism of the operon in the
absence and presence of lactose

a) Concept of operon

1. In prokaryotic cell protein synthesis is

controlled by operon system.

2. An operon is a coordinated regulation unit of
transcription in bacteria which consist of a
regulatory gene and an operon.

Regulatory Operon
gene
O S1 S2 S3
RP

P = promoter
O = operator
S1,S2,S3 = structural gene
R = regulator

a) Concept of operon

What is operon?
• A group of two or more gene having related

function.

OR

• Regulation of all gene as a single unit.

Operon

RP O S1 S2 S3

P = promoter
O = operator
S1,S2,S3 = structural gene
R = regulator

a) Concept of operon

Example for operon:
lac operon (lac for lactose)

a) Gene regulation and expression happen at
the transcription stage while its translation
will produce the polypeptides which act as
the enzymes to hydrolyse lactose

Operon

RP O S1 S2 S3 DNA
transcription
P = promoter 5’ 3’
O = operator mRNA
S1,S2,S3 = structural gene
R = regulator Enzyme Enzyme Enzyme translation
1 2 3
Polypeptide

a) Concept of operon

Example for operon:
lac operon (lac for lactose)

b)Consist of lacZ, lacY, and lacA
genes as structural gene, operator and
promoter. The operon is regulated by the
product of regulatory gene (lacI)

Operon

lacI (R) P O lacZ lacY lacA DNA

P = promoter 5’ Enzyme transcription
O = operator 2
S1,S2,S3 = structural gene 3’
R = regulator
mRNA

Enzyme Enzyme translation
1 3
Polypeptide

a) Concept of operon

Regulatory (R) A gene which has the genetic information for the
synthesis of the repressor protein.

Promoter (P) DNA region where the RNA polymerase bind to
Operator (O) start transcription

Structural The repressor protein binding site. Function as a
genes (S) “switch” which activates or deactivates a
sequences of structural gene.

A gene which has the genetic information for the
synthesis of a polypeptide (an enzyme to digest
lactose).

R P O S1 S2 S3

a) Gene regulation and expression: Francois Jacob
and Jacques Monod hypothesis.

The earliest study was done by Francois Jacob
and Jacques Monod (1961)

The experiment was on the synthesis of β-
galactosidase by E.coli in the presence of
lactose

Used E.coli that lives in the human colon &
depends on the host disaccharide lactose (milk
sugar); available if the human host drinks milk.

The bacteria absorb lactose and break it down
for energy use or use it as a source of organic
carbon for synthesizing other compounds.

a) Gene regulation and expression: Francois Jacob
and Jacques Monod hypothesis.

They cultured E.coli in media containing glucose and a few
inorganic salts.
Result: The bacteria grew very fast.

The bacteria were then transferred to media containing the
disaccharides sugar lactose.
Result: The cell began to produce new enzyme, called β-
galactosidase

So Jacob and Monod explained that β-galactosidase was only
produced when it was needed, although the corresponding gene
is always present within the bacterial chromosome.

They conclude that the gene for this enzyme is said to be induced
by the presence of appropriate substrate (lac operon-lactose)

b) Lactose/ Lac operon

• lac operon is a section of DNA that acts as an on/off

switch for genes that control the metabolism of lactose.

• Lactose is not a common sugar, so there is not a great

need for these enzyme.

• Lactose present = lac operon switch on (produces

enzyme)

• Lactose absent = lac operon switch off (no enzyme

produces)

b) Lactose/ lac operon

• lac operon found in E.coli consist of a promoter, an

operator, and three structural genes lacZ (S1), lacY (S2)
and lacA (S3).

• Regulatory gene (lacI) is located upstream of the lac

operon and encoded for repressor protein that bind to
the operator.

Operon

lacI P O lacZ (S1) lacY (S2) lacA (S3) Bacteria
(R) DNA

P = promoter
O = operator
S1,S2,S3 = structural gene
R = regulator

b) Lactose/ lac operon: components and its functions in
E.coli

Regulatory gene lac operon

R P O lacZ (S1) lacY (S2) lacA (S3)

Gene Function
P
O Promoter : site where RNA polymerase bind

lacZ Operator : Site where repressor protein bind
lacY
Gene encoding for ß- Converts lactose into glucose and
lacA galactosidase galactose
Gene encoding for Lactose
permease Transports lactose into the
cell and increase the permeability
Gene encoding for transacetylase of membrane to lactose.

Transfers an acetyl group from acetyl
CoA to β-galactosidase

c) Lactose/ lac operon: Mechanism

1. In the ABSENT of lactose
2. In the PRESENT of lactose.

c) Lactose/ lac operon: Mechanism

Lactose absent

1. Repressor protein

binds to the operator
of lac operon

2. Thus prevent RNA

polymerase from
binding to the
promoter

3. Transcription is

blocked by repressor-
operator complex.

c) Lactose/ lac operon: Mechanism

Lactose absent

4. lac Z, lacY and lac A are

not transcribed

5. β-galactosidase, lactose

permease and
transacetylase enzymes
are not produced.

c) Lactose/ lac operon: Mechanism

Lactose present

1. Some of lactose are

converted into
allolactose (isomer of
lactose)

2. Allolactose will binds to

(allosteric site on) the
repressor protein

3. Thus, changes the

molecular conformation
of repressor protein.

c) Lactose/ lac operon: Mechanism

Lactose present

4. Repressor protein

cannot binds to the
operator.

5. Allows RNA

polymerase to binds
to the promoter

6. lacZ, lacY and lacA

can be transcribed.

c) Lactose/ lac operon: Mechanism
Lactose present

7. ß-galactosidase, lactose permease and transacetylase

enzyme are produced.

8. Lactose is broken down into glucose and galactose.

How to memorize easily….
Mechanism of the lac operon

•In the present of lactose
•In the absent of lactose

Repressor protein
Lac Y

Regulatory
gene

Promoter Operator Lac Z Lac A

When lactose is present. Finger show “GOOD” sign

• will NOT bind at
• Lac operon is

Figure 3: Transcription occur
• Lac Z encode for β - galactosidase

Figure 4: Transcription occur
• Lac Y encode for

Figure 5: Transcription occur
• Lac A encode for

Figure 6: Lactose is absent.

•RP will bind at O.
• Transcription will NOT occur.
• Enzymes will NOT produce.