Transcript Inducible Genes - Lactose Operon

Regulation of gene
expression
References:
1. Stryer: “Biochemistry”, 5th Ed.
2. Hames & Hooper: “Instant Notes in
Biochemistry”, 2nd Ed.
1. Basic principles of gene control
What is gene expression?
It is a process by which a gene's
information is converted into the structures
and functions of a cell.
For a protein encoding gene, it is a process
of the transcription of a gene into mRNA,
and the latter is translated into protein.
How to control gene expression?
In prokaryotic cells, gene expression is
regulated primarily at the level of
transcription by metabolites;
In eukaryotes it is controlled by much
more complex mechanisms, including
the action of protein factors, hormones,
etc.
Control of gene expression in prokaryotes
Structural gene
R
DNA

R
mRNA
Enzyme
Product
Substrate
Control of gene expression in eukaryotes
Transcript.
control
1
DNA
Nucleus
RNA
synthesis
2
RNA
processing
control
RNA
transport
control
3
mRNA
Inactive
mRNA
Cytosol
5
mRNA
degradation
control
mRNA
Protein
activity
control
6
Translation4
control
Protein
Inactive
Protein
2. Regulation of Prokaryotic gene
expression– the operon theory
 Operons are groups of genes that
function to produce proteins needed by
the cell.
 There are two types of genes in operons
A. Structural genes code for proteins
needed for the normal operation of the
cell.
B. Regulator genes code for proteins that
regulate other genes.
Lactose Operon
PlacI
lac I
Plac
Olac
lac Z
lac Y
lac A
Plac : promoter; Olac: operator;
lac Z: b-galactosidase;
lacY: permease;
Lac A: transacetylase;
lac I: structural gene for lac repressor;
PlacI : promoter for lac I.
Regulation of the lac operon in E. coli.
p
i
Repressor
mRNA
Repressor
The lact Operon
p o
y
z
a
The repressor binds to the
operator region and prevents
RNA polymerase from
transcribing the structural
genes
In the presence of lactose
p
Repressor
mRNA
i
The lact Operon
p o
y
z
a
mRNA
b-galactosidase permease transacetylase
Repressor
Lactose
Lactose binds the repressor and
allows RNA polymerase access to
the operator, resulting in
transcription of the structural gene
In the presence of glucose & lactose
glucose

Adenylcyclase
cAMP
p
i
ATP
The lact Operon
p o
y
z
a
Lactose
Low level of transcription
In the absence of glucose and presence
of lactose
CAP
Adenylcyclase
cAMP
p
i
ATP
The lact Operon
p o
y
z
active
a
Lactose
High level of transcription
CAP: catabolite activator protein
Tryptophan Operon
The trp operon encodes the genes for the
synthesis of tryptophan
Genes for enzymes
P,O
L
a
E
D
C
B
A
P: promoter, O: operator, a: attenuator,
L: leader sequence, A,B,C,D,E: enzymes
of the Trp pathway
Regulation of the trp operon in E. coli.
Binding of Trp to the trp repressor
increases the activity of the repressor,
and therefore Trp is known as a corepressor.
Expression of the trp operon is also
regulated by attenuation. The attenuator
region contains tandem Trp codons,
which play a role to attenuate the
transcription of the following genes.
Regulation of the trp operon