Lac operon sohil

Synthesis of protein under the influence of gene is
called gene expression
All genes of cell are not expressed at all the time
for example any hormone ( Eg Insulin)

It is a combined process of the
transcription of a gene into mRNA , the
processing of that mRNA and its
translation into protein
The gene is expressed in the form of
protein
Prokaryotic gene is controlled mainly
at the level of transcription m-RNA
synthesis

Regulation of gene expression is
absolutely essential for the growth,
development and the very existence
of an organism
They are of two type :-
1.Positive Regulation (Inducers)
2.Negative Regulation(Repressors)

There are two types of genes -
1.constitutive gene-they are present in almost each
and every cell , therefore also know as housekeeping
genes
 Example- citric acid cycle
2. Inducible gene – they are synthesized when
molecular signals are given
 Example – Tryptophan pyrrolase of liver is induced
by tryptophan
Inducible genes are subject to regulated expression

 The chemical product of gene expression is a protein which
may be an enzyme
Each gene codes for specific enzyme-
One gene –one enzyme , where as several enzymes are
composed of more than two non-identical subunits
 The cistron is the smallest unit of genetic expression .It is
the fragment of DNA coding for the subunit of a protein
molecule.
Thus the original concept one gene-one enzyme is replaced
by one cistron-one subunit

Prokaryotes have a simple mechanism
for coordinating the regulation of gene
.In prokaryotes the gene are clustered
on chromosome and transcribed
together
In prokaryotes the gene involved, in
metabolic pathway are often present in
a linear fashion called an operon
Example –
Lactose operon

OPERON
 Operon is a functioning unit of DNA containing a cluster
of genes under the control of single promoter
 Its Unit of gene expression, which includes structural
genes, control elements, regulator/inhibitor gene,
promoter and operator area

Schematic Diagram of LAC
OPERON

Regulatory
gene(Lac
i)
• Produces a
repressor
protein
Promoter
site(P)
• CAP site
• Site for entry of
RNA polymerase
Operator
site
• LAC repressor
binds here
Structural
genes
• Z-galactosidase
• Y-galactosidase
permease
• A-Transacetylase

Ai P O Z Y A
RNA
polymeras
e
Galactosidase

Ai P O Z Y A
RNA
polymeras
e
Galactosidase
Permease

Ai P O Z Y A
RNA
polymeras
e
Galactosidase
Permease
Transacetyla
se

Ai P O Z Y A
RNA
polymeras
e
Galactosidase
Permease
Transacetylase

Type of
regulation
Regulated
by
Lactose Glucose
Negative
regulation
Lac
repressor
Absent Present
Positive
regulation
Induction of
expression
Present Absent
Positive
control
Catabolite
repression
Present Present

Ai P O Z Y A
Repressor
protein

Ai P O Z Y A
Repressor
protein
RNA
polymer
ase

Ai P O Z Y A
Repressor
protein
RNA
polymer
ase
Lactose

Ai P O Z Y A
Repressor
protein
RNA
polymer
ase
Lactose
Galactosidase
Permease
Transacetyla
se

Ai P O Z Y A
RNA
polymer
ase
Lactose
Lactose
Lactose
Lactose
Lactose
glucose
glucose
glucose

Ai P O Z Y A
RNA
polymer
ase
Lactose
Lactose
Lactose
Lactose
Lactose
cAMP

Ai P O Z Y A
RNA
polymer
ase
Lactose
Lactose
Lactose
Lactose
Lactose
cAMPCAP

Ai P O Z Y A
RNA
polymer
ase
Lactose
Lactose
Lactose
Lactose
Lactose
cAMPCAP
Galactosidase
Permease
Transacetyla
se

Lactase in human intestine is an inducible enzyme.
Clinical manifestations of lactase deficiency and lactose
intolerance are described.
Examples of derepression in human beings:
1. Induction of tryptophan pyrrolase by tryptophan
2. Transaminases by glucocorticoids
3. ALA synthase by barbiturates
4. Glucuronyl transferase by barbiturates.
Clinical Applications of Derepression

Repression is the mechanism by which the presence of excess
product of a pathway shuts off the synthesis of the key
enzyme of that pathway.
Heme synthesis is an example. It is regulated by repression of
ALA synthase, the key enzyme of the pathway.
Regulation of Genes by Repression

Repression of heme synthesis by heme.
(ALA: aminolevulinic acid; RNAP: RNA polymerase)

5 structural genes
2 polycistronic
mRNAs
3 enzymes

TRP Operon is a group of genes encode
biosynthetic enzyme for amino acid Trp
Found in E Coli.
Repressible Operon
Eg of Feedback Inhibition
Trp Operon expressed on- when trp level
is low
Repressed when- trp level is high

Tryptophan
repressor
2
Molecules
of
tryptophan
Repressor

content
 TRP operon consists of:
1. 5 structural genes:
 Trp e
 Trp d
 Trp C
 Trp B
 Trp a
2. Regulatory Gene
3. Promoter
4. Operator
5. Terminator

Turning on and turning off
 Trp operon is normally ON.
 If surrounding medium contains Large amount of Trp:
Switched OFF.
 Trp repressor regulates Synthesis of Trp

Mechanism of on and off
 Trp repressor is bind with DNA of operator and block the
transcription of structural gene.
 Trp repressor doesn’t always bind to DNA instead it bind and block
Transcription only when Trp is present
 When Trp is around it, attaches to the repressor molecule and
change their structural Configuration
 Small Molecule like Trp Which Switches a repressor into its Active
state is called Co-represser
 Trp repressor(inactive) + Co-represser = Active repressor.
 Active repressor bind with Operator to block Transcription

Attenuation
 Mechanism based on Coupling of Transcription and
Translation.
 Trp operon is a classical example of Attenuation.
 Attenuator Region: Additional control site for regulation
of transcription

Continued…
 If any of RNA polymerase escape repression at the
operator, transcription begins at the Promoter.
 But, due to presence of TRP a region, transcription is
prematurely terminated at Attenuter region
 In absence of TRP, attenuter has no effect on
transcription and Transcription proceed as usual

GENE EXPRESSION IN EUKARYOTES
 The important features of eukaryotic gene expression
along with the regulatory aspects are described in the
eukaryotic cells also employ variety of other mechanisms
to regulate gene expression. The most important ones
are listed below , and briefly described next

Continued…
 1. Gene Amplification
 2.Gene Rearrangment
 3.Processing of RNA
 4. Alternate mRNA splicing
 5.Transport of mRNA from nucleus to cytoplasm
 6.Degradation of mRNA

Gene Amplification
 Methotrexate resistance by cancer cells
 Methotrexate inhibits the enzyme dihydrofolate
reductase.
 The malignant cells develop drug resistance to long
term administration of methotrexate by amplifying the
genes coding for dihydrofolatereductase
 Result for active folate more replication

Processing of mRNA
 Splicing
 Capping
 Poly (A) Tail

Alternate mRNA splicing
For example, tropomyosin (TM) is an actin filament-binding
protein that regulates the functions of actin in both muscle
and nonmuscle cells.

Degradation of mRNA
 Prevented by Capping and poly (A) Tail
 Also by Stem loop in Non coding region
 Certain hormone acts by acting on protein that
regulates formation of above factors

Co and Post transcriptional
regulation-mRNA Editing
 apoprotein (apo) B—an essential protein component of
chylomicrons and very low density lipoproteins.
 Apo B mRNA is made in the liver and the small intestine;
however, in the intestine only, the C residue in the codon
(CAA) for glutamine is deaminated to U, changing the sense
codon to a nonsense or stop codon (UAA)
 This results in a shorter protein (apo B-48, representing 48%
of the message) being made in the intestine (and
incorporated into chylomicrons) than is made in the liver
(apo B-100, full-length, incorporated into VLDL)

Trans acting regulation –
locally such as TFs

Cis Acting regulation-
Hormone ( Not localised)

At translation level
Phosphorylation of eIF-2 inhibits
its function and so inhibits
translation at the initiation step

Post Translationally
Hypermethylation inhibits
gene expression

Lac operon sohil

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