Gene regulation can be defined as any kind of alteration in the gene to give rise to a different expression which might result in a change in the synthesized amino acid sequence.” Gene expression is basically the synthesis of the polypeptide chain encoded by a particular gene. Therefore the expression of the gene can be quantified in terms of the amount of protein synthesised by the genes.

1. REGULATION OF GENE EXPRESSION IN
PROKARYOTES – LAC OPERON
Dr. M.Sankareswaran
Assistant professor, Department of Microbiology.

2. REGULATION OF GENE EXPRESSION
 Gene regulation can be defined as any kind of alteration in
the gene to give rise to a different expression which might
result in a change in the synthesized amino acid
sequence.”
 Gene expression is basically the synthesis of the
polypeptide chain encoded by a particular gene.
 Therefore the expression of the gene can be quantified in
terms of the amount of protein synthesised by the genes.

3. Importance of Gene Regulation
 There are two types of gene action
 constitutive and
 regulated.
 The constitutive gene action occurs in those systems which operate all the
time and the cell cannot live without them, e.g., glycolysis. It does not
require repression. Therefore, regulator and operator genes are not
associated with it.
 In regulated gene action all the genes required for a multistep reaction
can be switched on or off simultaneously.
 The genes are switched on or off in response to particular chemicals
whether required for metabolism or formed at the end of a metabolic
pathway.

4. Regulation of Gene Expression
Takes place in
 Replication– Any error in copying the DNA may result in an altered expression.
 Transcriptional – During transcription, any error in the polymerization may again
lead to a change in expression of the gene.
 Post-transcriptional – During the post-transcriptional modification i.e., RNA splicing,
there may be some changes.
 Translational - During translation, if there is an error in the attachment of mRNA to
the tRNA molecules, there may arise some changes.

5. Types of regulation
1.Inducible Regulation- Generally for regulation of catabolic
pathways Inducer molecule is needed for switching on the
operon. The product of regulator gene ,the active repressor
switch offs the operon . Example –Lac (Lactose ) Operon:
Negative control
2. Repressible Regulation Generally for regulation of anabolic
pathways, the product of regulator gene is inactive
repressor ,So the operon is switched. Example
Trp(Tryptophan) Operon: Positive control.

6. Operon
 In bacteria cistrons or structural genes, producing enzymes
of a metabolic pathway are organised in a cluster whose
functions are related.
 Polycistronic genes of prokaryotes along with their
regulatory genes constitute a system called operon.
 Operon is a unit of expression and regulation.

7. Terms involved in Operon
Repressors are proteins that suppress transcription
Activators are proteins that increase the transcription
of a gene
Inducers are small molecules that either activate or
repress transcription
Repressible enzymes are necessary for synthesis.
Regulatory proteins Proteins that exert either negative
or positive control.
Activator proteins protein promotes transcription
initiation
Operator specific sites of DNA binding (Orgin)
The structural genes — the genes coding for
polypeptides.

8. Lactose Operon or Lac Operon
 Lactose is a disaccharide molecule.
 In order to utilize lactose as a carbon and energy source, the lactose
molecules must be transported from the extracellular environment into the
cell, and then undergo hydrolysis into glucose and galactose.
 “Lac operon is an operon or a group of genes with a single promoter that
encode genes for the transport and metabolism of lactose in E.coli and other
bacteria.”
 This is a negative control mechanism.
 In 1961 Francois Jacob and Jacques Monod proposed operon model for the
regulation of gene expression in E. coli.

9. Components of Lac operon

10. Regulatory genes
 The regulatory genes of lac operon includes
 Promoter gene
 Operator gene,
 Regulator - Lac I, and
 Catabolite activator protein (CAP) binding site.

11.  i. The promoter is the binding site for RNA polymerase, the
enzyme that performs transcription.
 ii. The operator is a negative regulatory site where the lac
repressor protein binds. It is located between the promoter and
the structural genes.
 iii. Lac I (repressor) gene codes for the lac operon repressor,
which is located adjacent to the promoter of the lac operon.
LacI encodes an allosteric repressor protein that keep the Lac
operon “off”. The lac repressor binds DNA and repress
transcription only in the absence of lactose
 iv.Catabolite Activator Protein (CAP) is a dimer protein, which
has binding sites for cAMP and DNA. CAP acts as an activator.
CAP binds DNA and activate the lac genes in the absence of

12. Structural genes
 The genes, which code for these enzymes lie in a cluster and are called cistrons or
structural genes.
 They are transcribed simultaneously into a single mRNA chain, which has codons for
all the three enzymes.
 There are three structural genes Z, Y and A.
 lac Z gene — codes for enzyme β galactosidase which breaks lactose into
galactose and glucose
 lac Y gene — codes for permease which transports lactose into the cell
 lac A gene — codes for transacetylase which transfer the acetyl group from
acetyl CoA to galactose.

14. Mechanism
 When the inducer (lactose) in supplied from outside, the
inducers binds to the repressor.
 The lactose on entering the bacteria changes into
allolactase.
 Allolactose changes the shape of the repressor
(conformational changes) which renders it inactive and
unable to bind to the operator.
 The operator becomes free and is “turned on” and thus
transcription starts.

15. Control of Gene Expression in prokaryotes
 In prokaryotes, the control mechanism is controlled by two ways:
Positive control
Negative control

16. Positive Control of Lac-Operon
 It also refers to a Positive inducible system.
 The regulatory gene is expressed by the repressor.
 After expression of a regulatory gene, the repressor proteins produces.
 Repressor protein has binding sites for the operator and the inducer i.e. allolactose.
 Therefore, when allolactose is present as an inducer it binds with the repressor
protein and forms “R+I complex”.
 After the binding of inducer to the repressor, it blocks the binding of the repressor to
the operator.
 As the repressor protein does not block the operator, the RNA polymerase binds to
the promotor and moves further to transcribes mRNA.
 This concept is known as “switch on” of Lac-operon (by the presence of inducer).

18. Negative Control of Lac-Operon
 This also refers to Negative control of repressor system
 First, the regulatory gene is expressed by the repressor.
 After expression of a regulatory gene, the repressor proteins produces.
 In the absence of inducer or lactose the, repressor protein directly binds to an
operator.
 This blocks the movement of RNA polymerase and its attachment to the promoter.
 At last, inhibits the mRNA transcription.
 This concept is known as “switch off” of Lac-operon (by the absence of inducer).

20.  In presence of lactose
repressor protein binds to allactose and forms Repressor-Lactose complex.
RNA polymerase binds to P site and genes transcribed.
 In absence of lactose
repressor protein binds to O site of lac operon , block the RNA
polymerase from reaching P site. No transcription

21. Lac operon in the presence of Glucose
 When glucose is present, E. coli does not need to use lactose as a carbon
source and so the lac operon does not need to be active.
Lac operon in the absence of Glucose
 When glucose is absent, adenylate cyclase is not inhibited, the level of
intracellular cAMP rises and binds to CRP.
 Therefore, when glucose is absent but lactose is present, the CRP–cAMP
complex stimulates transcription of the lac operon and allows the lactose
to be used as an alternative carbon source.
 .

22. CRP/CAP
 High-level transcription of the lac operon requires the presence of a specific
activator protein called catabolite activator protein (CAP), also called cAMP
receptor protein (CRP).
 This protein, which is a dimer, cannot bind to DNA unless it is complexed with 3’5′
cyclic AMP (cAMP).
 The CRP–cAMP complex binds to the lac promoter increases the binding of RNA
polymerase and so stimulates transcription of the lac operon.
 Whether or not the CRP protein is able to bind to the lac promoter depends on
the carbon source available to the bacterium.