it will be helpful to various medical course students

1. REGULATION OF GENE EXPRESSION
INTRODUCTION
• Genome refers to the total genetic information contained in a cell.
• The bacterium Escherichia coli contains about 4,400 genes present on a single
chromosome.
• The genome of humans is comprised of 23 pairs of (diploid) chromosomes
containing 6 billion (6x109) base pairs of DNA, with an estimated 30,000-
40,000 genes.
• Information encoded in DNA, is transcribed into RNA and then translated into
protein. It is called the gene expression.
• The living cells possesses a remarkable property to adapt to changes in the
environment by regulating gene expression.
• The expression of the genetic information must be regulated during
adaptation, development and differentiation.
• For instance, insulin is synthesized by specialized cells of pancreas and not by
cells of other organs (say kidney, liver), although the nuclei of all the cells of
the body contain the insulin genes.
• Molecular regulatory mechanisms facilitate the expression of insulin gene in
pancreas, while preventing its expression in other cells.

2. TYPES OF GENE REGULATION
• There are two types of gene regulation.
1. Positive regulation
• When the expression of genetic information is quantitatively increased by
the presence of a regulatory element, regulation is said to be positive.
• The element or molecule mediating the positive regulation is said to be an
activator or inducer.
2. Negative regulation
• When the expression of genetic information is diminished by the presence of a
specific regulatory element, regulation is said to be negative and element
mediating negative regulation is a repressor.

3. TYPES OF GENE
• There are two types of genes inducible and constitutive gene.
1. Inducible genes
• Inducible genes are expressed only when a specific positive regulatory
substance, i.e. an inducer or activator is present, e.g. the production of the
enzyme β-galactosidase is induced by the presence of lactose in the
prokaryotes or insulin is an inducer of the gene glucokinase of glycolysis in
human being.
2. Constitutive genes
• Constitutive genes refer to genes whose expression is not regulated.
• They are expressed at a constant rate.
• These are often referred to as housekeeping genes.

4. REGULATION OF GENE
EXPRESSION IN PROKARYOTES

5. THE OPERON
• In prokaryotes, the genes involved in a metabolic pathway are often present in a
linear fashion, called an operon i.e. a set of clustered genes in bacteria.
• An operon includes
1. The adjacent structural genes that code for the related enzymes or associated
proteins.
2. A regulatory gene that encodes a repressor protein which binds to the operator
site.
3. Control elements that are the sites on the DNA near the structural genes, at which
regulatory proteins act.
• The concept of operon was introduced by Jacob and Monod in 1961 based on their
observations on the regulation of lactose metabolism in E.coli.
• For example:
i. Lactose operon (Lac operon for regulation of lactose metabolism).
ii. Arabinose operon (Ara operon for regulation of arabinose metabolism).
iii. Galactose operon (Gal operon for regulation of galactose metabolism).
iv. Tryptophan operon (Trp operon for regulation of tryptophan metabolism)

6. LACTOSE (Lac) OPERON
STRUCTURE OF LAC OPERON
• The lac operon consists of a regulatory gene (I; I for inhibition), operator gene
(O) and the structural genes (Z, Y, A).
• Besides these three genes, there is a promoter site (P), next to the operator
gene, where the enzyme RNA polymerase binds.
• The structural genes Z, Y, A respectively, code for the enzymes β-galactosidase,
galactoside permease and galactoside acetylase.
• β-galactosidase hydrolyzes lactose (β-galactoside) to galactose and glucose
while permease is responsible for the transport of lactose into the cell.
• The function of acetylase (coded by A gene) remains a mystry.
• The structural genes Z, Y and A transcribe into a single large
mRNA(polycistronic mRNA) with 3 independent translation units for the
synthesis of 3 distinct enzymes.

7. REPRESSION OF LAC OPERON
• The regulatory gene (I) is constitutive.
• It is expressed at a constant rate leading to the synthesis
of lac repressor.
• Lac repressor is a tetrameric (4 subunits) regulatory
protein which specifically binds to the operator gene (O).
• This prevents the binding of the enzyme RNA polymerase
to the promoter site (P), thereby blocking the
transcription of structural genes (Z, Y, A).
• This is what happens in the absence of lactose in E.coli.
• The repressor molecule acts as a negative regulator of
gene expression.

8. DEREPRESSION OF LAC OPERON
• In the presence of lactose (inducer) in the medium, a small amount of it can
enter the E.coli cells.
• The repressor molecules have a high affinity for lactose.
• The lactose molecules bind and induce a conformational change in the
repressor.
• The result is that the repressor gets inactivated and, therefore, cannot bind to
the operator gene (O).
• The RNA polymerase attaches to the DNA at the promoter site and
transcription proceeds, leading to the formation of polycistronic mRNA (for
genes Z, Y and A) and, finally, the 3 enzymes.
• Thus, lactose induces the synthesis of the three enzymes β-galactosidase,
galctoside permease and galactoside acetylase.
• Lactose acts by inactivating the repressor molecules, hence this process is
known as derepression of lac operon.

10. GRATUITOUS INDUCERS
• There are certain structural analogs of lactose
which can induce the lac operon but are not the
substrates for the enzyme β-galactosidase. Such
substances are known as gratuitous inducers.
• Isopropylthiogalactoside (IPTG) is a gratuitous
inducer, extensively used for the study of lac
operon.

11. THE CATABOLITE GENE ACTIVATOR PROTEIN
• The cells of E. coli utilize glucose in preference to lactose; when both of them are
present in the medium.
• After the depletion of glucose in the medium, utilization of lactose starts.
• The attachment of RNA polymerase to the promoter site requires the presence of
a catabolite gene activator protein (CAP) bound to cyclic AMP.
• CAP is a DNA-binding protein, also called cAMP-response protein (CRP), since CAP
can bind to cAMP and form a complex with it.
• It is the CAP-cAMP complex that binds to the promoter and not the CAP alone.
• The presence of glucose lowers the intracellular concentration of cAMP by
inactivating the enzyme adenylyl cyclase responsible for the synthesis of cAMP.
• Due to the diminished levels of cAMP, the formation of CAP-cAMP is low.
• Therefore, the binding of RNA polymerase to DNA ( due to the absence of Cap-
cAMP) and the transcription are almost negligible in the presence of glucose.
• Thus, glucose interferes with the expression of lac operon by depleting cAMP
levels.
• Addition of exogenous cAMP is found to initiate the transcription of many
inducible operons, including lac operon.

13. TRYPTOPHAN OPERON
• The tryptophan operon of E. coli contains five structural genes
(trpE, trpD, trpC, trpB, trpA), and the regulatory elements –
primary promoter (trpP), operator (trpO), attenuator (trpa),
secondary internal promoter (TrpP2), and terminator (trpt).
• The five structural genes of tryptophan operon code for three
enzymes (two enzymes contain two different subunits)
required for the synthesis of tryptophan from chorismate.
• The tryptophan repressor is always turned on, unless it is
repressed by a specific molecule called corepressor. Thus
lactose operon is inducible, whereas tryptophan operon is
said to be derepressed when it is actively trnascribed.

14. Tryptophan operon regulation by a repressor
• Tryptophan acts as a corepressor to shut down the synthesis of enzymes from
tryptophan operon. This is brought out in association with a specific protein,
namely tryptophan repressor.
• Tryptophan repressor, a homodimer (contains two identical subunits) binds
with two molecules of tryptophan, and then binds to the trp operator to turn
off the transcription. It is of interest to note that tryptophan repressor also
regulates the transcription of the gene (trpR) responsible for its own synthesis.
• Two polycistronic mRNAs are produced from tryptophan operon- one derived
from all the five structural genes, and the other obtained from the last three
genes.
• Besides acting as corepressor to regulate tryptophan operon, tryptophan can
inhibit the activity of the enzyme anthranilate synthetase. This is referred to as
feedback inhibition and is brought out by binding of tryptophan at an allosteric
site on anthanilate synthetase.

15. Attenuator as the second control site for
tryptophan operon
• Attenuator gene (trpa) of tryptophan operon lies upstream of trpE gene.
Attenuation is the second level of regulation of tryptophan operon. The
attenuator region provides RNA polymerase which regulates transcription.
• In the presence of tryptophan, transcription is prematurely terminated at the
end of attenuator region. However, in the absence of tryptophan, the
attenuator region has no effect on transcription. Therefore, the polycistronic
mRNA of the five structural genes can be synthesized.

17. Parameter Prokaryotes Eukaryotes
Genome type Smaller; 4.6 x 106 bases in
E.coli
Larger; 3000 x 106 bases in
humans
Cell types Usually a single type Different cell types (e.g.
liver cells, pancreatic cells);
each contains the same set
of genes but in different
cells they are differentially
expressed.
Operons Present Absent
Transcription and
translation
Coupled uncoupled
Untranscribed DNA Very small amount large amount
Overall control of gene
expression
Simple complex