2. INTRODUCTION
What is gene regulation?
Regulation of gene expression, or gene regulation, includes a wide range of
mechanisms that are used by cells to increase or decrease the production of
specific gene products like proteins.
• It is the process of turning genes ‘on’ and ‘off’.
• Gene regulation ensures that the appropriate genes are expressed at the
proper times.
• Gene regulation in prokaryotes is most extensively observed at the initiation
of transcription or occurs at the transcriptional level.
3. TRANSCRIPTIONAL
REGULATION
• To understand how gene expression is regulated,
we must first understand how a gene codes for a
functional protein in a cell.
• Prokaryotic cell lack nucleus and their DNA
therefore floats freely in the cell cytoplasm.
• So to synthesize a protein,the processes of
transcription and translation occur almost
simultaneously.
• When the resulting protein is no longer needed,
transcription stops, and when more protein is
required, more transcription occurs.
4. CONSTITUTIVE, INDUCIBLE , AND
REPRESSIVE GENE EXPRESSION
1. Constitutive gene expression:- Genes are always active in this type. Gene
products such as tRNA molecules.,ribosomal protein ,rRNA molecule that are referred as
cellular housekeeping function are essential components of all living cells.So the Genes that
specify this type of gene products which are continuously expressed in most cell are known
as constitutive genes.
2. Inducible gene expression:- The process of turning on the expression of Genes in
response to a substance in the environment is called induction. Genes whose expression is
regulated in this manner are called inducing genes;and their products ,enzymes ,are called
inducible enzymes.
3. Repressive gene expression:- When E.coli cells are present in an environment
containing enough tryptophan to support optimal growth,the continued synthesis of the
tryptophan biosynthetic enzymes would be waste.Thus a regulatory mechanism has evolved
in E.coli that turns off the synthesis of tryptophan.So a gene whose expression has been
turned off in this way is said to be “repressed” and the process is called Repression.
5. PRINCIPLES OF TRANSCRIPTIONAL
REGULATION
• Gene regulation is controlled by regulatory proteins which are of two types:
1. Positive regulator or activators
2. Negative regulator or repressor
• TYPES OF GENE REGULATION
1. Positive regulation:- In this case ,the genes are expressed only
when an active regulator protein e.g. an activator, is present,if
not operon will turn off.
2. Negative regulation:- in this case ,the genes in the operon are
expressed unless they are switched off by a repressor
protein.Thus the operon will turn on when the repressor is
inactivated.
6.
7.
8. • In prokaryotes, operon model is
used explain gene expression.
• In 1961,Francois Jacob and
Jacques Monod proposed the
operon model of gene regulation in
bacteria.
• The model was based on their study
of the genes in E.coli that code for
enzymes that affect the breakdown
of lactose.
• Lac operon
1. An operon is a regulatory
unit consisting of a few
structural genes under the
control of one promoter.
2. It encodes polycistronic
mRNA that contains the
coding sequences for two or
more structural genes.
9. • Lac operon consist of two types of
genes
1. Structural genes :- lac Z:-encodes
the enzyme beta-galactosidase
which hydrolyzed lactose to form
glucose and galactose and also
converts lactose into allolactose.
• lacY:-It encodes the enzyme beta-
galactoside permease which facilitate
the entry of lactose into the cell
• lacA:- encodes the enzyme beta-
galactoside transacetylase. It transfers
an acetyl grp from acetyl CoA to
toxic beta-galatosidase
• Regulatory genes include
1. LacI :-It encodes repressor
protein that binds to the
operator.
2. Promoter :-It serves as the
binding site for RNA
polymerase.
3. Operator :-It serves as the
binding site for repressor
protein
• Regulatory genes regulate the
transcription of structural
genes.
10. • Lac operon occurs only when two conditions are favoured:-
1. Availability of lactose. (detected by lac repressor act as lactose sensor)
2. Glucose not available.( detected by CAP acts as glucose sensor)
• In presence of lactose:-
• When lactose is available, some molecules will converted into allolactose (an
isomer of lactose) which binds to the lac repressor and change its conformation
to inhibit the binding to DNA.
• The lac repressor is the protein that inhibits the transcription and loses its ability
to bind DNA and floats off the operator in presence of lactose.
• Allolactose is an inducer that turns on the lac operon which is usually off, so it is
also known as inducible operon.
13. • In absence of lactose:-
When lactose is not present ,
the lac repressor binds tightly
to the operator and prevents
transcription of RNA
polymerase (sigma 70
factor)and lacZ, lacY and lacA
genes.
14. 1. Presence of high glucose:-When glucose is present,
then the rate of transcription initiation is very low,
resulting in synthesis of only low levels of lac mRNA
and the protein.
• Glucose inhibits the synthesis of the cAMP , and CAP
cannot bind DNA without cAMP and remains
inative.(cyclic adenosine monophosphate).
2. Absence of glucose:- Once the glucose depleted
from the media and the intracellular glucose conc.falls
.cells respond by synthesizing cyclic cAMP.
• As the concentration of cAMP increases ,it binds to the
CAP protein , causing a conformational change that
allows the protein to bind to the CAP site in the lac
transcriptional control region.
• The bound CAP- cAMP complex interacts with the
polymerase bound to the promoter,stimulating the rate
of transcription initiation.This activation leads to
synthesis of high levels if lac mRNA and subsequently of
the protein encoded by lac operon.
15. ROLE OF TRYPTOPHAN REPRESSOR IN GENE
REGULATION
• Ecoli can synthesize tryptophan using enzymes that are encoded by
five structural genes located next to each other in trp operon.
• When environmental tryptophan is low,the operon is turned on,
this means that transcription is initiated ,the genes are expressed.
• The repressor itself does not bind to the operator and tryptophan
is synthesized.
• However, when tryptophan accumulates in the cell,two molecule
bind to the trp repressor molecule, which changes its
shape,allowing it to bind to the operator and blocks RNA
polymerase from transcribing the structural genes, stopping
expression of the operon.
22. TRANSLATIONAL CONTROL OF GENE
EXPRESSION
• Gene expression in prokaryotes is regulated predominantly at the level of
transcription,fine-tuning often occurs at the level of translation.
• In prokaryotes, mRNA molecules are multigenic ,carrying the coding sequences of
several genes.For example , the E.coli lac operon mRNA harbors nucleotide
sequences encoding beta-galactosidase,beta –galactoside permeate etc .thus the
three genes encoding these proteins must be turned on and off together at the
transcriptional level.
• Nevertheless, the three genes products are not synthesized in equal amount as in
Ecoli cell that is growing on rich medium with lactose as the carbone source
contains about 1000 molecule of beta-galactosidase, 1500 molecule of beta-
galactoside permeate and 600 molecule of beta-galactoside transacetylase.
• Clearly, the different molar quantities of these proteins per cell must be controlled
posttranscriptionally.
23. DIFFERENCE BETWEEN PROKARYOTIC
AND EUKARYOTIC GENE REGULATION
• It completely occurs in the
cytoplasm.
• Prokaryotic DNA occurs in
the cytoplasm.
• Prokaryotic genes are small.
• Results in polycistronic
mRNA.
• Equipped with a single type
of RNA polymerase.
• Transcription occurs inside the
nucleus and translation occurs inside
cytoplasm.
• Eukaryotic DNA occur inside the
nucleus.
• Eukaryotic genes are larger.
• Results in monocistronic mRNA.
• Three RNA polymerase are
employed in Eukaryotic gene
regulation.
