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Introduction Proteins play an important role in the structures and
functions of living cells. Proteins form enzymes whichcontrol many cell metabolisms.
Prokaryotes (e.g :bacteria) live in direct contact with
their environment which may vary at different times.
The bacteria have evolved regulating mechanisms to
control gene transcription to adapt to the different
levels of nutrients in the changing environment.
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How do Organisms Control the Level of
Gene Expression?
Gene expression (transcription, translation)takes up large amounts of cellular energy andresources
Cells live frugal lifestyles they conserveenergy and resources
Cells must only express genes when needed
So genes will only be expressed when theirproducts are needed
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What is Gene Expression?
It is the process by which information from a
gene is used in the synthesis of a functional
gene product.
These products are often proteins, but in non-
protein coding genes such as rRNA genes or
tRNA genes, the product is a functional RNA.
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Terminologies,components of an operon
and function of each component A operon system consists of a regulator gene and
an operon.
A typical operon consists of a functional complexon the same length of DNA. The operon consistsof a cluster ofstructural genes, operator, and apromoter.
a) Structural gene (S) : is a gene which has genetic
information for the synthesis of a polypeptide. Ifmore than one structural genes are present inan operon, then all the genes will be activated atthe same time and function as a single
transcription unit.
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b) Operator (O) : short region of DNA situated
between the promoter and the first
structural gene. It acts as a switch for startingand stopping the transcription process. The
operator is the binding site for the
regulatory protein.
c) Promoter (P) : short sequence of DNA next
to the operator. The promoter is the binding
site to which the RNA polymerase molecule
first attaches to initiate transcription of the
structural genes.
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d) Repressors are protein molecules that can
bind to the operator and block the
transcription of structural genes.e) Activators can bind to specific sites on DNA
to stimulate the initiation of transciption.
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Experiments of Jacob and Monod
Francois Jacob and Jacques Monod carriedout a research on gene regulation for protein
synthesis in the human gut bacterium,
Escherichia coli. The DNA ofE. Coliis sufficient
to encode about 4000 proteins, but only a
fraction of these are made at any one time. E.
Coliregulated the expression of many of its
genes according to the substrates that areavailable to it.
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Principles of Gene Regulation:
Most prokaryotic genes are regulated in
units
called operons.
Francois Jacob & Jacques Monod, 1961.
This is largely based on regulation of
lactose metabolism. By intestinal bact.E.coli.
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A regulator gene codes for a repressor
protein.
Transcription of the regulator gene producesmRNA. Translation of mRNA produces the
repressor molecule.
In the presence of an inducer (allolactosefrom lactose), it binds to the repressor
molecule to form a repressor-allolactose
complex. The conformation of the repressor isaltered so that it does not fit into the operator
site. The repressor molecule is inactivated.
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The RNA polymerase binds to the promoter.
The operon is switched on thereby initiatingthe transcription of the structural genes lac Z,
lac Yand lac A. Enzymes ofor lactose metabolism -
galactosidase, permease and transacetylase
are produced. The lactose operon is an inducible operon.
The regulatory switch is turned on in thepresence of lactose and turned off in its
presence.
This is an adaption by E. colitowards itsenvironment, to conserve its cell nutrients and
to prevent energy wastage.
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General structure of an OPERON
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Explanation of the lactose (lac) operon as
a model for control of gene expression
Effect of the absence of lactose on the lactose
operon
A certain DNA segment in the bacterium
Escherichia colifunctions as the lac operon.
Lac operon is an example of an inducible system.
The repressor molecule does not affect the
structural genes directly. When lactose is absent,the active repressor molecule which has a strong
affinity for the operator binding site, binds to the
operator and also covers part of the promoter.
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This prevents the binding of PNA polymerase
to the promoter site.
The operator acts as a switch and the lactose
operon is switched off.
There is no transcription of the genes oflac Z,
lac Y and lac A. Enzymes -galactosidase ,
lactose permease and transacetylase are not
produced.
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Absence of lac operon:
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Effect of the presence of lactose on the
lactose operon
When lactose (milk sugar) is present in theenvironment, a small amount is converted toits isomer, allolactose.
Allolactose acts as an inducer and binds to
the allosteric site of the lactose repressormolecule produced by the regulator gene.
This causes a change in the conformation of
the repressor. The repressor- allolactose complex is unable
to attach to the operator.
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Presence of lac preron:
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Lac OPERON an inducible Operon
In the absence of
lac
In the presence
of lac
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Catabolic repression (the glucose effect)
The presence of glucose in the surrounding
prevents the induction of the lac operon andother operons, for example, the galoperon,which regulates the enzymes involved in thecarbohydrates metabolism.
This glucose effect or catabolic repressionoccurs because the bacterial cellspreferentially metabolise glucose as the main
energy source. In catabolic repression, the lac operon is
repressed while glucose is being used.
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High levels of glucose cause a sharp decreasein the levels of cAMP in the cell and vise versa.
The cAMP acts as the effector molecule whichregulates the effect of CAP on transcription inthe lac operon.
When concentration of cAMP is low, there is
insufficient cAMP to bind to CAP to form thecAMP-CAP complex.
When the cAMP=CAP complex does not bind
to the CAP site on the DNA, the RNApolymerase cannot bind well to the lac operonpromoter.This prevents activation of the lacoperon.
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When glucose is absent (or low
concentration) , there is a high concentration
of cAMP. cAMP is abundant and binds to CAPto form cAMP-CAP complex.
The cAMP-CAP complex then binds to the
CAP binding site (catabolic activator proteinbinding site). The binding causes the DNA to
bind around the complex to stimulate more
efficient binding of RNA polymerase to
promoter.
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CRP or CAP is positive regulator of Lac and some
other catabolic Operons:
CRP= Catabolic gene regulatory Protein
CRP= cAMP receptor Protein
CAP= Catabolic gene Activating Protein
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Tryptophan (trp) operon
Tryptophan operon in E. coliis an example of
a repressible operon.
Tryptophan acts as a repressor and attaches
to the allosteric site of the repressor. This
alters its conformation and the active
repressor binds to the operator site.
The RNA polymerase is unable to attach to the
promoter. The trp operon is turned off and theenzymes are not produced. (Look at the figure
in the next slide)
Th O ibl
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The trp Operon - repressible operon
The trp operon is always on and is turned off only by end product
tryptophan. This is why it is called a repressible operon.
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Tryptophan therefore switches off its own
synthesis (end-product repression) andprevents expenditure of energy to synthesise
excessive proteins which are not required by
the cells.
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There is repression of the synthesis of the tryptophan
biosynthetic enzymes when the surrounding environment
contains concentrations of the tryptophan amino acid
sufficient to sustain optimal growth of the bacterial cells.
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