2. SYNOPSIS:-
•INTRODUTION
•BACTERIAL OPERON
•LAC OPERON
•Structure of Lac operon
•The first control mechanism is the regulatory response to lactose
•The second control mechanism is a response to glucose
•Lactose analogs
•Classification of regulatory mutant
•Multimeric nature of repressor and the complex operator
•Regulation by cyclic AMP
•Mechanism of induction
•Application
•CONCLUSION
•REFERENCES
3. INTRODUCTION:-
A bacterial cell lives in direct contact
with its environment, which may change dramatically in
chemical composition from one moment to the next. At certain
times, a particular compound may be present, while at other
times that compound is absent. A number of strategies an
organism use to regulate expression of individual genes. By
controlling transcription initiation, a cell can regulate which
proteins it produces and how rapidly.
(G.Karp, Lodish, T.Brown)
4. The Bacterial Operon:-
In bacteria, the genes that encode the
enzymes of a metabolic pathway are usually clustered together
on the chromosome in a functional complex called an operon.
•A typical bacterial operon consists of:-
Structural genes
Promoter
Operator
Regulatory
(G.Karp)
5. Lac OPERON:-
The lac operon—the cluster of genes that
regulates production of the enzymes needed to degrade lactose
in Escherichia coli and some other enteric bacteria.
Fig:-Structure of Lac opreon
7. • Lactose is a disaccharide composed of galactose and glucose
linked in a b configuration (1->4).
• In its natural environment, the lac operon allows for the
effective digestion of lactose.
• It achieves this with the lac repressor which halts the
production in the absence of lactose.
• The Catabolite activator protein(CAP), which assists in
production in the absence of glucose.
8. Structure of Lac operon:-
The lac operon consists of three structural
genes, and a promoter, a terminator, regulator, and an operator.
Fig:- Structure of Lac operon
10. The first control mechanism is the regulatory response
to lactose:-
This uses an intracellular regulatory protein also
called the lactose repressor to hinder production of β-
galactosidase in the absence of lactose.
The repressor gene produces repressor, which binds to the
operator. This blocks the action of RNA polymerase, thereby
preventing transcription.
11. Fig :- Binding of repressor in absence of lactose
• If lactose is absent in the medium:-
12. Fig :- Binding of RNA polymerase in presence of lactose
• If lactose is present in the medium:-
13. The second control mechanism is a response to glucose:-
It is not enough for lactose to be present to
induce the lac operon.
•RNA polymerase has a low affinity for the promter of the lac
operon unless helped by a regulatory proten - cAMP receptor
protein (CRP) .
•Glucose inhibits the formation of cAMP
• If the concentration of glucose is high, the concentration of
cAMP is low
• If the concentration of glucose is low, the concentration of
cAMP is high.
14. Fig:-Control of lac operon by CRP-cAMP complex
(www.wikipedia.com., www.lect15.h
15. Lactose analogs:-
A number of lactose derivatives or analogs have
been described that are :-
• Isopropyl-β-D-thio-galactoside (IPTG)
Isopropyl-β-D-thio-galactoside (IPTG)
18. Classification of regulatory mutant:-
To analyze regulatory
mutants of the lac operon, Jacob developed a system by which a
second copy of the lac genes could be introduced into a single
cell.
•This experiment, in which genes or gene clusters are tested
pairwise, is called a complementation test.
(www.wikipedia.com.)
19.
20. Multimeric nature of repressor and the complex operator:-
• The lac repressor is a protein tetramer, where all four
identical components are 360 amino acids in length.
• When associated into its active tetramer form, the repressor
has a molecular weight of 154, 520 Daltons.
• The repressor protein binds to a palindromic sequence of
DNA on the lac promoter at the NH2 terminus.
• In this molecule, the DNA is bound to a 21 base-pair
symmetric DNA duplex (GAATTGTGAGC-
GCTCACAATT).
(www. MASTER.HTM.)
21. Regulation by cyclic AMP:-
The experimental microorganism
used by François Jacob and Jacques Monod was the common
laboratory bacterium, E. coli.
• The key idea is that proteins are not synthesized when they
are not needed.
• During World War II, Monod was testing the effects of
combinations of sugars as nutrient sources for E. coli. He found
that bacteria grown with two different sugars often displayed
two phases of growth.
23. •Two other genes, cya and crp:-
The cya gene encodes adenylate
cyclase, which produces cyclic AMP
The second gene, crp, encodes a
protein called catabolite activator protein (CAP) or cAMP
receptor protein (CRP).
• The cAMP level is related not to intracellular glucose
concentration but to the rate of glucose transport, which
influences the activity of adenylate cyclase.
24. Mechanism of induction:-
The repressor is an allosteric protein, i.e.
it can assume either one of two slightly different shapes, which
are in equilibrium with each other. In one form the repressor will
bind to the operator DNA with high specificity, and in the other
form it has lost its specificity.
(www.wikipedia.com
25. APPLICATION:-
The lac gene and its derivatives are amenable to
use as a reporter gene in a number of bacterial-based selection
techniques in genetic engineering. It is most commonly used in
following method:-
•Blue-White Screening
(R.C.Dubey)
26. CONCLUSION:-
Lac operon is a cluster of gene that regulate
lactose metabolism. It is an inducible operon. Lac operon is
transcribed only when lactose is single source of energy, in
order to conserve cellular resource and energy by not
producing the enzyme, when there is no need to metabolize
lactose.
27. REFERENCES:-
Karp.Gerald, 2010, The Cell Nucleus and the Control of
Gene Expression, Cell and Molecular Biology Concepts and
Experiments, John Wiley & Sons, Inc., 6th Edition, Page
No.-499-501.
Lodish M.Baltimore, Basic Molecular Genetic Mechanism,
Molecular Cell Biology, John Wiley & Sons, Inc, Page No.-
115-116.
Watson.J.D., Baker.T.A., Bell.S.P., Gann.A, Levine.M.,
Losick.R., 2004, Gene Regulation in Prokaryotes,
Molecular Biology of Gene, Pearson Education Inc., 5th
edition, Page No.-488-493.
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