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Lac Operon
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Lac operon

Apr 12, 2017

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Rachana Tiwari
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Page 1: Lac operon

Lac Operon

Page 2: Lac operon

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

Page 3: Lac operon

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)

Page 4: Lac operon

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 PromoterOperator Regulatory

(G.Karp)

Page 5: Lac operon

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

Page 6: Lac operon

Figure: Chromosomal organization of the lac operon and controlling elements.

Chromosomal organization of the lac operon and controlling elements.

Page 7: Lac operon

• 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.

Page 8: Lac operon

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

Page 9: Lac operon

•The three structural genes are: • lacA encodes β-galactoside

transacetylase (LacA)• lacY encodes β-galactoside permease

(LacY)• lacZ encodes β-galactosidase (LacZ)

• Promoter (P)

• Operator (O)

• Repressor (lacI) gene

(Watson et.al,, www.wikipedia.com., lect15.htm.)

Page 10: Lac operon

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.

Page 11: Lac operon

Fig :- Binding of repressor in absence of lactose

• If lactose is absent in the medium:-

Page 12: Lac operon

Fig :- Binding of RNA polymerase in presence of lactose

• If lactose is present in the medium:-

Page 13: Lac operon

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.

Page 14: Lac operon

Fig:-Control of lac operon by CRP-cAMP complex (www.wikipedia.com., www.lect15.htm.)

Page 15: Lac operon

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)

Page 16: Lac operon

•Phenyl-β-D-galactose (phenyl-Gal)

•Other compounds serve as colorful indicators of β-galactosidase activity.

ONPG (orthonitrophenol)

Phenyl-β-D-galactose (phenyl-Gal)

Ortho nitro phenol(ONPG)

Page 17: Lac operon

X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactoside).

X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactoside).

• Allolactose is an isomer of lactose and is the inducer of the lac operon. Lactose is galactose-(β1->4)-glucose, whereas allolactose is galactose-(β1->6)-glucose.

(www.wikipedia.com., www.lect15.htm.)

Page 18: Lac operon

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.)

Page 19: Lac operon
Page 20: Lac operon

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.)

Page 21: Lac operon

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.

Page 22: Lac operon

Fig:-Monod's "bi-phasic" growth curve

Page 23: Lac operon

•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.

Page 24: Lac operon

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.)

Page 25: Lac operon

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)

Page 26: Lac operon

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.

Page 27: Lac operon

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.

www.lect15.htmwww. MASTER.HTMwww.wikipedia.com

Page 28: Lac operon

Acknowledgement

Thank You