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GENE REGULATION

Lac Operon &Trp Operonin

Bacteria

Salam Pradeep

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Gene Regulation• Gene Regulation refers to the processes that

cells use to turn the information on genes into gene products.

• This functional gene product may be RNA or Protein & the majority of known mechanisms regulate protein coding genes.

• Gene regulation is essential for viruses, prokaryotes & eukaryotes as it increases the versatility & adaptability of an organism by allowing the cell to express protein when needed.

• The first example of gene regulation system was the lac operon, discovered by Jacob & Monod, in which protein involved in lactose metabolism are expressed by E.coli only in the presence of lactose and absence of glucose.

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June 1961 - Lac Operon

• Jacques Monod (born February 9, 1910 – May 31, 1976 in Paris) : A French biologist , Awarded Nobel Prize in Physiology/ Medicine in 1965.

• François Jacob (born 17 June 1920 in Nancy, France)

• Famous for their work on the Lac operon• Jacob F; Monod J (June 1961) "Genetic

regulatory mechanisms in the synthesis of proteins". J Mol Biol. 3: 318–56.

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What is Lac Operon

• Lac Operon is an Operon required for the transport and metabolism of lactose in Escherichia coli and some other enteric bacteria.

• Consists of 3 adjacent structural genes, a promoter, an operator, a regulator and a terminator.

• Regulated by several factors including the availability of glucose and of lactose.

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Structure of the Operon• Lac operon consists of 3 structural genes, a

promoter, an operator, a regulator and a terminator. They are lacZ, lacY, and lacA.

• lacZ encodes β-galactosidase (LacZ): An intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose.

• lacY encodes β-galactoside permease (LacY)- a membrane-bound transport protein that pumps lactose into the cell.

• lacA encodes β-galactoside transacetylase (LacA)- an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.

• Only lacZ and lacY appear to be necessary for lactose catabolism.

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• Specific control of the lac genes depends on the availability of the substrate lactose to the bacterium.

• The proteins are not produced by the bacterium when lactose is unavailable as a carbon source.

• The lac genes are organized into an operon; they are oriented in the same direction immediately adjacent on the chromosome and are co-transcribed into a single polycistronic mRNA molecule.

• Transcription of all genes starts with the binding of the enzyme RNA polymerase (RNAP).

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Lac Operon Organization

• 3 structural genes involved in lactose metabolism in E.coli

• These lac genes are organized into the lac Operon

• An Operon is a cluster of genes along with an adjacent promoter & operator that control the transcription

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Structural Gene Transcription

A Single mRNA is Produced

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Polycistronic mRNA - Because it carries the information for more than one type of protein

Lac Z+

Lac Y+

Lac A+

Breaks down lactose into glucose & galactose

Transports lactose into the cell

Not fully understood

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Lac Operator

Lac O+ is a short region of DNA that lies partially within the promoter

It interacts with the regulatory protein that controls the transcription of the Operon.

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Lac Regulatory

Lac I+ produces an mRNA & synthesized a repressor protein

The lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural gene

This repressor protein can bind to the operator of the lac operon

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In Absence of Lactose

In the absence of lactose, lac repressors binds to the operator

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In Presence of LactoseSmall amount of it are converted into

Binds Changed its Shape & cannot bind to the promoter

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Positive Control of Lac Operon

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Catabolite Repression

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Trp Operon

• Trp Operon is an operon in bacteria which promotes the production of tryptophan when tryptophan isn't present in the environment.

• First repressible operon to be discovered.

• If the amino acid is present, then the operon is repressed & biosynthetic enzymes are not produced.

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Trp OperonConsists of Regulatory & Structural Genes

5 Structural Genes – trpE to trpA (Downstream) of Promoter Region

Transcribed as Polycistronic mRNA – Responsible for tryptophan biosynthesis

Promoter & Operator region regulates the level of transcription

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Between the Promoter/Operator & the Structural gene is the Leader Region, trpL

Transcribed into an mRNA called – Leader mRNA

Contains a short ORF & translated into a leader peptide

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Leader mRNA contains the Attenuator consisting of 4 regions (1,2,3 &4) that fold & forms 3 different loop secondary structure.

1 & 2 pairing results in transcription pause signal

2 & 3 pairing results in an antitermination signal that allows transcription to continue

3 & 4 pairing results in a termination signal which stops further transcription

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Since there is no nuclear membrane in bacteria transcription & translation occur at the same time

Translation of trpL mRNA proceeds while RNAP transcribe the attenuator mRNA

After RNAP has transcribed region 2, regions 1 & 2 pair to form a stem loop structure.

This structure causes RNAP to pause & allows the ribosomeo catch up to it. Transcription is tightly coupled with translation

Ribosome’s position along the leader mRNA plays an important role in the regulation of transcription

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Trp Starved

When the cell is starved for Trp, the amount of Trp-tRNA drops dramatically. In the leader mRNA there are 2 Trp codons in tandem. When the ribosome reaches these codons, it stalls because the amino acid is short supply & the leader peptide cannot be completed.

Since the ribosome covers region1 of the attenuator, region2 will pair with region3. This pairing is an antitermination signal, thereby allowing RNAP to proceed & translation follows to synthesize enzymes necessary to produce trp

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When the cell is not starved for tryptophan, there is enough Trp-tRNA for translation

When the ribosome reaches the 2 Trp codons , 2 Trps are added to the leader peptide. The ribosome then continues to the stop codon at the end of region 1 of the leader peptide. This prevents region2 from pairing with 3

Instead region 3 pairs with 4 which is a termination signal fort RNAP

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The key signal for attenuation is the concn of Trp-tRNA gene in the cell

If there is an abundance of Trp & Trp-tRNA, then a short leader mRNA transcript is synthesized & transcription is then terminated

No Tryptophan biosynthesis enzymes are made

If the cell is deficient in Trp & Trp-tRNA , then RNAP continues transcription past trpL & through the 5 trp genes. Ribosome translate this polygenic mRNA

to produce the enzymes for Tryptophan biosynthesis

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More slides at http://biotech-job.blogspot.com/