Gene Regulation - Lac Operon & Trp Operon
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Transcript of Gene Regulation - Lac Operon & Trp Operon
04/11/23 2
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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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 Presence of LactoseSmall amount of it are converted into
Binds Changed its Shape & cannot bind to the promoter
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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