16 Questions
Which protein negatively regulates the lac operon?
Lac repressor protein
What happens to the lac operon when glucose is present?
The genes are not transcribed
What effect does the CAP protein have on the lac operon?
It promotes transcription of the genes
In what condition is the CAP protein produced?
When glucose is not present
What is the role of the lac repressor protein when glucose is not present?
It is not produced
What type of operon is the lac operon classified as?
Repressible operon
Which condition allows the CAP protein to promote transcription effectively?
Presence of lactose and absence of glucose
What happens to the lac operon when lactose is not present?
The CAP protein is not produced
Which condition triggers the induction of genes for lactose utilization?
Absence of glucose and presence of lactose
What is the role of β-galactosidase in lactose metabolism?
Hydrolysis of lactose into monosaccharides
Where does the lac repressor primarily bind?
O1 operator site
What molecule serves as the inducer in the lac operon?
Allolactose
Which structural gene encodes β-galactosidase?
lacz
Under what condition do cells produce low levels of lactose metabolism enzymes?
Presence of glucose and lack of lactose
What is the function of galactoside permease?
Transport of lactose into the cell
How does the inducer affect the lac repressor?
Binds allosterically, causing the repressor to release from the operator
Study Notes
Induction of Genes for Lactose Utilization
- The activity of enzymes involved in lactose utilization increases rapidly after 10 minutes when lactose is added.
- Gene expression is induced when glucose is absent and lactose is present.
The Operon Model
- Consists of regulatory sequences, genes transcribed as a unit, an activator binding site, promoter, and repressor binding site (operator).
The Lactose Operon
- Contains 3 linked structural genes: lacz, lacy, and lacA, which encode β-galactosidase, galactoside permease, and thiogalactoside transacetylase, respectively.
- The 3 structural genes are under the control of their own promoter Plac and an operator sequence.
- Control of structural genes is by negative regulation, which utilizes a repressor.
Induction of the lac Operon
- In the absence of inducer, the repressor binds to the lac operator and represses transcription of the structural genes.
- The inducer is allolactose, a minor reaction product of β-galactosidase.
- The inducer binds to the repressor, causing the latter to be released from the operator, and thus transcription of the structural genes is turned on.
Lactose Metabolism in E.coli
- Uptake and metabolism of lactose require the activities of galactoside permease (uptake) and β-galactosidase (metabolism).
- β-galactosidase also catalyses conversion of lactose to allolactose by transglycosylation.
- When glucose is aplenty and lactose is lacking, cells make very low levels of enzymes for lactose metabolism.
- If cells are fed lactose but not glucose, cells can use it as their energy source given that lactose is effectively entering the cell and being hydrolysed into monosaccharides.
Regulation of the lac Operon
- The lac operon is negatively regulated by the lac repressor protein.
- The lac repressor protein binds to the operator region of the operon and prevents transcription of the genes.
- The lac operon is also positively regulated by the catabolite activator protein (CAP).
- The CAP protein binds to the activator region of the operon and promotes transcription of the genes.
- The lac operon is regulated by both glucose and lactose.
- The lac operon is a repressible operon because it is regulated by a repressor protein.
Explore the regulation of gene expression in prokaryotes, specifically the induction of genes involved in lactose utilization. Learn how the presence of lactose triggers the rapid increase in enzyme activity.
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