What is an operon?
In prokaryotes like E.coli, genes of structural proteins that have related functions is clustered or grouped under the control of
a single promoter called operon.
Watch our simplified summary video: LAC operon Simplified
An operon is a DNA sequence
that includes a promoter, an operator, and
the genes that are regulated together.
- The promoter is a DNA sequence where RNA polymerase binds and initiates transcription.
- The operator is a DNA sequence that a repressor or activator can bind to, these proteins can either allow or inhibit transcription.
- The set of genes in an operon are transcribed together by way of the promoter, creating a polycistronic transcript. In other words, multiple proteins are encoded by a single mRNA molecule.
Lac Operon
An operon in E.coli
involved in lactose metabolism
The lacZ gene encodes β-galactosidase, the enzyme that breaks lactose, a
disaccharide, into galactose and glucose.
The lacY gene encodes lactose permease,
a membrane protein that facilitates lactose entry into the cells.
The role of the lacA gene (encoding a transacetylase)
in lactose energy metabolism is not well understood.
Catabolite Activation of Lac operon (CAP-cAMP complex)
When glucose is low or absent, ATP synthesis reduces and some ATP is converted to cAMP, a signal molecule that suggest the deficiency of glucose. cAMP binds to a protein called CAP (Catabolite Activator Protein).
This
binding acts like a "booster," helping RNA polymerase bind strongly
to the promoter enhancing the rate of transcription of structural genes.
Result:
Much higher levels of transcription of the lac operon.
Basically, allolactose ensures the operon is on and allowed to function, while cAMP-CAP binding near the promoter site ensures high level of transcription of structural genes in the absence of glucose and presence of lactose.
How the Lac operon works under different conditions?
Case 1: Glucose Present, Lactose Absent
- The Lac repressor protein is active and binds to the operator.
- RNA polymerase cannot bind to the promoter effectively.
- Transcription of the lacZ, lacY, and lacA genes is blocked.
- Therefore, the enzymes needed for lactose metabolism are not produced.
- This occurs whether or not glucose is present, since the default state of the operon is off. That is why called as negative regulation.
Case 2:
Lactose Present, Glucose Present:
Genes of the E. coli lac operon regulate
the use of lactose as an alternative nutrient to glucose. Glucose is the
preferred nutrient to lactose.
Lactose is converted to allolactose, which acts
as an inducer.
Allolactose binds to the Lac repressor, causing
it to change shape and detach from the operator.
RNA polymerase can bind to the promoter, and
transcription of the lac genes occurs.
However, because glucose is preferred, and
glucose levels are high, the concentration of cyclic AMP (cAMP) is low.
cAMP is necessary to bind to CAP. CAP is a
protein that greatly enhances the binding of RNA polymerase to the promoter.
Because of low cAMP, CAP does not bind, and
transcription occurs at a low level.
Therefore, some lactose metabolizing enzymes are
produced, but not at a very high level.
Case 3: Glucose Absent (Low Glucose), Lactose Present
Lactose is converted to allolactose, which binds to the Lac repressor, causing it to detach from the operator.
Low glucose levels lead to high levels of cAMP.
cAMP binds to the Catabolite Activator Protein (CAP).
The cAMP-CAP complex binds to a site upstream of
the promoter, enhancing RNA polymerase binding.
Transcription of the lac genes occurs at a high
level.
Therefore, large amounts of lactose-metabolizing
enzymes are produced.
This is the ideal condition for high level
lactose metabolism.
Hope you are clear.
Check out these worksheets also
- You can access free worksheets or purchase relevant ones to support this site.
- Lac Operon Labeling worksheet, Crossword and Word Search Activity
- Lac Operon Concept Poster and Word Search Activity (Free )
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