Lac Operon
What is a repressor?
A repressor is a regulator molecule responsible for negative regulation of a gene.
How do prokaryotes regulate multiple genes in a Metabolic pathway?
They group the genes on one operon. These are a defined group of genes on the control of one promoter. This produces polycistronic mRNA.
What is a constitutive gene?
A gene that is transcribed at a relatively constant level regardless of the cell environmental conditions.
What does it mean for a gene to be repressible?
A repressible gene is a gene whose transcription and expression are stopped when expression is no longer needed.
What is inducer for the lac operon?
Allolactose
What happens in the presence of lactose?
Allolactose binds the repressor protein and transcription is initiated.
What is an activator?
An activator is a regulator molecule responsible for positive regulation of a gene.
The lac operon repressor protein can be described as what kind of protein?
An allosteric protein.
What does the LacI product do?
Binds the operator
Lactose is a ___________.
Disaccharide.
What scientists first figured out the Lac Operon? What else are they known for?
Jacob and Monod. They also first postulated the existence of mRNA.
What is the repressor gene of the lac operon?
LacI
What is the chemical break down of lactose?
Lactose----> Glucose + Galactose
What is the function of transacetylase?
Likely protects the cell from toxic products created by action β-gal
The lac operon is under what kind of regulatory conrol?
Negative (but is also inducible)
Describe negative regulation.
Negative regulation occurs when the regulatory molecule bind to the DNA and prevents the RNA polymerase from binding, thereby blocking transcription.
What does the repressor protein bind?
Operator
What is encoded by the lacY gene?
Permease
What is the permease function?
Permease transports lactose into the bacterium.
Describe positive regulation.
Positive regulation occurs when a regulatory molecule activates transcription. It binds the DNA and helps RNA polymerase bind to the promoter.
What constitutes the regulatory region of the lac operon?
Promoter & Operator.
Where does the regulatory protein bind?
Promoter and operator
What is the function of RNA polymerase?
RNA pol binds the promoter to begin transcription.
If the repressor protein binds the operator, ________________________
RNA polymerase cannot initiate transcription.
Regulation of gene expression permits what?
Regulation of gene expression improves efficiency.
What are the products of the LacI?
Repressor protien
Which transcription regulators are trans-acting?
Repressor, Activator, TF
What is the molecule responsible for repressing gene expression of repressible genes?
Repressor. The repressor is often the end product of the biosynthetic pathway controled by the enzymes expressed by the gene.
What are regulatory elements?
Segments of DNA involved in regulation of structural gene transcription.
Give an example of a constitutive gene.
The lacI gene is constitutive. (It is the structural segment of the lac operon that is inducible)
What is the operator region?
The region between the promoter and structural genes to which the repressor protein binds. It enables control of structural gene expression.
The repressor protein has ______ binding sites, the _____________ and ___________.
The repressor protein has 2 binding sites, the operator region and allolactose region.
What defines the behavior of regulation molecules?
Their binding.
What is so great about Monosaccharides?
They are useable; they are faster to metabolise than disaccharides.
In the absence of lactose, the repressor protein ________
binds the operator.
What are the structural genes of the lac operon?
lacZ - beta-galactosidase (necessary to turn lactose into monosaccharides) lacY - lactose permease (necessary to get lactose into the cell) lacA - thiogalactoside transacetylase (not necessary for lactose metabolism)
Glucose and galactose are both______________
monosaccharides
When lactose is present, ezymes are required to metabolize it in order to produce allolactose. How does the cell get around this problem when the lac operon is repressed in the absence of lactose?
β-gal is made even in the absence of lactose because this enzyme is needed in case lactose becomes available. Transcription for the lac operon is not 100% inactive in the absence of lactose; enough transcription occurs to counter this conundrum.
What converts lactose to allolactose?
β-galactosidase
How many regulatory elements does the lac operon have?
2
What is β-galactosidase?
Galactosides are a particular group of organic molecules. β-galactosidase is encoded by the lacZ gene. It is an enzyme that cleaves lactose in to monosacharides and converts lactose to allolacose.
How may gene expression be regulated?
Gene expression may be under positive or negative control.
What does it mean for a gene to be inducible?
Inducible genes are expressed only for specific conditions; they must be turned on. They often encode enzymes that catabolize a molecule.
Give an example of an inducible gene
Lac operon
Give a general description of the lac operon.
The Lac Operon has structural genes, with an upstream regulatory region consisting of an operator (lacO) and a promoter (lacP)
What is the lacI gene?
The LacI gene encodes the repressor protein, which is a negative regulator of the transcription of the three structural lac genes.
What role do metabolites play in gene expression?
The presence or absence of metabolites can turn a gene on or off.
What happens when Allolactose binds the repressor protein?
This binding causes a conformational change. The change alters the operator binding site and the repressor protein falls off the operator. RNA pol can now transcribe structural genes.
What molecule is responsible for turning on a gene that is currently off?
This molecule is called the inducer; it induces transcription of the inducible gene, activating gene expression.
What is encoded by the lacA gene?
Transacetylase
The lac operon is involved in the metabolism of ___________.
lactose