Unit 6.1: The Lac Operon and Lactose
Repressor?
A regulatory protein that binds to the DNA and inhibits transcription
Activator?
A regulatory protein that increases the rate of transcription
What happens if there is lactose in environment?
A small amount of it is taken up and converted to allolactose by b-galactosidase. THe allolactose binds to the lac repressor, causing it to fall off at the operator site. Transcription will occur.
Allolactose?
Acts as an inducer.
Common processes regulated at genetic level?
Metabolism, Response to environmental stress, cell division
Operon?
A regulatory unit consisting of a few structural genes under the control of one promoter.
Corepressor?
A small effector molecule that binds to repressor protein, thereby causing protein to bind to the DNA
Inducer?
A small effector molecule that causes transcription to increase.
Inhibitor?
A small effector molecules that Binds to an activator protein and prevents it from binding to the DNA
Activator protein, inducer molecule, inducible gene?
Activator protein can't bind to DNA unless an inducer is present. No transcription will occur without activator. When inducer is bound to activator protein, this enables that activator protein to bind to DNA and activate transcription. Positive inducible.
Activator protein, inhibitor molecule, repressible gene?
Activator protein will bind to DNA without aid of effector molecule. Presence of an inhibitor causes a conformational change that releases activator protein from the DNA. Inhibits transcription. Positive repressible.
Anabolism?
Anabolic enzymes are not active/produced when end products are available. Endproduct repression. Tryptophan operon.
Catabolism?
Big to small. Catabolic enzymes are active/induced when substrate is available. Substrate induction. Lac operon.
How can lac operon be transcriptionally regulated?
By an activator or repressor protein.
How can small effector molecules inhibit transcription?
By either being a corepressor or an inhibitor
How can an inducer cause transcription to increase?
By either binding to repressor protein and preventing it from binding to DNA, or it could bind to an activator protein and cause it to bind to DNA.
What is the most common way to regulate gene expression in bacteria?
By influencing the rate at which transcription is initiated. Can be increased or decreased.
DNA elements?
CAP site - binds the catabolite activator protein (CAP)
Both ____________ and _______________ act to reduce the rate of transcription.
Corepressors, Inhibitors
Components of the actual lac operon?
DNA elements and structural genes
LacZ?
Encodes b-galactosidase. Enzymatically cleaves lactose and lactose analogues. Also converts lactose to allolactose (an isomer)
LacA?
Encodes galactoside transacetylase. Covalently modifies lactose and analogues. Its functional necessity remains unclear.
LacY?
Encodes lactose permease. Membrane protein required for transport of lactose and analogoues.
Constitutive genes?
Frequently encode proteins that are continuously needed for survival of bacterium. Have essentially constant levels of expression in all conditions over time.
LacI gene?
Further upstream of lac operon. Not considered part of lac operon. Has its own promoter, the I promoter. Constitutively expressed at fairly low levels. Encodes lac repressor
Repressible genes?
Genes regulated by corepressors and inhibitors
Inducible genes?
Genes that are regulated by inducers.
Metabolism and gene regulation?
Includes catabolism and anabolism
Structural genes?
Includes lacZ, lacY, and LacA
Repressor protein, inducer molecule, inducible gene?
Inducer molecule will bind to repressor to prevent repressor from binding to DNA. Transcription will occur. Negative inducible.
Inducible, negative control mechanism of lac operon?
Involves repressor protein. Inducer is allolactose. Binds to the lac repressor and inactivates it.
What happens if there's no lactose in the environment?
Lac operon is repressed, therefore no allolactose
What happens after transcription occurs (lac operon)?
Lac operon proteins are synthesize. Promotes the efficient metabolism of lactose.
What happens after lactose metabolism?
Lactose is depleted. Allolactose levels decrease. Allolactose is released from the repressor, allowing to bind to the operator site. Then most proteins involved with lactose utilization are degraded.
Negative repressible?
Negative - means repressor protein, repressible - means regulated by corepressors and inhibitors. Product makes repressor active.
Negative inducible?
Negative- means repressor protein, inducible - means regulated by inducers. Substrate makes the repressor inactive
Small effector molecules?
Often play a critical role in transcription regulation. Do not bind directly to the DNA to alter transcription. Binds to activator or repressor.
An operon encodes a _________________ that contains the coding sequence for two or more structural genes.
Polycistronic mRNA. This allows a bacterium to coordinately regulate a group of genes that encode proteins with a common functional goal.
Positive inducible?
Positive - means activator protein, inducible - means regulated by inducers. Substrate makes activator active.
Positive repressible?
Positive - means activator protein, repressible - means regulated by corepressor and inhibitors. Product makes activator inactive.
What DNA sequences does an operon contain?
Promoter; terminator; structural genes; operator
Lac repressor?
Protein that functions as a tetramer. Will bind to lac operon.
Two types of regulatory proteins involved in gene regulation?
Repressor and activator
Repressor protein, corepressor molecule, repressible gene?
Repressor protein can't bind to DNA unless corepressor is present. Transcription will occur without repressor. When corepressor is bound to repressor protein, this enables that repressor protein to bind to DNA and inhibit transcription. Negative repressible.
T/F: Genetic regulatory proteins that respond to small effector molecules have two functional domains. One domain is a site where protein binds to DNA; the other domain is the binding site for effector molecule.
T
T/F: Only a small amount of protein is needed to repress the lac operon. There are usually ten homotetramer proteins per cell.
T
T/F: Repressor does not completely inhibit transcription. So very small amounts of the lac operon proteins are made.
T
T/F: the binding of the effector molecule causes a conformational change in the regulatory protein and thereby influences whether or not the protein can bind to the DNA.
T
Enzyme adaptation?
THe observation that a particular enzyme appears within a living cell only after the cell has been exposed to the substrate for that enzyme
Two distinct transcriptional units of the operon?
The actual lac operon and the lacI gene
Gene regulation?
The phenomenon in which the level of gene expression can vary under different conditions
Negative control?
Transcriptional regulation by a repressor protein
Positive control?
Transcriptional regulation by an activator protein