CONTROL OF GENE EXPRESSION IN PROKARYOTES (LECTURE 8) Flashcards
For the regulation of metabolic pathways, there is regulation occurring at what 2 levels?
(1) Regulation at the protein level (negative feedback inhibition) and (2) regulation at the gene level.
How are prokaryotic genes organized?
Prokaryotic genes are organized into operons.
What can be said about genes within the same operon?
Operons contain genes that code for proteins (mostly enzymes) that function in the same metabolic pathway.
What are the two elements of an operon?
The promotor and the genes.
Describe the promoter of an operon.
Promoters contain an on/off switch called the operator.
The genes in an operon are controlled by the same what?
By the same operator within the promoter of that operon.
Since the genes of an operon share a promoter, what can be said about the product of their transcription?
They are transcribed as one single mRNA molecule.
Although the genes within an operon are transcribed as one single mRNA molecule, each gene has its own what?
Each gene has its own start and stop codon for translation of each polypeptide.
Describe the activity of prokaryotic DNA within an operon when that operon is turned on.
RNA polymerase binds to the promoter, all genes of the operon are transcribed as one long mRNA and each gene on the mRNA is translated into a polypeptide.
Describe the activity of prokaryotic DNA within an operon when that operon is turned off.
A repressor protein binds to the operator, preventing RNA polymerase from binding to the promoter and thus transcription is blocked.
What is a repressor protein?
A protein that binds to an operator to turn off the operon containing that operator.
The repressor protein is the product of what?
Of a regulatory gene.
Describe the regulatory gene that codes for the repressor protein of an operon.
The regulatory gene is located upstream (5’) of the operon promoter and the gene is always being transcribed.
The binding of repressor to operator is ____.
The binding of repressor to operator is reversible.
Can repressor proteins be regulated? By what?
Repressor proteins can be regulated by allosteric regulators.
Where do allosteric regulators bind to on a repressor protein?
On the repressor protein’s allosteric site.
Allosteric regulators are _____ in the cell.
Allosteric regulators are metabolites in the cell.
Provide two examples of metabolites that function as allosteric regulators.
Tryptophan and allolactose.
What difference is there between an active repressor and an inactive repressor protein?
An active repressor protein can bind to the operator, an inactive repressor protein cannot.
Operons can be ____ or ____.
Operons can be repressible or inducible.
Repressible operons are ____ by default.
Repressible operons are on by default.
Inducible operons are ____ by default.
Inducible operons are off by default.
Describe the regulation of transcription in repressible operons.
Transcription is turned off by the product of a metabolic pathway.
Provide an example of a repressible operon.
The Trp operon.
Describe the regulation of transcription in inducible operons.
Transcription is turned on by the presence of a reactant in a metabolic pathway.
Provide an example of an inducible operon.
The Lac operon.
Describe the trp operon (repressible).
It codes for enzymes involved in the production of tryptophan. Repressor protein is active when bound to tryptophan - tryptophan is a corepressor.
Describe the activity of the trp operon in low tryptophan concentration.
The repressor protein is inactive and does not bind to the operator. Operon is turned on and mRNA is produced.
Describe the activity of the trp operon in high tryptophan concentration.
Repressor protein is active and binds to the operator. Operon is turned off and mRNA is not produced.
Describe the state of transcription in the trp operon in high versus low concentrations of tryptophan.
In high concentrations, transcription is off. In low concentrations, transcription is on.
Describe the lac operon (inducible).
Produces enzymes involved in lactose metabolism (breaking down lactose for energy). Repressor protein is inactive when lactose is present - allolactose acts as an inducer (lactose is converted into allolactose in the cell).
Describe the activity of the lac operon when lactose is not present.
Repressor protein is active and binds to the operator. Operon is turned off.
Describe the activity of the lac operon when lactose is present.
Allolactose binds to repressor and inactivates it. Repressor protein is inactive and does not bind to operator. Operon is turned on and mRNA is produced.
The control of gene expression can either be ____ or ____.
The control of gene expression can either be positive or negative.
Describe positive control of gene expression.
Positive control is when transcription can be increased by an activator.
Provide an example of positive control of gene expression.
CAP-CAMP is an activator for the lac operon.
Describe negative control of gene expression.
Negative control is when the operon can be turned off by a repressor protein, which is the case for both repressible and inducible operons.
Provide 2 examples of negative control of gene expression.
Trp operon and lac operon both have repressor proteins that can turn off transcription.
Even when lactose is present, what can cause transcription levels of the lac operon to be very low?
High glucose concentration can cause lac operon transcription levels to be very low despite the presence of lactose.
Why does a high concentration of glucose decrease transcription levels of the lac operon?
Because glucose is a better energy source than lactose, so there is no point is breaking down lactose when glucose is present.
In addition to an operator, what else is located on the lac operon promoter?
The lac operon promoter also has a CAP - binding site.
What must bind to the lac operon promoter to stimulate transcription?
Active CAP binds to the promoter to stimulate transcription of the lac operon.
When is CAP in its active conformation?
When bound by cyclic AMP (cAMP).
When are cAMP levels high?
When glucose levels are low.
Glucose acts as an inhibitor to what what enzyme?
Glucose inactivates / inhibits adenylyl cyclase.
What chemical reaction is catalyzed by adenylyl cyclase (or just adenyl cyclase)?
ATP –> cAMP
Describe, step by step, how low glucose concentrations increase the transcription levels of the lac operon.
low [glucose] –> high [cAMP] –> Active CAP –> CAP binds to promoter –> transcription increases!
Describe, step by step, how high glucose concentrations decrease the transcription levels of the lac operon.
High [glucose] –> low [cAMP] –> Inactive CAP –> CAP leaves promoter –> transcription decreases.
Will transcription be turned on if there is no glucose but no lactose in the cell?
No.
When transcription of an operon is on, this means the repressor is ____ and ____.
When transcription of an operon is on, this means the repressor is inactive and not bound.
When transcription of an operon is off, this means the repressor is ____ and ____.
When transcription of an operon is off, this means the repressor is active and bound.
The lac operon is ____ in the presence of lactose.
The lac operon is on in the presence of lactose.
The lac operon is ____ when no lactose is present.
The lac operon is off when no lactose is present.
The trp operon is ____ in the presence of trp.
The trp operon is off in the presence of trp.
The trp operon is ____ when no trp is present.
The trp operon is on when no trp is present.
