Gene Regulation Flashcards
Prokaryotes
What are the 2 means by which bacterial transcription can be regulated (in general)?
- How well their promoters match consensus sequence
- Through various signals that are mediated by repressors and activators
What is a repressor?
What is an activator?
Repressors inhibit transcription by binding to an operator DNA sequence near the promoter. These signals can be modulated by effector molecules or co-repressors.
Activators enhance transcription by binding to a DNA sequence near the promoter sequence and may or may not be modulated by an effector
What is feedback inhibition? What is its relationship to gene expression?
Feedback inhibition is when a molecule inhibits the activity of the enzyme(s) that prodcued the molecule. This is a means by which the products of genes can be regulated without directly regulating transcription. It is related to gene expression because sometimes (but not always) the molecule that inhibits enzymatic activity can also inhibit genetic expression and downregulate its own production at the genetic level.
What is an operon?
A unit of genentic information that includes the operator, the promoter, the leader (if present) and the genes that are being controlled. The operator is the regulatory element of the operon, the promoter is the binding region, the leader is an area involved in attenuation. The operator is often found inside the promoter (but not always).
Describe how the trp operon works.
- The trp operon contains the genes necessary for tryptophan synthesis.
- The trp operon is an example of a repressible operon. This means it is always on unless the repressor is active.
- The repressor requires a co-repressor (tryptophan) to be active.
- When levels of trp are low, the repressor will not be bound to trp and will be inactive. It will not bind to the operator and transcription will continue without issue.
- When levels of trp increase, the repressor will begin to bind more to trp and will activate. It will bind to the operator and prevent RNA poly from transcribing the rest of the information in the operon.
Why does attenuation only occur in prokaryotes?
Because transcription and translation are coupled in prokaryotes but they are segregated in eukaryotes.
What is attenuation?
- A form of gene regulation
- Transcription begins, proceeds to the leader sequence, terminates at the leader sequence under the right conditions
The trp operon contains a leader sequence with an attenuator. Describe how this sequence attentuates the transcription of DNA –> RNA for the trp operon.
- No translation
No translation
- If there is no concatenated translation of the mRNA that is being transcribed, the leader sequence is able to form 2 hairpin loops (1–>2 and 3–>4). The second of these loops contains a U rich sequence at the end of the hairpin that de-stabilizes the RNA polymerase, which then falls of the RNA and transcription terminates at the attenuator sequence (poly U).
The trp operon contains a leader sequence with an attenuator. Describe how this sequence attentuates the transcription of DNA –> RNA for the trp operon.
Low Trp
Low Trp
RNA polymerase transcribes just ahead of translation
When ribosome reaches the leader sequence, it encounters two trp codons that are close by. Because Trp is low, the ribosome will stall at this position because it is waiting for a tRNA carrying trp but there is not much trp around so it will take a longer time for that to happen. This alters the formation of the hairpins (only 2–>3 forms), moving the destabilizing poly-U sequence to a different position where it does not cause RNA polymerase to fall off the mRNA. As such, transcription continues and the remainder of the operon is transcribed.
The trp operon contains a leader sequence with an attenuator. Describe how this sequence attentuates the transcription of DNA –> RNA for the trp operon.
High Trp
High Trp
RNA polymerase transcribes just ahead of translation
When ribosome reaches the leader sequence, it enounters the two trp codons close by. Trp levels are high, so the tRNA carrying trp is readily available. The ribosome is able to move past these codons until it reaches a stop codon at the end of the leader sequence. The progression of the ribosome prevents the formation of the 1–>2 hairpin and the 2–>3 hairpin but allows the formation of the 3–>4 hairpin. This places the poly-U attenuator sequence in a destabilizing location so when the RNA polymerase encouters the sequence it falls off the RNA and transcription stops.
Which genes tend to be found in inducible operons?
Which genes tend to be found in repressible operons?
Inducible: catabolic
Repressible: anabolic
What is an inducible operon?
An operon that is always off (repressor is always bound) unless an inducer molecule binds to the repressor to remove it from the operon
Describe how the lac operon works.
Lactose absent
When lactose is absent, the repressor is bound to the operator and transcription is inhibited. When lactose is present, it is brought into the cell and converted to allolactose, which acts as an inducer and binds to the repressor causing the repressor to become inactive and dissociate from the operator. This allows transcription to occur.
Describe how catabolite activator protein (CAP) activates transcription.
Low Glucose
Low Glu –> increased cAMP –> binds to and activates CAP –> binds to promoter –> increases affinity of RNA polymerase for promoter –> accelerates trasncription of genes for utilizing alternate sugars like lactose
High Glucose
High Glu –> decreased levels cAMP –> less cAMP binding to CAP –> less CAP binding to promoter –> transcription of alternative sugar utilization genes return to basal rates
Describe the various stages at which genes can be regulated in eukaryotes.
Describe the role of activators in the initiation of transcription.
In Eukaryotes, one gene only encodes for one product. However, multiple products from different genes often need to be expressed simultaneously for normal function b/c genes can be scattered across different chromosomes. Explain how eukaryotic cells facilitate the transcription of genes that are needed to accomplish a cellular task given that they are not polycistronic genes.
Eukaryotic cells use transcription factories.
Loops of chromatin extend from individual chromosomes at specific sites in nucleus. Loops may congregate at specific sites which are rich in transcription factors and RNA polymerases. These may be areas specialized for a common function.
