Topic: Control of gene expression in bacteria: positive vs negative control and inducible vs repressible
Group: Isabela Alesna, Bo-Sung Kim, Midori Yoshino
We’ll be covering the “Control of gene expression in bacteria”:
This happens through transcriptional control and operon control .
Before we jump into the process, let’s define things first.
In the control of gene expression, there are a few key players: the operon, promoter, operator, structural genes, regulatory gene, and regulatory protein.
An operon is a group of bacterial structural genes and sequences that control transcription of those genes into a single mRNA.
Within the operon is then a promoter, a sequence that RNA polymerase can recognize and bind to.
An operator, which is the place where a regulatory protein can bind to turn the operon on or off.
And structural genes, the sequences that encode for protein.
A regulatory gene, located outside of the operon, is a sequence that makes the regulatory protein which as we said before binds to the operator
This will all make more sense in a bit! So back to talking about the actual gene expression!
There are 2 types of transcriptional control.
Negative control and Positive control.
In both cases the regulatory protein acts like an on/off switch.
Negative control is when the regulatory protein is a repressor. It binds to the operon and inhibits transcription. Like a switch, the regulatory protein turns gene expression off.
And Positive control is when the regulatory protein is an activator. It binds to the operon and and stimulates transcription. Like a switch, the regulatory protein turns gene expression on.
Now there are 2 types of operon control.
Inducible operon and Repressible operon.
First, there is the inducible operon.
Here, gene expression is usually turned off either by an active repressor or an inactive activator.
It can be induced, aka turned on.
Then there is the repressible operon.
Here, gene expression is usually turned on either by an inactive repressor or an active activator.
It can be repressed, aka turned off.
Let’s put it all together! In summary, there is Negative Inducible, Positive Inducible, Negative Repressible, and Positive Repressible.
Negative Inducible is when the regulatory protein is an active repressor is bound to the operon, and transcription is off. Positive Inducible is when the regulatory protein is an inactive activator and is not bound to the operon, and transcription is off. In both of these cases, they can be INDUCED (or encouraged) to be turned on.
Negative Repressible is when the regulatory protein is an inactive repressor, does not bind to the operon, and transcription is on. Positive Repressible is when the regulatory protein is an active activator, binds to the operon, and transcription is on. In both of these cases, they can be REPRESSED (or restrained) to be turned off.
Let’s look at a specific example of negative inducible.
The most popular negative inducible system is the lac operon. The lac operon is responsible for making enzymes that break down lactose. When there isn’t any lactose around, the lac operon is turned off--there’s no need to make that protein if it won’t be doing anything. Therefore the repressor is bound to the operator, inhibiting transcription of the gene.
BUT. when a lactose (allolactose) is present, it binds to the repressor. The repressor protein changes its shape and becomes inactive, meaning it cannot bind to the operator. Therefore, the gene is turned on and makes a series of enzymes which can then break down the lactose.
Let’s look at an example for negative repressible, dealing with Tryptophan.
The trp operon is responsible for controlling biosynthesis of the amino acid tryptophan, in E. coli. When tryptophan level is low, the repressor is inactive and cannot bind to the operator. So the gene expression is turned on. Transcription takes place and more tryptophan is synthesized. When tryptophan level is high, tryptophan binds to the inactive repressor, making it active. The repressor then binds to the operator and turn off the gene expression.
Hope you liked our video on the Control of gene expression in bacteria!