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Inducible Operons

by Georgina Cornwall, PhD
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    00:00 Let us look at an example. The lac operon is a very common example in looking at gene regulation or gene expression. And in the lac operon, let us start with the knowledge that in E. coli they will generally metabolize glucose as a favorite source. We know how cellular respiration works and so they can utilize glucose just as we can as a primary source. However when there is no glucose present, they need to be able to metabolize lactose. This operon is an inducible operon and that means it is OFF and we need to induce this to be ON and allow transcription of the genes that are involved in digesting lactose. In the absence of lactose, we will look at that version first. We have our repressor gene, which is coded for upstream and the repressor gene is actually in an active form and so that repressor gene will bind right onto the operator region and it is sitting right in the way of our RNA polymerase. So RNA polymerase cannot move down the track and then we see that there is no production of the proteins involved in digesting lactose. It is OFF in its normal state, but what, when are out of glucose, bacteria needs to metabolize lactose, need to turn the system ON. As it happens, there is a very neat thing that comes into play. Lactose itself is going to get in the way of this protein binding to the operon, the repressor protein and so when lactose binds to the repressor protein, the repressor protein can no longer bind to the operator region. It is no longer in the way off the RNA polymerase and so RNA polymerase can make its way all the way along the gene sequence and make a nice mRNA transcript and make all three proteins that are involved in the digestion of lactose. That is great. We have now a cell that can metabolize lactose. But another system actually comes into play.

    02:29 This is a recently discovered system in which further things based on the level of glucose can accelerate the production of the proteins associated with digesting lactose. What happens in this case? We are going to take a look at activation of the lactose digesting genes when glucose is not there. So we're going a little bit backwards, but start with this principle that if there is no glucose, then there is going to be probably a lot of cyclic AMP because glucose would have been made into ATP, but ATP has been used, so we are running out of glucose to make ATP from and so we have lots of cyclic AMP or adenosine monophosphate floating around. And that adenosine monophosphate will bind to these CAP responsive proteins.

    03:36 CAP is catabolite activator protein. I shortened it to CAP because it is easier and when that CAP protein is activated by lots of cyclic AMP because there is not much glucose, it will go ahead and bind to the CAP binding site back by the operator and the promoter region and when it does that somehow it accelerates the production or the rate of movement of that polymerase so we can increase the production in the presence of lactose and absence of glucose, where accelerating the production of the enzymes that are necessary to metabolize lactose rather than glucose. The opposite is the case when we have glucose.

    04:27 We don't need to make those enzymes any faster that breakdown lactose. This cyclic AMP concentration is low and thus, we do not have activation of the CAP protein and the CAP protein doesn't push polymerase along any further. This portion is the summary of CAP activation on top of the lac operon. So a relatively recent set of discoveries on further levels of regulation of gene expression, so CAP proteins can accelerate things if they need to.


    About the Lecture

    The lecture Inducible Operons by Georgina Cornwall, PhD is from the course Gene Regulation.


    Included Quiz Questions

    1. not induced in the presence of both glucose and lactose.
    2. a form of negative control, mediated by a repressor.
    3. controlled by the expression of three downstream genes.
    4. only induced when there is glucose but not lactose.
    5. preferentially utilizing lactose as a carbon source.
    1. …cAMP-CAP complex binding to the CAP site to stimulate the RNA polymerase activity on lac operon.
    2. …cAMP-CAP complex binding on the repressor gene to stop repressor protein production.
    3. …cAMP-CAP complex binding on the promoter site to stimulate the DNA polymerase activity on lac operon.
    4. …cAMP-CAP complex binding on the repressor protein to prevent its binding to the operator site of the lac operon.
    5. …lactose-CAP complex binding on the promoter site to facilitate the RNA polymerase binding on the lac operon.

    Author of lecture Inducible Operons

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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