Lac Operon – RNA Basics

by Kevin Ahern, PhD

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    00:00 On the top of this figure you see a representation of the lac operon and the messenger RNA that's made from that. These are the top two sets of lines.

    00:10 In the lac operon we see, of course, that we have several things.

    00:12 First of all, we have the three genes in the lac operon that I have described.

    00:18 These are the lacZ, the lacY and the lacA genes.

    00:22 From this operon is transcribed the messenger RNA that is shown in the line immediately beneath it that has the AUGs within it.

    00:30 Each AUG is the starting coding region for each gene.

    00:36 AUG being the coding for the first of the amino acid that go in to there.

    00:40 So there are three different genes that are made and each gene starts with an AUG within that larger messenger RNA.

    00:48 Now another gene that is in the region, I just showed it, because, of its interest is the coding gene for the lac repressor.

    00:54 It's located immediately adjacent to the operon. It doesn't have any influence on the operon but just for information, it's located in the same region.

    01:03 We also see the control regions that I have been describing. These include the binding site for CAP, the promoter, that is the binding site for RNA polymerase, and the O site, that's the binding site for the lac repressor.

    01:15 Now I would like to take you through some various scenarios to describe what happens in these scenarios.

    01:20 So let's imagine we have a situation first that the cell finds itself in a condition of low glucose and lactose is available. Now in low glucose cyclic AMP would be made and cyclic AMP binds to the CAP.

    01:36 So when cyclic AMP binds to CAP that creates the circumstances that starts the CAP binding to the CAP sequence.

    01:42 You remember that when CAP binds to its sequence it favors the binding of RNA polymerase and so we have the scenario that we see on the top.

    01:51 Low glucose, lactose available, RNA polymerase binds, transcription will occur.

    01:58 So under the circumstances where the cell has lactose available and it needs it it will make abundant amounts of this transcript and make the genes necessary to breakdown lactose.

    02:09 Now the RNA polymerase, you see here, is bound to the P site and this can only happen if the O site is open. If the O site is bound by the repressor we will see that, that isn't even possible.

    02:23 Now in the second scenario we have a circumstance where the cell finds itself under conditions of high glucose meaning it has plenty of energy and lactose is not available.

    02:32 For the cell to turn the transcription of this operon on at this time would not make much sense; because, there would be no lactose to metabolize and the cell would be wasting energy making the RNA and the protein for this operon.

    02:49 So under these circumstances when there is no lactose available the lac repressor is not bound to lactose. And when it's not bound to lactose it binds to the O site. So you see that happening on this schematic right here.

    03:03 The binding to the O site precludes the RNA polymerase from binding to the P site.

    03:10 Now in the third scenario under conditions of low glucose and lactose being unavailable we have an interesting circumstance.

    03:17 You recall that under conditions of low glucose, cyclic AMP is made.

    03:20 So the CAP binds to the cyclic AMP and it binds to the CAP site as we have seen before.

    03:26 However, when lactose is not available the lac repressor has no allolactose to bind to which means that the lac repressor can bind to the O site.

    03:36 When this happens, we basically have the CAP trying to turn on transcription and we have the lac repressor trying to turn off transcription.

    03:43 And guess what? Well, the lac repressor wins and the reason it wins is because as you could imagine in that little tiny region between there the RNA polymerase cannot bind.

    03:53 Now from a metabolic perspective this makes pretty good sense; because, low glucose and lactose not being available would be a very bad scenario in which to turn on the transcription of this operon because the cell already has low energy and would be wasting even more energy if the operon would turn on.

    04:13 The last scenario is also an interesting scenario.

    04:15 This is where the cell has high glucose, so it has plenty of energy and it also has lactose available. Now when this occurs cyclic AMP, of course, will not be made. So this CAP will not bind to the CAP binding site.

    04:29 But further lactose is available so allolactose is being made which means that the repressor won't bind to its site either.

    04:39 When that happens we have a sort of a bare DNA that you can see in this picture at the very bottom.

    04:45 And interestingly under these conditions the RNA polymerase can actually get to the promoter and bind it to a limited extent and make a small amount of transcription.

    04:54 Since there is high amounts of glucose and there is lactose available, it's not really a waste of energy.

    05:00 But it shows lot about the dynamics of this system and how it's setup to preclude the operon from being made when it shouldn't be and allowing it to be made when it can be.

    05:14 So there are several different metabolic circumstances that an E-Coli cell might find itself into and the lac operon adjust accordingly.

    05:21 So when lactose is absent the lac repressor binds to the O site and stops RNA polymerase from transcribing the operon.

    05:28 When lactose is present, allolactose is being made that binds to the lac repressor and stops the repressor from binding to the O site; therefore, allowing other things to happen.

    05:40 When cyclic AMP is abundant and bound to the CAP, this occurs when the glucose concentration is low.

    05:46 This will help facilitate transcription if the cell has lactose available to it.

    05:52 So the expression will be highest then, when lactose is available and glucose is low.

    05:58 And expression will be lowest when lactose is absent.

    06:02 Thereby giving the cell exactly what it needs when it has lactose available or doesn't have lactose available.

    06:09 I conclude the slide by showing you a beautiful representation of the CAP binding to the lac operon. And you can see the cyclic AMP being bound in the middle of this protein.

    06:21 RNA is diverse, RNA is important for cells, and RNA is essential for making protein.

    06:27 I hope in this presentation you've gained that understanding about how these important functions of RNA help the cell to do what it does.

    About the Lecture

    The lecture Lac Operon – RNA Basics by Kevin Ahern, PhD is from the course RNA and the Genetic Code.

    Included Quiz Questions

    1. The CAP protein binds to DNA when it binds to cAMP
    2. The lac repressor binds DNA when it binds to allo-lactose
    3. The RNA polymerase binds to the promoter when it is bound to lactose
    4. It is turned off when the cell has low energy and lactose is present
    1. Very low basal levels of lac operon gene expressions will be observed in the cell
    2. Very high levels of lac operon gene expressions will be observed in the cell
    3. The β-galactosidase enzyme will be expressed in the cell, while other two genes will not be transcribed
    4. The β-galactoside permease enzyme will be expressed in the cell, while other two genes will not be transcribed
    5. The β-galactoside permease and β-galactoside transacetylase enzymes will be produced, while β-galactosidase enzyme will not be transcribed
    1. The binding of the lac repressor protein to O-site will oppose the action of CAP protein present on CAP site, and hence the lac operon will not be expressed
    2. The lac operon will be expressed as CAP protein will be present at the CAP site
    3. The high levels of cAMP will facilitate the lac gene expressions by facilitating the RNA polymerase binding to the promoter site
    4. The high levels of CAP protein on the promoter site will cause the over-expression of lactose metabolism-related genes
    5. The repressor protein binding to the O-site will not occur, and hence the lac operon will be expressed at its fullest

    Author of lecture Lac Operon – RNA Basics

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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