Microbiology, part 40: Genetics - Gene Regulation

Hi, I'm Cathy with Level Up RN. In this video, we will be discussing gene regulation, including a discussion of repressible operons and inducible operons. At the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key facts I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards on gene regulation so you can follow along with me.

An operon is a group of related genes that are transcribed together and are under the control of a single promoter. Each operon contains a promoter site, an operator site, and structural genes. The promoter is the site on the operon where RNA polymerase binds to initiate transcription. The operator on the operon is the site where a repressible molecule binds. And when the repressor binds to the operator site, this blocks transcription. And then the structural genes on the operon are genes that encode for enzymes. Outside of the operon, we also have a regulatory gene, which is a DNA sequence that codes for proteins that control transcription of the operon. An activator is a regulatory protein that helps RNA polymerase bind to the promoter site, which increases transcription of a gene, whereas a repressor is a regulatory protein that binds to the operator site, which blocks transcription of a gene. Of note, in some prokaryotic cells, there are operons that are constitutively expressed. This means they are expressed continuously without any regulation. So housekeeping genes are often constitutively expressed because they are involved in basic cellular functions such as DNA replication and metabolism.

Next, let's talk about repressible operons. This means that the genes in the operon are transcribed by default unless they are turned off by a repressor. So in other words, they are usually on but can be turned off. One important repressible operon that you definitely need to know is the trp operon, which codes for enzymes that are needed to synthesize the amino acid tryptophan. Let's take a look at how this operon works. So when levels of tryptophan are low in the environment, the cell needs to make tryptophan. And when tryptophan is absent, it does not bind to the repressor. This means that the repressor is inactive and does not bind to the operator. And that means that there is nothing in the way to block transcription of the genes in the trp operon. However, if tryptophan is present in the environment, the cell does not need to waste energy and resources to make more tryptophan. So when tryptophan is present, it binds to the repressor, which then binds to the operator site. And as you can see, this blocks transcription of the genes in the operon.

Let's now talk about inducible operons, which contain genes that are only transcribed when there is an inducer present. So this means they are usually off, but can be turned on, which is the opposite of the repressible operons that we just talked about. So one important inducible operon that you definitely have to know is the lac operon, which codes for enzymes involved in the breakdown of lactose into simple sugars. The cell only wants to make enzymes needed to break down lactose when lactose is actually present or else it's a waste of the cell's resources. In addition, the cell prefers to use glucose when it is available over breaking down lactose. Therefore, the cell only wants to transcribe genes in the lac operon when lactose is present and glucose is not available. So let's take a closer look at how this inducible operon works. In the top image, lactose is absent in the environment. Therefore, the cell does not want to waste resources making enzymes to break down lactose. Consequently, the lac repressor is bound to the operator site, which blocks transcription of the genes in the operon. However, in the bottom image, lactose is present in the environment, which is converted into allolactose inside the cell. Allolactose binds to the repressor, and this changes the shape of the repressor such that it can no longer bind to the operator site. This, in turn, allows for the transcription of the genes in the operon. And you'll notice that in both of these scenarios, cAMP and CAP complexes are bound at the promoter site of the operon. Let's talk about why that is the case.

As I mentioned before, transcription of the genes in the lac operon only occurs when levels of glucose are low. And when levels of glucose are low, this causes an increase in cAMP, which stands for cyclic adenosine monophosphate. cAMP binds with CAP, which stands for catabolite activator protein. These cAMP-CAP complexes then bind to the promoter site on the lac operon, which stimulates RNA polymerase activity and increases the rate of transcription. Conversely, if glucose levels are high, then we will have decreased cAMP. And this means that those cAMP-CAP complexes are not going to form and are not going to bind to the promoter site on the operon, and therefore, transcription will occur at a low rate.

All right. It's quiz time, and I have five questions for you. Question number one. RNA polymerase binds to the blank site on the operon, which initiates transcription. The answer is promoter. Question number two. A repressor binds to the blank site on the operon, which blocks transcription. The answer is operator. Question number three. Is the trp operon an example of a repressible or inducible operon? The answer is repressible. Number four. What two key conditions must be met to allow for the transcription of genes in the lac operon? The answer is lactose must be present, and glucose levels must be low. And number five, does the presence of cAMP-CAP complexes indicate glucose levels are high or low? The answer is low. All right. That's it for this video. I hope it was helpful for you. Take care and good luck with studying.

[BLOOPERS]

cAMP binds with CAP which stands for-- glucose. Then what we had.