Biology: Control Mech: E Coli Lactose Metabolism

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You know, I always worry about something. I worry about losing sight of the forest for the trees. With all of our talk of protein synthesis and all the details everyone knows about protein synthesis and transcription and translation and the whole bit. Let's not lose track of what this is supposed to be about. This is supposed to be about genetics. This is supposed to be about genes. This is supposed to be about control of cell processes. That's what this is supposed to be about - figuring out who's the boss and how. And so the bottom line is we need to start talking about how genes are regulated, because you can have all the DNA in the world and all the RNA in the world, but you and I both know that they're not all turned on and off. Do you realize that in your cells, way over 97 percent of your DNA does not make proteins? Now possibly, a lot of that used to make proteins, back when you were an embryo, but probably a lot of it never, ever turns on. Why? Well, that's a good question. I hope some of you find out the answer to that. Some of the answers are some of them are used to regulate genes. Some of them are used to turn genes on and off.
Now, I want to start out simple. You've heard me throughout this entire molecular genetics unit - I keep bringing up "but in prokaryotes it's this way and in eukaryotes it's this way." You have to understand something, that until recent times, the mechanism of learning about genes has been prokaryotic cells. They're simple. There's a lot less DNA. There are no nuclear membranes. There is no endomembrane system. So generally speaking, we simply have a circular chromosome - remember that transcription and translation is simple. The RNA polymerase simply lands on there and starts cooking to make transcription and you don't have all of the factors that you have in eukaryotic cells. So prokaryotes are important. And when all is said and done, we're going to spend some time really making a list, comparing and contrasting prokaryotes and eukaryotes. But we're not going to do that now.
What I want to do right now is talk to you about control mechanisms. And the organism I want to talk about is a prokaryote. And at first, before we do that, I want to tell you about a kind of an interesting story about prokaryotes. Living in your intestine, as we speak, are a lot of bacteria. They're good. Don't go getting all nervous on me about this. It's called your bacteria flora, and you have billions of E. coli, and some other bacteria, living in there.
E. coli - not necessarily a harmless bacteria. We don't like to find I in our drinking water because that means that intestinal stuff got into our drinking water. And with intestinal stuff, meaning feces - and if it's human feces and human disease can be transmitted, that's why we worry about E. coli in our drinking water, because that just means there's been a source of contamination. But E. coli in your body, generally speaking, is harmless.
But here's the thing. Let's talk about these E. coli living in your large intestine. And let's talk about the enzyme systems that they have. Now you know that they get their food from you. As you eat, they're going to get some stuff. They're going to get some glucose. They're going to get materials that come down to them and they're going to use that as a source of food. But it's not necessarily anything that's going to deprive you. And one of the systems that was greatly investigated in E. coli is their metabolism of a sugar called lactose. And what I just want to do right now - and then later on, we'll talk about how this works - but first I want to tell you about how lactose is metabolized in E. coli.
First of all, they're there living in your large intestine and they have a series of enzymes that they make. Let me tell you about these three enzymes. It's important to understand that what I'm about to tell you is enzymes. The first enzyme they make is called betagalactocidase - sometimes called B-gal, because it's shorter that way. It's a digestive enzyme based on lactose.
The second one is, shortened, called E-permease. It allows them to absorb the digested products. The third one, we know it's produced when lactose is present but we don't know exactly what it does. It's called trans-acetylase. There's a lot of different theories as to what it does, but you know what? That's not important in this discussion.
Simply stated, these are used in the metabolism of lactose. Now, I present these to you because I want to ask you a question and I want to make you think. And here's what I want you to think about. Is it efficient to make these enzymes all the time? Now think about it - they're proteins. Why would it be not a good thing to make these enzymes all the time? Well let's think about enzyme synthesis, or in other words, protein synthesis. Let's think about ribosomes. Let's think about endergonic reactions. Let's think about making peptide bonds. Let's think about the fact that you need phosphorylated intermediates - energy costs. So if an enzyme is not going to be used, there's two things you can do. Number one, you can make them, in very small amounts, all the time and keep them around. But wouldn't it be more efficient to make these three enzymes when you need them? Wouldn't that be selected for, in a Darwinian situation? If lactose is present, let's make these enzymes. And if lactose is not present, let's not make these enzymes.
Now as a final thought, if that's true - and we're going to write that down: lactose present - make the enzymes. I want you to ask yourselves some questions - and I'll give you the answers - as to how and what's going on here. Okay, "lactose present" - let's change that to "protein synthesis." Well, if therefore lactose present is going to make protein synthesis happen, then it must be doing something. There must be some kind of regulation here. And that regulation is either going to occur at the level of the RNA or the level of the DNA. You're either going to turn RNA on and off or the DNA on and off. Now, I'm going to ask you still another question. Which one would be more efficient? To make the RNA and not use it or to not make it? I kind of phrase that question so there's only one answer. That's right. We are at a point now where we are ready to look at regulation of DNA transcription of RNA. We, by using this whole lactose system in the E. coli, came up with a way, in prokaryotes, that DNA making RNA can be stopped. We call that the lac operon. That's going to be the topic of our next lesson.
Molecular Genetics
The Lac Operon
Control Mechanisms: Lactose Metabolism in E. coli Page [1 of 2]