Lecture 6b - Operons

Hi, I am Dr. Lesley Blankenship-Williams and this lecture continues with the topic of control

of gene expression and gene arrangement. The previous lecture focused on eukaryotes;

here we look at how bacteria or prokaryotes do it. So prokaryotes take a more simplistic approach.

Like eukaryotes, there are usually multiple genes that need to be turned on in order to

achieve some outcome. For instance let's say you're a little bacterium that's swimming along

and you encounter a whole bunch of new nutrients - nutrients that are plentiful. And you want them,

but you haven't encountered that nutrient in a long time, so you don't have the machinery

available yet to transport that nutrient into the cell and then break apart that nutrient so

you can make ATP or use it for parts. So if you do want to capture that nutrient, and use it,

you're going to have to express some genes that will help you do that. Namely you're going to

need a transporter enzyme to bring that nutrient into the cell, and then you're going to need some

enzymes to break it down. So that's multiple genes that you're looking at. Transporters are probably

two or three different genes, and enzymes are each going to at least be one if not more

than one gene. So where are those genes located on the single circular chromosome?

Well they're all going to be located together. So we know that eukaryotes have a tendency to spread

their genes around to different chromosomes, but prokaryotes are going to put them together.

So if the gene for transporting that sugar is located here,

then you can bet that the genes for metabolizing that sugar are also located in that same spot.

And moreover, all of those genes are going to be controlled by one "master switch",

and that master switch is known as an operator. The collection of genes in tandem

that all go to some outcome and their single control that precedes them,

are collectively known as an operon. Let's take a closer look. So here I have an enlargement of an

operon. This operon has only three genes with it, so we have hypothetical gene A, gene B,

and gene C, and we presume that all of these genes would need to be expressed in order for the cell

to achieve some outcome. Notice that preceding these genes - which are all located one right

after another - is something called an operator and then in front of that is something called a

promoter. And these are just other regions of DNA. The operon is all of these put together.

So this represents a single operon. Now, in addition, we're going to need to bring

RNA polymerase into this picture, because we know that gene expression is controlled at the

transcriptional level, and more specifically it's controlled by whether or not RNA polymerase is

allowed to transcribe genes A, B and C. We also learned in a prior lecture that RNA polymerase

is attracted to the promoter region, and that tends to be where RNA polymerase likes to start.

So let's assume that RNA polymerase comes down here to the promoter

if RNA polymerase is now allowed to move along this double-stranded DNA helix, it will transcribe

genes A, B, and C. However if RNA polymerase is somehow not allowed to move past this region,

then no transcription of genes A, B or C will happen. So unlike eukaryotes which control the

gene expression largely by whether or not RNA polymerase was even allowed to the promoter site,

in prokaryotes RNA polymerase tends to be permitted to any promoter site. The control

happens with an interaction between the operator and another molecule known as a repressor.

So the operator is a section of DNA in between the promoter

and the genes that need to be transcribed. The repressor is a physical big bulky protein usually

that is attracted to this operator section, and if it sits down on the operator like so, it

prevents RNA polymerase from moving downstream and transcribing genes A, B and C. It is a physical

blockade. So if we assume that my fist is RNA polymerase, and RNA polymerase is running along,

the repressor is literally a wall. It's like uh uh no, can't go past can't go past, can't go past.

So if the repressor is sitting on the operator, no transcription happens.

Note that the transcription of genes A, B and C are all linked together. In other words,

if the repressor is removed - so we'll just quickly remove it here - and RNA polymerase

is allowed down, then gene A is going to be transcribed, gene B is going to be transcribed,

and gene C is going to be transcribed. There's no picking and choosing. It's not as if you can

transcribe gene A, but not B and C. It is an all or nothing deal. Either all of the genes

in the operon are transcribed or none of them are and that is what we mean by the "master switch".

It turns out that operons come in two different designs. In other words,

they use their repressor in different ways. The two designs are called inducible and repressible.

Let's take a closer look at what this means. I'm going to start with the inducible operon.

The word inducible suggests to "turn on" like to induce delivery of a woman who is pregnant. So

typically women who are induced are not progressing in their labor

and therefore delivery, and so you induce labor and delivery by adding Pitocin.

In the same way, inducible operons have a default position of off,

but if you give them a little something special, you can turn them on. So inducible operons

are by default off. And what that means is by default, the repressor

is sitting on the operator, and preventing RNA polymerase from transcribing genes A, B, and C.

But, an inducible operon can be turned on. In other words, it can be induced.

How is it induced? Well usually there's another molecule,

in this case I have a little red dot called the signal molecule,

that will bind to the repressor. And when that signaling molecule binds to the repressor

it causes a conformational change in the repressor that allows the repressor to leave the operator

when the repressor leaves the operator there is no longer a physical blockade for RNA polymerase,

and RNA polymerase starts moving downstream and doing its transcription.

So again, an inducible operon is typically not experiencing gene expression. But if an

appropriate signal is given, then that signaling molecule will bind to the repressor. The repressor

detaches from the operator, and now RNA polymerase is free to start transcribing.

And that will continue to happen until the repressor

rebinds the operator. So that's one design. A second way that operons can be designed is

where the default position is on, and these are referred to as repressible operons.

So in this case, RNA polymerase is going to keep making RNA transcripts over and over and

over again, until it encounters a block. Well when would it encounter a block? Well when the

repressor binds of course. So the term repressible has in its root "repress". To repress is to

minimize, or to stop, or to or to make negligible. So in other words a repressible operon is one

that can be turned off when needed. And like the other case, there's usually a signaling molecule

that will bind to the repressor, and once that happens, that creates a conformational change

in the repressor, that makes it more attracted to the operator area,

and then it binds. And now once that blockade is in place, RNA polymerase is

no longer capable of transcribing, and therefore transcription is turned off.

The terms inducible and repressible can be a little bit counterintuitive when

you first learn them, but think of the terms this way. Inducible. To induce.

If you are inducing something you're turning it on, like to induce a delivery is to turn

on the labor and delivery so that that woman gives birth quickly and easily (hopefully).

But that means that she wasn't already progressing in her labor and delivery. To induce it means

to make it start, make it go faster. In other words her default position was not giving birth,

and then you had to induce it to start that process. In the same way, an operon that is

classified or designed as inducible is not on by default, but you can turn it on if you need to.

The term repressible can be viewed with the same lens. Repressible means to repress, or turn off,

but that implies that the default position is on. Now operons are designed this way depending on the

needs of the cell. Repressible operons - because their default position is on - tend to be operons

that the cell needs all of the time. In other words the cell needs those products all the time.

Inducible operons are ones that typically the cell doesn't need, and only needs in special

circumstances. If you had to guess, which kind you think is more common in a genome?

And surprisingly, repressible operons are slightly more common. So for instance,

in E. coli it is estimated that around 60% of the operons are repressible and 40% percent are

inducible meaning that repressible operons are a little bit more common than inducible ones.

Okay the last lecture objective is one that you are tasked with,

and that is to look up and do your own research on the lac operon, which deals with lactose - a

rare sugar. And what you want to do is look at that operon and determine.... is it an inducible

operon or a repressible operon? Alright that's the end of our lecture thanks for watching.