PROFESSOR: Hi.
In this clip we're going
to demonstrate the gene
regulation of the E.
coli lac operon.
As you remember from the lecture
clips, the lac operon
is a collection of genes that
produces the lactose
metabolism machinery
when expressed.
Now, when these are expressed,
E. coli is able
to metabolize lactose.
However, it prefers to
metabolize glucose, to use
that as its sugar.
Thus, when there's glucose
available to the cell, it's
pointless for it to create this
machinery, because it's
not going to actually use it.
It's going to metabolize
the glucose instead.
But when we remove glucose and
add lactose, the E. coli
absolutely needs to be able to
metabolize this lactose,
otherwise it's not going to
be able to grow with the
available energy source.
Thus, we have a couple of
different situations.
When we have glucose present
but not lactose, we do not
want to express the
lac operon.
However, when we remove glucose
and we add lactose, we
do want to express
the lac operon.
Now we've covered when the E.
coli would like to express
this operon.
However, how can it express this
operon only when it wants
to and not when it doesn't?
That's where the promoter
comes into play.
The promoter is where RNA
polymerase binds.
And RNA polymerase needs to bind
to this promoter in order
to transcribe and express
these genes.
In this particular promoter,
we have a site where a
repressor can bind and a site
where an activator can bind.
So I have a repressor and
I have an activator.
So when the repressor is bound,
polymerase cannot bind
to the promoter.
Polymerase also needs the
activator to be bound to the
promoter in order to bind.
So how can the E. coli regulate
when the repressor
and activator are bound
to the promoter?
Well, when we have lactose in
the system, I have lactose
right here, the repressor
is able to bind lactose.
The repressor only binds
to the promoter
when there is no lactose.
And when binding to lactose,
the repressor comes undone
from the promoter.
How does the cell tell whether
there's glucose
in the cell or not?
When glucose is low in the
cell, the cell makes a
molecule called cyclic
AMP, or CAMP.
I have a molecule of
CAMP right here.
And so CAMP is the cell's way
of indicating that there is
low glucose in the cell.
The activator needs
CAMP to bind to be
activator binding site.
When CAMP is removed, or
glucose is added, the
activator cannot bind to
the promoter anymore.
Ok, so let's give it a try.
In this first situation
we have glucose,
we don't have lactose.
So I have neither my lactose
nor my CAMP in the system.
Repressor, can you bind?
The Repressor is bound.
Activator, can you bind?
No, the activator
is not bound.
The repressor is bound, and so
when RNA polymerase comes to
the promoter, RNA polymerase
is not able to
bind at this promoter.
And thus, it's not able to
express these genes.
And so, as we can see here,
there is no expression.
In this second situation,
we have no glucose
and we do have lactose.
Since I'm removing glucose,
I am adding back CAMP.
Activator, can you bind?
Activator is now bound.
And I'm also adding lactose
to the system.
Repressor can't bind anymore.
So what do I have at
my promoter now?
RNA polymerase comes along
and finds only an
activator at the promoter.
The polymerase can thus bind
to the promoter and move
downstream to transcribe and
express the lactose metabolism
machinery that we have here.
So we have now covered how the
E. coli organism is able to
regulate the transcription
of the
lactose metabolism machinery.
This is only one example of many
genes and many organisms
that are regulated by mechanisms
such as this.
Gene regulation is a very
widespread mechanism, in order
to keep a cell from producing
proteins that
are a waste to produce.
Thank you for watching.
