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.
In the lac operon we see, of course,
that we have several things.
First of all, we have the three genes
in the lac operon that I have described.
These are the lacZ, the
lacY and the lacA genes.
From this operon is transcribed
the messenger RNA that is shown
in the line immediately beneath
it that has the AUGs within it.
Each AUG is the starting
coding region for each gene.
AUG being the coding for the first
of the amino acid that go in to there.
So there are three different
genes that are made
and each gene starts with an
AUG within that larger messenger RNA.
Now another gene that is in the region,
I just showed it, because, of its interest
is the coding gene
for the lac repressor.
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.
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.
Now I would like to take you through some various
scenarios to describe what happens in these scenarios.
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.
So when cyclic AMP binds to CAP that
creates the circumstances that
starts the CAP binding
to the CAP sequence.
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.
Low glucose, lactose available,
RNA polymerase binds, transcription will occur.
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.
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.
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.
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.
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.
The binding to the O site precludes the
RNA polymerase from binding to the P site.
Now in the third scenario under conditions of
low glucose and lactose being unavailable
we have an interesting circumstance.
You recall that under conditions
of low glucose, cyclic AMP is made.
So the CAP binds to the cyclic AMP and it binds
to the CAP site as we have seen before.
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.
When this happens, we basically have the
CAP trying to turn on transcription
and we have the lac repressor
trying to turn off transcription.
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.
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.
The last scenario is also
an interesting scenario.
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.
But further lactose is available
so allolactose is being made
which means that the repressor
won't bind to its site either.
When that happens we have a sort of a bare DNA that
you can see in this picture at the very bottom.
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.
Since there is high amounts of glucose and there is
lactose available, it's not really a waste of energy.
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.
So there are several different metabolic circumstances
that an E-Coli cell might find itself into
and the lac operon adjust accordingly.
So when lactose is absent the lac
repressor binds to the O site and stops
RNA polymerase from
transcribing the operon.
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.
When cyclic AMP is abundant and bound to the CAP, this
occurs when the glucose concentration is low.
This will help facilitate transcription
if the cell has lactose available to it.
So the expression will be highest then, when
lactose is available and glucose is low.
And expression will be lowest
when lactose is absent.
Thereby giving the cell
exactly what it needs
when it has lactose available or
doesn't have lactose available.
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.
RNA is diverse, RNA
is important for cells,
and RNA is essential for making protein.
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.