Now we know, of course, that RNA is produced by
the transcription of DNA, the
copying of DNA-RNA polymerase.
And if that is done to make the coding for a gene
then, of course, we are thinking about gene expression
In prokaryotic systems we have an interesting
situation with respect to the layout of genes.
Now in prokaryotic system, genes are very
closely spaced to each other within the genome.
Meaning that if we have a gene at
one place and then the end of the gene
the beginning of the next gene is not very far
away. So they are very close to each other.
And that's different in the arrangement
that we see in eukaryotic systems;
because, of this close spacing bacteria actually
use this somewhat to their advantage
and in an interesting way that
eukaryotes do not do.
So in prokaryotic systems genes are commonly
organized in what are called operons.
So an operon is a sequence
that has multiple genes
that are under the control
of a single promoter.
And when that single promoter is activated, as
it is in the figure you see on the right here,
the RNA polymerase copies all
of those genes onto a single messenger RNA.
Now that's different from the eukaryotic system
where the widely spaced eukaryotic genes
when transcription occurs, only one gene
makes it onto the final transcript. So the
operon arrangement in bacteria
is unique to bacteria.
Operons are transcribed and
translated when they are needed
and this organization means that
it's important on a given operon
that the genes that are there have related functions. And we
will see in an example of that at the end of this lecture.
The RNAs that are made in bacteria comes
from transcription and the transcription
that is controlled in bacteria
uses very simple promoter elements.
When we talk about elements and we are actually talking
about the sequence of DNA that comprise the promoter.
So the simple needs of bacteria actually work out fairly well;
because, these simple controls means that there aren't too many
different variations that bacteria use
or need in fact during their life style.
So in prokaryotes I would like
to go through and show you an example
of a system involving a fairly simple control
and it involves an operon
called the lac operon.
Now the lac operon is a
segment of the E-Coli genome
that contains coding for three genes
necessary for the metabolism of lactose.
Lactose as we have seen
in other discussions here
is a disaccharide that is an energy
source; because, it provides sugar.
In order for the bacterium
to use the lactose
they have to be able to break it down;
because, lactose contains two sugars.
So one of the genes that's
contained in the lac operon
contains a gene called beta
galactosidase that breaks lactose
into glucose and galactose,
its constituent sugars.
So glucose and galactose can be metabolized in the
metabolic pathways that we have discussed before.
The operon contains the 3 genes
for lactose metabolism, as I said.
These include the lacZ gene, the
lacY gene and the lacA gene,
Now for our purposes here the only
gene that really matters is the
lacZ gene, because, that's the beta
galactosidase that breaks down lactose.
Nonetheless all three genes are made
in the organism and all these genes
are needed for the proper metabolism
and proper functioning of
the overall lactose operon.
Now lacZ, as I said, cleaves
lactose into glucose and galactose.
And the cell really has this need to
do this. But it's important that the cell
be making this operon only when it needs it;
because, cells don't have a lot of energy to
throw into things that they don't need.
And the synthesis of proteins, the synthesis of RNAs
is a very energy intensive process.
So ideally the cell is only making
genes when it needs the genes
and in the case of a lac operon
the cell is ideally only making
this operon when lactose is present
and when the cell is needing it.
So the transcription of the lac
operon is controlled by several proteins.
It's not very complicated but it's
not simply just an RNA polymerase.
The first protein that performs a function
here is called the lac repressor.
Now this is a protein that can bind to the
site on the lac operon called the O site
and it is highlighted on the DNA before.
The O site is the part of the overall
promoter or control sequence for the operon.
We will see that the control sequence
for the operon has several components
and these are managed by the individual proteins.
The function of the lac repressor
is to bind to the O site.
And when it binds to the O site it
stops transcription from occurring.
Now we will see there are times when it
can bind and other times when it cannot bind.
The second protein that plays a role in this
overall transcription is called the CAP.
So the CAP is a protein
that binds to the CAP site
and the CAP site, you can see,
is located near the O site.
The function of the CAP is
to bind to the CAP site
and in doing so facilitate the
binding of the RNA polymerase
to the P site which you can
see is adjacent to the CAP site.
So lac repressor and CAP
have opposite functions.
Lac repressor to block transcription
and CAP to activate transcription.
The RNA polymerase of course
binds to the P site which we can
describe here as the promoter. Although
this entire region is technically a promoter.
Now the factors that influence
whether binding of these proteins
occur to the control sites on the DNA or not
are mediated by small molecules.
Now one of the small molecules that
plays a role here is cyclic AMP.
Cyclic AMP is bound by CAP.
And when cyclic AMP binds to CAP,
CAP can bind to the CAP site.
And if cyclic AMP is not present then
CAP cannot bind to the CAP site.
Allolactose is related to lactose and for
our purposes we can think of it like lactose.
But the important thing about allolactose
is it binds to the lac repressor.
Allolactose is present
when lactose is present.
So that means that when lactose is present,
allolactose will bind to the lac repressor.
The importance of that is that when
allolactose is bound to the lac repressor,
the lac repressor cannot bind to the O site.
However, when allolactose is absent as
it is when lactose is not present
then lac repressor will bind to
the O site and stop transcription.
So we have really two scenarios here.
One scenario where we are activating
transcription, this is when cyclic AMP is present.
And one scenario where we are stopping
transcription and that's when lactose is absent.