Now I have drawn this figure on the
screen here to give you an idea
of the transcriptional activation
of a eukaryotic gene.
Now it illustrates a couple of different terms
that I wanna say something about.
So there are three main things that I
wanna communicate in this image and
of course we are keeping in mind that the complexity
of eukaryotic transcription is considerable more
than it's shown in the slide. The first of this
is the enhancer sequences that you can see here.
Now enhancers are sequences in DNA
and the sequences in DNA are located
within usually a few thousand
base pairs of a target gene
that the cell will ultimately want to express.
In some cases enhancer sequences are known to
be as far as a million base pairs away but
usually they are within a few thousand.
The enhancer sequences are targets for binding
by proteins called activator proteins.
And as the name is suggest the activator
proteins are activating something
and these activator proteins are there to help
activate the process of transcription.
Well transcription doesn't happen at the
site of the enhancer, as you can see.
Instead transcription is happening a long way away.
I said it could be few thousands base pairs for example.
So how does an enhancer sequence
activate transcription that is a long ways away
and how those activator proteins accomplish this?
Well they accomplish it, as you can see,
on the screen by a bending of the DNA
to bring the activator proteins and the enhancer
close to the promoter. Now the promoter
for the gene of interest here
is on the bottom strands and it's where that
handful of individual proteins are found.
These are called transcription factor proteins.
And these transcription factor
proteins form a complex
at the site of the promoter
that is the place close to where the RNA
polymerase is going to start transcription.
Now in eukaryotic cells the RNA polymerase that
catalyzes transcription is called
RNA polymerase II.
And the proteins that are the transcription
factors that you see in the box
have designation of TF
as in transcription factor the roman
numeral 2 (II) and the variety
of letters corresponding to each of the
individual transcription factors.
So for example TFIID is one of
the first proteins that binds
to the promoter;
because, it recognizes the 80 rich
sequence that we saw earlier
in promoters of prokaryotic cells.
This complex of proteins is helped to
form by the activators. That's part of the
function what the activators are doing.
This large complex that you see here and
they can actually be quite complicated
is necessary for the RNA
polymerase II to bind.
The RNA polymerase II will not bind
to bare DNA. It will not bind to DNA
by itself. Now that's in contrast
to what we saw in E-Coli in another presentation;
because, in E-Coli all that it
takes for RNA polymerase to
bind in many cases is the sigma factor
helping it to recognize the promoter.
Eukaryotes are a lot different
and the number and complexity of proteins allows
for either activation or inactivation of
transcription for a variety of reasons
as might be needed by eukaryotic cells.
Well there is a third factor
that's involved in helping to
control the transcription here.
And this third factor is another
sequence that can be adjacent
to the promoter and adjacent to the enhancer
and it's called an insulator sequence.
An insulator sequence works
opposite to an enhancer sequence.
It works probably through
a protein called CTCF.
So what CTCF and the insulator
sequence are thought to do,
is to alter the 3D configuration of this
entire complex that you see here.
And that alteration doesn't
allow for the looping back
which is what I described
earlier, the looping back
of the enhancer to help form
the transcriptional complex.
In this way CTF is thought to stop the
transcription of individual genes.
Now factor affecting the binding
of CTF to the insulator sequence may well be
the modification chemically of the DNA.
So we are starting get pretty complex
here in terms of what all is happening.
So the modification of the
DNA may affect the insulator,
the insulator may affect the enhancer and the
enhancer may not be able to affect the promoter.
It's complicated but eukaryotic
transcription is very complicated.
This schematically shows what I have
described to you on the last figure.
We have an enhancer and when
the insulator is not active,
the enhancer can help to activate individual
genes by the looping I have described here.
The insulator can block the sequences if
CTCF binds and changes the 3D
configuration that I described.
Blocking the insulator, as I said,
allows the enhancer to work
but letting the insulator
function stops transcription.