Voiceover: So what makes a cell that's located
inside of your nose responsible
for smelling, say, a slice of pizza look and act
differently from a cell that lines your gut
and is responsible for absorbing
the nutrients from that pizza?
They have the exact same DNA
so the differences can't be attributed to that fact alone.
The answer actually lies in the expression of that DNA,
which genes are actively transcribed and which ones aren't
and there are several ways in which gene regulation
occurs at the level of transcription and so we're
going to be talking about the main ones here.
Now let's draw out a hypothetical gene here
and associated with this gene is a sequence upstream
so towards the three prime region of the antisense strand,
also called the template strand.
And this sequence is called the promoter
and there is another sequence in between
the promoter and the gene called the operator.
The operator is the sequence of DNA to which
a transcription factor protein combined
and the promoter is the sequence of DNA to which
the RNA polymerase binds to start transcription.
Now first off in prokaryotes we have what are
called general transcription factors,
which are a class of proteins that bind to
specific sites on DNA to activate transcription.
General transcription factors plus RNA polymerase
and another protein complex called
the mediator multiple protein complex
constitute the basic transcriptional apparatus,
which positions RNA polymerase right at the start
of a protein coding sequence or a gene
and then releases the polymerase to transcribe
the messenger RNA from that DNA template.
Now there's another type of DNA binding protein
called activators and these enhance the interaction
between RNA polymerase and a particular promoter,
encouraging the expression of the gene
and activators can do this by increasing the attraction
of RNA polymerase for the promoter through interactions
with sub units of the RNA polymerase
or indirectly by changing the structure of the DNA.
An example of an activator is
the catabolite activator protein or CAP
and this protein activates transcription
of the lac operon in E. coli.
In the case of the lac operaon and E. coli,
cyclic adenosine monophosphate or cAMP
is produced during glucose starvation
and so this cAMP actually binds to
the catabolite activator protein or CAP
which causes a confirmational change that
allows the CAP protein to bind to a DNA site
located adjacent to the promoter and then this activator,
the CAP, then makes a direct protein to protein
interaction with RNA polymerase that recruits
the RNA polymerase to the promoter.
Now enhancers are sites on the DNA that
are bound to by activators in order to loop the DNA
in a certain way that brings a specific promoter
to the initiation complex and as the name implies
this enhances transcription of the genes
in a particular gene cluster.
And while enhancers are usually what
are called cis-acting, cis meaning the same
or acting on the same chromosome,
an enhancer doesn't necessarily need to be
particularly close to the gene that it acts on
and sometimes it's not even located
on the same chromosome.
Enhancers don't act on the promoter region itself,
but are actually bound by activator proteins
and these activator proteins can interact with
that mediator complex I mentioned earlier
which recruits RNA polymerase
and the general transcription factors which then
can lead to transcription of the genes.
So here I've drawn a little schematic of what
it might look like to have the enhancer
actually change the structure of the DNA
so that the DNA is now looping around.
Here you still have your promoter sequence,
the operator sequence, the gene sequence,
and the enhancer sequence, and having the DNA
looped in such a way so that you could then
recruit RNA polymerase, the transcription factors,
the mediator protein complex, and then you have
transcription begin of this gene here.
Now let's talk about repressors.
Repressors are proteins that bind to the operator,
impending RNA polymerase progress on the strand
and thus impeding the expression of the gene.
Now if an inducer, which is a molecule
that initiates gene expression, is present,
then it can actually interact with the repressor protein
in such a way that causes it to detach from the operator
and then this frees up RNA polymerase to then
transcribe the gene further down on the DNA strand.
One example of a repressor protein is
the repressor protein associated again
with the lac operon operator, which prevents
the transcription of genes used in lactose metabolism
unless lactose, which is the inducer molecule,
is present as an alternative energy source.
Now silencers are regions of DNA that are bound
by repressor proteins in order to silence gene expression
and this mechanism is very similar to that of
the enhancer sequences that I just talked about.
And similarly, silencers can be located several bases
upstream or downstream from
the actual promoter of the gene
and when a repressor protein binds to
the silencer region of the DNA,
RNA polymerase is prevented from
binding to the promoter region.
Now a few notes about the differences
between prokaryotes and eukaryotes
when it comes to transcriptional regulation.
In prokaryotes, the regulation of transcription
is really needed for the cell to be able to
quickly adapt to the ever-changing outer
environment that it is sitting in.
The presence, the quantity,
the type of nutrients actually determines
which genes are expressed and in order to do that,
genes must be regulated in some sort of fashion
so a combination of activators, repressors,
and rarely enhancers, at least in the case
of prokaryotes, determines whether a gene is transcribed.
In eukaryotes, transcriptional regulation
tends to involve a combination of interactions
between several transcription factors
which allows for a more sophisticated response
to multiple conditions in the environment.
And another major difference between
eukaryotes and prokaryotes is the fact that
eukaryotes have a nuclear envelope which
prevents the simultaneous transcription
and translation of a particular gene
and this adds an extra spacial
and temporal control of gene expression.