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In this video we're going to discuss regulation and gene expression in the bacteria
Gene expression is the process in which the information contained within a gene
is used to synthesize a protein.
Most bacterial genes are expressed all the time however some genes are regulated, meaning
their expression is controlled. Genes are regulated in order to help a cell conservative
its energy and resources.
In prokaryotic cells gene regulation often occurs through the use of operons. An operon
is a group gene and its associated regulators. There
are four main components to an operon. A regulator gene which will be located upstream
from the other components of the operon. The regulator gene codes for the repressor protein.
There's a promoter region, where the RNA polymerase binds. RNA polymerase is needed for the transcription
process. The third component is an operator region
and it serves as the binding site for the repressor protein.
Finally there are structural genes. Structural genes code for the enzymes and proteins needed
for the metabolic processes. There can be one or more structural genes in an operon.
There are two different types of operons, inducible operon and repressible operons. In the inducible
operon the gene is not normally being transcribed and must be induced. In other words it is
normally off and must be turned on, whereas in a repressible operon, the gene is normally
being transcribed and must be repressed. So in a repressible operon the operon is normally on
and must be turned off. Let's look at an example of each.
First is the lactose operon, the classical example of an inducible operon. In a lactose
operon. It is normally off and must be turned on. It will be turned on when it needs to
make the structural proteins, in the case of the lac operon there are three structural genes each which codes
for enzymes necessary for the transport and digestion of lactose. As the operon is normally
off and it must be induced or turned on, in order to turn the operon on we need an inducer
and in the case of the lactose operon, lactose is the inducer. So let's look at the parts
of the lactose operon. Again we have the regulator and it is found upstream from the other components
of the operon. The regulator codes for the repressor proteins, the promoter region which
is going to be the binding site for RNA polymerase. The operator region which is going to be the
binding site for the repressor protein and the structural genes. Again in the lactose
operon there are three structural genes, each coding for the enzyme needed for the digestion
of lactose. Looking at the repressor protein, notice that it has two unique binding sites,
one binding site allows the repressor to bind to the operon, the other binding site
is for the inducer, or lactose, in this case. The lactose operon is an inducible operon
therefore normally the lac operon is in the off mode and does not initiate transcription. As long
as lactose is not present the repressor protein is able to bind to the operator region of
the operon. When RNA polymerase attaches to the operon at the promoter region it tries
to move downstream, but it is blocked by the repressor protein. The RNA polymerase is not
able to transcribe the structural genes because transcription cannot occur
then translation cannot occur. Without the structural genes being transcribed no enzymes
are being produced. What happens when lactose becomes available? Lactose then initiates
the events to turn the operon on. Lactose binds to the repressor protein at the lactose
binding site, the repressor protein undergoes a conformational change, and notice now that it is
no longer able to bind to the operator. The repressor protein then falls off of the operon.
Now the RNA polymerase can bind to the promoter region and move along the operon, transcription
can occur. Transcription occurs, messenger RNA is made and translation is able to occur at
the site of the ribosomes. The enzymes are then made. The enzymes, being digestive
enzymes, are used to break down and digest lactose. The cell is now able to use the lactose
sugar for its metabolic processes. Once the lactose has been fully digested it is no longer
able to bind to the repressor protein. The repressor protein returns to its normal conformation
and is able again to bind to the operator region and the operon is now turned off. The
lactose operon is an example of an inducible operon, it's normally off and must be turned
on. Let's look at an example of a repressible protein. Tryptophan operon is
a repressible operon. In the tryptophan operon, the operon is normally on and must be turned off. When
the cell no longer needs to make the particular enzymes or proteins. In the case of tryptophan operon
there are five structural genes all coding for enzymes needed to synthesize tryptophan.
In order to turn the tryptophan off a co-repressor is needed. In the case of tryptophan operon,
the co-repressor is tryptophan itself. Let's look at the components of the trp operon.
It, too, has a regulator region upstream from the rest of the operon and it codes for the repressor
protein, the promoter region is the binding site for RNA polymerase. The operator region
again will bind to a repressor protein and there are structural genes. In the case for
trp operon there are five structural genes each coding for an enzyme needed to synthesize
the amino acid tryptophan. There is the repressor protein it will have two binding site, one
for the co-repressor and another for the operator. And there is the co-repressor itself, in this case
is the amino acid, tryptophan. As a repressible operon
8:10 Tryptophan is typically in the on
mode synthesizing the enzymes needed for making the amino acid tryptophan.
When tryptophan is absent from the environment the repressor protein is not able to bind
to the operator region because it does not have a binding site or a shape that will allow
it to bind. The RNA polymerase is able to initiate transcription, translation can occur, the
proteins can be made and tryptophan can be produced. However when tryptophan becomes
abundant in the environment it acts as a co-repressor activating the repressor protein
and turning the operon off. As a co-repressor tryptophan binds to the repressor protein
the repressor protein undergoes a conformational
change, this change in shape allows the repressor protein to attach to the operator region of
the operon. Now when RNA polymerase binds to the promoter. The repressor protein blocks
it from moving forward, transcription is blocked, translation cannot occur and thus the enzymes
are not synthesized and tryptophan cannot be produced. Once the
cell uses the tryptophan from the environment tryptophan will no longer be available to
attach to the repressor. The repressor will then return to its normal shape and will no
longer be able to bind to the operator. The operon will be turned on and the cell will
be able to synthesize tryptophan again. And that concludes our discussion on bacterial
gene expression and operons.