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Gene Regulation
I. Gene regulation in Prokaryotes
A. Many related genes in prokaryotes are arranged sequentially along the chromosome.
When one is transcribed, all are transcribed into one long (polycistronic) mRNA.
B. Transcription begins when RNA polymerase first binds to a set of DNA bases that serve as the
point of attachment
1. this sequence of bases are called the Promoter
a. another kind of gene – a regulatory gene. Note this gene is not transcribed.
b. genes that are transcribed are called structural genes
C. Three possible situations
1. Promoter always capable of binding to RNA polymerase and therefore the genes in
question are always transcribed (“on”) >genes that are always on are called constitutive
genes
2. Promoter usually incapable of binding to RNA polymerase and therefore the genes are
usually not transcribed (“off”) but can be turned on
3. Promoter usually capable of binding to RNA polymerase and therefore the genes are
usually transcribed (“on”) but can be turned off
D. The second and third situations imply that genes can be controlled (turned on or off). How?
1. Control of a prokaryote gene was first worked out by two French scientists Jacob
and Monod
a. the model they developed was the called the lac operon for E coli
1. E. coli can digest and utilize lactose if it is present in the culture
2. three proteins are essential for this process.
3. They are made only when lactose is present. Therefore the genes for
these proteins must be capable of being turned on when lactose
present.
2. Jacob and Monod proposed that a functional units called an Operons existed on the
chromosome of E. coli.
a. an operon consists of:
1. two regulatory genes consisting of the:
a. operator gene
b. promoter gene
2. one or more structural genes.
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b. In the lac operon three structural genes coded for the three proteins needed to
digest, transport and utilize lactose.
c. In addition to the genes of the operon another gene that is usually located some
distance from the operon codes for another structural gene that was always
on – the regulatory gene – which produced a protein called the “repressor”
1. repressor can be made in one of two forms depending on the particular
regulator gene
a. active repressor
b. inactive repressor
d. In the lac operon the regulatory gene lies next to the operon (an unusual
situation) and makes active repressor
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E. The lac operon – an inducible operon
1. Active repressor binds to the operator and prevents RNA polymerase from binding to
the promoter
2. the active repressor molecule has a binding site to which an chemical called the
inducer can bind. When “induced”, the repressor becomes inactive.
3. inactive repressor falls way from the operator and RNA polymerase can bind to the
promoter causing the structural genes to be transcribed.
4. Remember we want this to happen when lactose is present. The inducer for the
lac operon is lactose and certain of its analogs (collectively referred to as
allolactose)
5. The inducible operon model accounts for the situation when the gene is usually off.
What about the situation when the gene is usually on?
F. The trp operon – a repressible operon.
E. coli can synthesize the amino acid tryptophan. Certain enzymes are necessary
for this and the genes for these enzymes must be “on” for this to occur.
If tryptophan is placed in the culture medium, the genes for these enzymes turn off.
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2. the regulator gene, therefore, makes inactive repressor.
3. inactive repressor cannot bind to the operator and, therefore, RNA polymerase binds to
the promoter and the structural genes are transcribed.
4. When tryptophan is present, it binds to the inactive repressor activating it. The
tryptophan is said to be a corepressor
5. the active repressor binds to the operator turning off the promoter.
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G. The inducible and repressible operons are examples of negative gene regulation. The basic
method of action is to prevent transcription.
H. Positive gene regulation in prokaryotes - Catabolite Activator Protein (CAP)
1. The lac operon not only requires the presence of lactose but also the absence of glucose
a. by itself, the lac operon is has a very inefficient promoter sequence.
2. In order to work, the lac operon requires a positive regulator called CAP to be present
a. CAP binds to a site immediately adjacent to the promoter sequence.
b. makes it easier for RNA polymerase to bind to promoter. Without CAP very
little transcription takes place even in the presence of lactose
3. CAP must be activated by first binding to cAMP. It is actually a CAP-cAMP complex
that activates the promoter. This is the key to how E. coli “knows” if glucose is
present.
4. If glucose present, very little cAMP present and, therefore, very little active CAP
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