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Summary Unit 4 Summarized
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Extensive step-by-step summary of the textbook (Brooker) and Study Guide combined. All outcomes are covered in this summary of Unit 4.
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GNE2601 SUMMARY | Fransché BeukesLearning Unit 4: Gene regulation in prokaryotes
TRANSCRIPTIONAL REGULATION
is a regulatory protein that binds to DNA and inhibits transcription – the repressor exerts
.
is a regulatory protein that increases the rate of transcription when it binds to DNA – the
activator causes .
Inducer is a small effector molecule that causes transcription to increase. It does this by binding to a
repressor molecule and preventing it from binding to the DNA, or it can bind to an activator protein and
cause it to bind to the DNA. Either way, transcription is increased. Genes that are controlled in this
manner are called inducible genes.
Corepressor is a small effector molecule that binds to a receptor, thereby causing the molecule to bind
to the DNA. Transcription is inhibited/reduced.
Repressible genes are the genes that are controlled by corepressors and inhibitors.
operon is a group of two or more genes which are under the transcriptional control of a single promoter.
Polycistronic mRNA contains the coding sequence for two or more genes. Transcription results in
polycistronic mRNA. The genes that make up the operon are usually related in function.
The lactose operon:
Has two transcriptional units:
o Regulatory gene just before
o Operon
Operon starts with CAP site, then promoter sequence (lacP), the operator site (lacO) then three
structural genes (lacZ, lacY and lacA).
lacZ
o Encodes β-galactosidase – enzyme which cleaves lactose into glucose and galactose.
o Enzyme converts small amount of lactose into Allolactose
lacY
o Encodes lactose permease – membrane protein responsible for transporting lactose into
the cell.
lacA
o Encodes galactoside transacetylase.
CAP site and operator sites are involved in regulation.
Operator is where repressor molecule binds.
CAP site is where activator protein Catabolite Activator Protein binds.
Transcriptional unit (appears ahead of operon) is regulatory gene. Here lac repressor (lacl) is
encoded.
Gene which is not part of operon has its own promoter, which is called I promoter – gene is
constitutively expressed at very low levels.
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, GNE2601 SUMMARY | Fransché Beukes
Negative control Positive control
Lac operon is switched ON when there is
If there is NO LACTOSE in the medium, the i LACTOSE in the medium and switched OFF when
promoter is bound by RNA polymerase and the there is GLUCOSE in the medium.
lacI gene is transcribed and translated into lac Glucose and lactose both PRESENT in medium –
repressor molecules. lac operon will be repressed.
Repressor molecules bind to the operator site. No lac mRNA is made in the presence of glucose
Once bound, they prevent RNA polymerase from because in addition to an inducer to inactivate
transcribing the structural genes in the operon. the repressor, another component is necessary
for initiating lac mRNA synthesis.
If there is lactose in the medium, some of it will Small effector molecules – cyclic AMP (cAMP).
be converted into Allolactose by β-galatosidase. cAMP – made from ATP using enzyme adenylyl
The Allolactose will bind to the repressor cyclase. Concentration of cAMP is affected by
molecules. This causes a conformational change glucose concentration. Effect of cAMP on the lac
which prevents the repressor from binding to operon is mediated by the CAP (catabolite
the operator. activator protein).
If there is no repressor bound to the operator, Interaction between CAP and cAMP is:
RNA polymerase can bind and transcribe the If there is LACTOSE in the medium, NO
structural genes lacZ, lacy and lacA. Thus, GLUCOSE (cAMP concentration is high):
operon has been induced by effector molecule, Allolactose will bind to the repressor,
Allolactose. inactivating it. CAP binds to cAMP – this
complex then binds to the CAP site. The
binding of RNA polymerase to the
promoter is enhanced by the presence of
CAP-cAMP in the CAP site.
If there is NO LACTOSE, NO GLUCOSE
(cAMP concentration is high): CAP will
bind to cAMP – this complex will bind to
the CAP site. The repressor will bind to
the operator. No transcription occurs.
If there is LACTOSE AND GLUCOSE (cAMP
concentration is low): CAP is inactive
beaus ether is no cAMP – the CAP site will
be open. Allolactose will bind to the
repressor and inactivate it. No
transcription occurs because there is no
CAP-cAMP complex bound to the CAP site,
which is needed for RNA polymerase
binding.
If there is NO LACTOSE, GLUCOSE (cAMP
concentration is low): The CAP-cAMP
complex does not form – the CAP site is
open. Because there is no Allolactose, the
repressor binds to the operator. No
transcription occurs.
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