Molecular Biology

1. Sir. Stymass Kasty
Sokoine University of Agriculture.
Morogoro-Tanzania
GENE REGULATION
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2. REGULATION OF PROTEIN SYNTHESIS IN
PROKARYOTES
Relationship of protein synthesis to nutrient supply
Prokaryotes respond to changes in their supply of nutrients in a
way that allows them to obtain or conserve energy most
efficiently.
(1) Prokaryotes, such as E. coli, require a source of
carbon, which is usually a sugar that is oxidized for energy.
(2) A source of nitrogen is also required for the synthesis
of amino acids from which structural proteins and enzymes are
produced
molecular biology
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3. E. coli uses glucose preferentially whenever it is available. The
enzymes in the pathways for glucose utilization are made constitutively
(i.e., they are constantly being produced).
If glucose is not present in the medium but another sugar is
available, E. coli produces the enzymes and other proteins that allow the
cell to derive energy from that sugar.
The process by which the synthesis of the enzymes is regulated is
called induction.
If an amino acid is present in the medium, E. coli does not need to
synthesize that amino acid and conserves energy by ceasing to produce
the enzymes required for its synthesis.
The process by which the synthesis of these enzymes is regulated is
called repression
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4. OPERONS
An operon is a set of genes that are adjacent to one another in the
genome and are coordinately controlled; that is, the genes are either all
turned on or all turned off.
The structural genes of an operon code for a series of different
proteins.
(1) A single polycistronic mRNA is transcribed from an operon.
This single mRNA codes for all the proteins of the operon.
(2) A series of start and stop codons on the polycistronic mRNA
allows a number of different proteins to be produced at the translational
level from the single mRNA.
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5. Transcription begins near a promoter region, located
upstream from the group of structural genes. Associated with
the promoter is a short sequence, the operator, which
determines whether the genes are expressed or not.
Binding of a repressor protein to the operator region
prevents the binding of RNA polymerase to the promoter and
inhibits transcription of the structural genes of the operon.
Repressor proteins are encoded by regulatory genes, which
may be located anywhere in the genome.
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6. .
INDUCTION
Induction is the process whereby an inducer (a small
molecule) stimulates the transcription of an operon
The inducer is frequently a sugar (or a metabolite of the
sugar), and the proteins produced from the inducible operon
allow the sugar to be metabolized.
(1) The inducer binds to the repressor, inactivating it.
(2) The inactive repressor does not bind to the operator.
(3) RNA polymerase, therefore, can bind to the promoter
and transcribe the operon.
(4) The structural proteins encoded by the operon are
produced
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8. REGULATION OF OPERONS BY REPRESSORS.
When the repressor protein is bound to the operator, RNA
polymerase cannot bind, and transcription therefore not occur
The lactose (lac) operon is inducible.
(1) A metabolite of lactose, allolactose, is the inducer.
(2) Proteins produced by the genes of the lac operon allow the
cell to oxidize lactose as a source of energy. Gene Z produces a a-
galactosidase; gene Y, a lactose permease; and gene A, a transacetylase.
(3) The lac operon is induced only in the absence of glucose. It
exhibits catabolite repression
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10. REPRESSION
Repression is the process whereby a corepressor (a small
molecule) inhibits the transcription of an operon
An inducible operon (e.g., the lac operon).
If the inducer is absent, the repressor is active and binds
to the
operator, preventing RNA polymerase from binding. Thus,
transcription
does not occur.
If the inducer is present, it binds to and inactivates
the repressor, which then does not bind to the operator.
Therefore, RNA polymerase can bind and transcribe the
structural genes.
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11. A repressible operon. The
repressor is inactive until a small
molecule, the co-repressor,
binds to it. The repressor-co-
repressor complex binds to the
operator and prevents
transcription.
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12. The co-repressor is usually an amino acid, and the
proteins produced from the repressible
operon are involved in the synthesis of the amino
acid.
(1) The corepressor binds to the repressor, activating
it.
(2) The active repressor binds to the operator.
(3) RNA polymerase, therefore, cannot bind to the
promoter, and the operon is not
transcribed.
(4) The cell stops producing the structural proteins
encoded by the operon.
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13. The tryptophan (trp) operon is
repressible.
(1) Tryptophan is the co-repressor.
(2) The proteins encoded by the trp
operon are involved in the synthesis of
tryptophan.
(3) The trp operon is repressed in the
presence of tryptophan, since cells do not need
to make the amino acid if it is present in the
growth medium.
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14. POSITIVE CONTROL
Some operons are turned on by
mechanisms that activate transcription.
When the repressor of the arabinose (ara)
operon binds arabinose, it changes the
conformation and becomes an activator that
stimulates the binding of RNA polymerase to
the promoter. The operon is then transcribed,
and the proteins required for the oxidation of
arabinose are produced.
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15. CATABOLITE REPRESSION
Cells preferentially use glucose when it is available.
Some operons (e.g., lac and ara) are not expressed when
glucose is present in the medium.
These operons require cAMP for their expression.
(1) Glucose causes cAMP levels in the cells to decrease.
(2) When glucose decreases, cAMP levels rise.
(3) cAMP binds to the catabolite-activator protein
(CAP).
(4) The cAMP–protein complex binds to a site near the
promoter of the operon and facilitates binding of RNA
polymerase to the promoter
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16. THE LAC OPERON EXHIBITS CATABOLITE
REPRESSION.
(1) In the presence of lactose and the
absence of glucose, the lac repressor is
inactivated, and the high levels of cAMP
facilitate the binding of RNA polymerase to the
promoter.
(2) The operon is transcribed, and the
proteins that allow the cells to utilize lactose are
produced.
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17. ATTENUATION
a. In bacterial cells, transcription and translation
occur simultaneously.
b. Attenuation occurs by a mechanism by which
rapid translation of the nascent transcript causes the
termination of transcription.
c. As the transcript is being produced, if
ribosomes attach and rapidly translate the transcript,
a secondary structure is generated in the mRNA that
is a termination signal for RNA polymerase.
.
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18. CATABOLITE REPRESSION.
The operon is transcribed only when
glucose is low. Cyclic adenosine monophosphate
(cAMP) is elevated, and the inducer binds to the
repressor, inactivating it. Under these conditions, the
cAMP-CAP complex forms and binds to the DNA,
facilitating the initiation of transcription by RNA
polymerase. As shown in this figure, the lac operon
exhibits catabolite repression.
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19. IF TRANSLATION IS SLOW,
This termination structure does not form, and
transcription continues.
(1) Multiple codons for the amino acid are
located near the translation start site of the
mRNA.
(2) When cells contain low levels of the amino
acid (which is produced by the enzymes
encoded by the operon), less aminoacyl-tRNA is
available to bind to these codons, and
translation slows down.
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20. Factors, such as sigma,
affect RNA polymerase
activity. These factors bind
to the core RNA polymerase
and increase its ability to
bind to specific promoters
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21. Email. Stymass1@gmail.Com
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22. .
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