Download Regulation of gene expression

Genetic regulation
• Genotype is not phenotype: bacteria possess many
genes that they are not using at any particular time.
• Transcription and translation are expensive; why
spend ATP to make an enzyme you don’t need?
• Operon
– Genes physically adjacent regulated together
• Regulon
– Genes dispersed but controlled by same proteins
– Operator sequences must be same/similar
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More on Regulation
• Two important patterns of regulation: Induction and
repression.
– In induction, the genes are off until they are needed.
– In repression, the genes normally in use are shut off
when no longer needed.
• Negative control
– Binding of protein to the DNA prevents transcription
• Positive control
– Binding of protein to DNA promotes transcription
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Repressible operons
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• Operon codes for enzymes that make a needed amino
acid (for example); genes are “on”.
– Repressor protein is NOT attached to DNA
– Transcription of genes for enzymes needed to
make amino acid is occurring.
• The change: amino acid is now available in the
culture medium. Enzymes normally needed for making
it are no longer needed.
– Amino acid, now abundant in cell, binds to repressor protein
which changes shape, causing it to BIND to operator region
of DNA. Transcription is stopped.
• This is also Negative regulation (protein + DNA = off).
Repression picture
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Transcription by RNA
polymerase prevented.
Regulation can be fine tuned
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The more of the amino acid present in the cell, the more
repressor-amino acid complex is formed; the more likely
that transcription will be prevented.
Structure of the Lac operon
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KEY:
P O are the promoter
and operator regions.
lac Z is the gene for
beta-galactosidase.
lac Y is the gene for
the permease.
lac A is the gene for
a transacetylase.
lac I, on a different
part of the DNA, codes
for the lac repressor,
the protein which can
bind to the operator.
Binding of small molecules to proteins causes
them to change shape
Characteristic of many DNA-binding proteins
Regulation of operons:
Inducible operons: Repressor protein comes off DNA
Repressible operons: Repressor protein attaches to DNA
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How the lac operon works
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When lactose is NOT present,
the cell does not need the
enzymes. The lac repressor,
a protein coded for by the
lac I gene, binds to the DNA
at the operator, preventing
transcription.
When lactose is present, and
the enzymes for using it are
needed, lactose binds to the
repressor protein, causing it
to change shape and come off
the operator, allowing RNA
polymerase to find the
promoter and transcribe.
http://www.med.sc.edu:85/mayer/genreg1.jpg
Lactose is not actually the inducer
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Low basal levels of betagalactosidase exist in the
cell. This converts some
lactose to the related
allolactose which binds to
the lac repressor protein.
Synthetic inducers such as
IPTG with a similar
structure can take the place
of lactose/allolactose for
research purposes.
http://www.search.com/reference/Lac_operon
Glucose is the preferred carbon source
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Positive regulation
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• Presence of lactose is not enough
– In diauxic growth graph, lactose is present from the start.
Why isn’t operon induced?
• Presence of glucose prevents positive regulation
– NOT the same as inhibiting
– Active Cyclic AMP receptor protein (CRP) needed to
bind to DNA to turn ON lactose operon (and others)
– Presence of glucose (preferred carbon source) prevents
activation of CRP.
www.answers.com/..
./cataboliteactivator-protein
Additional controls
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• Attenuation
– Seen w/ repressible operons, fine tuning
– Ribosome does not stall, transcription terminated
• mRNA rapidly degraded
– Signal “to make” stops, residual mRNA destroyed
• Examples of
– Antisense RNA: binds to mRNA, prevents use
– DNA rearrangements; genes flip in place, different gene
product produced
– Ribosome binding protein prevents translation
Global control: modulons
• Different operons/regulons affected by same
environmental signal
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Presence of glucose
Change from O2 to anaerobic growth
Nitrogen limitation; phosphate starvation
Growth rate control
Cell division
Stationary phase; entering starvation state
• One method of control: alternate sigma factors
– Sigma controls which promoters are used
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