Download Regulation of Gene Expression

Regulation of Gene Expression
• Chromosomal Map begins
at OriC; units of minutes.
– Only structural genes for
enzymes are shown here.
– Their control regions (promoter
and operator) determine
transcription.
– The complete organizational unit
is an operon.
• Transcriptional regulation:
– Negative Control by Repressors
• Repression
• Induction
– Positive Control by Activators
– Attenuation (involves translation)
Transcriptional Regulation by
Repression
• Regulatory protein
(repressor) is encoded on
a gene outside and away
from the operon it
regulates.
• Active repressor binds
operator region; RNA
Polymerase blocked =
negative control.
• Repressor becomes
active by a corepressor.
• Corepressor is often an
endproduct of pathway
enzymes encoded on the
operon.
Transcriptional Regulation by
Induction
• Active repressor binds
operator region; RNA
Polymerase blocked =
negative control.
• Gene transcribed when
inducer molecule is
present; binds and
inactivates repressor
(release from operator).
• Inducers are typically
substrate for a pathway
enzyme encoded on the
operon (e.g. allolactose for
the lac operon)
Lactose Catabolism (lac) Operon
Doesn’t work if glucose
is available! Why?
Transcriptional Regulation by
Catabolic Activator Protein (CAP)
• CAP = cyclic AMP
receptor protein (CRP).
• Active CAP binds
promotor and allows
transcription to proceed
= positive control.
• Activation of CAP
requires build-up of
cAMP to bind to CAP.
• cAMP builds-up in cells
not producing enough
ATP due to lack of
glucose availability.
• The lac operon requires
both lactose and cAMP.
lac Operon in
Action
(diauxic growth)
Separate cultures
• PEP-PTS at high glucose uptake
lowers adenyl cyclase activity;
low cAMP; CAP inactive.
• Exhaustion of glucose increases
cAMP, activating CAP; repressor
is inactivated; lac operon
transcribed!
Together
Tryptophan (Trp) Operon
(Trp synthesis (anabolic); regulated by repression and attenuation.)
Transcriptional Regulation by
Attenuation
•
In addition to a promotor and
operator, the operon has a leader
sequence with two pairs of selfcomplementing sequence sections
(#1&2 and #3&4). The first pair is
in what is called the leader peptide
gene.
•
The second pair (#3&4) is part of a
Rho-independent terminator region
upstream of any structural genes;
called an attenuator. Trp high.
•
Prevention of the first pair
complementing will result in a
hybrid complement of first and
second pair (sections #2 and #3).
Trp low.
Transcriptional
Regulation by
Attenuation
•
Attenuation of transcription
results when the attenuator
hairpin can form.
•
It forms when there is no
translation of leader sequence
mRNA & when there is ample
trp-tRNA.
•
Absence of trp-tRNA causes
ribosome to stall, blocking
section #1; hybrid forms.
•
No attenuation hairpin; RNA
polymerase proceeds to
transcribe genes.
1) No Translation; No
genes transcribed!
2) Trp & trp-tRNA available
3) Trp & trp-tRNA absent