gene expression

1. 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)

2. 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.

3. 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)

4. Lactose Catabolism (lac) Operon
Doesn’t work if glucose
is available! Why?

5. 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.

6. lac Operon in
Action
(diauxic growth)
• 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!
Separate cultures Together

7. Tryptophan (Trp) Operon
(Trp synthesis (anabolic); regulated by repression and attenuation.)

8. Transcriptional Regulation by
Attenuation
• In addition to a promotor and
operator, the operon has a leader
sequence with two pairs of self-
complementing 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.

9. 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