Lecture 7 1
Because gene expression is metabolically expensive.
Control of gene expression is essential to LIFE
Genes in prokaryotes regulated together transcriptionally
are called operons. Co-regulation of genes used for a
Genes must be regulated so that energy is used
Late 1950’s, the 1 example of a regulatory protein was
discovered during a genetic study of the E. coli lac operon.
Monod et al. noted mutants that showed constitutive
expression of β-galactosidase rather than inducible,
Induction could be restored by introducing genetic material
in trans. Postulated that the DNA encoded a repressor
This led to the notion that ALL regulation was due to
Not until the late 1960’s did experiments with arabinose
and maltose operons challenge this view. Lecture 7 2
Mutations in these pathways led to non-inducibility
Induction was restored by in trans DNA.
Now, hundreds of activators and repressors are known.
Some are specific acting at a few loci (lacI)
Others are “global” regulators controlling 100’s of genes
Sequence analysis shows that most activators an
repressors belong to a small number of families . Lecture 7 3
Both genetics and biochemistry have been used to identify
binding sites for transcription factors.
Base substitutions > binding interference and footprinting.
Sequence analysis and in silico methods nowadays.
In most cases, activator binding sites are located
upstream of repressor binding sites.
For σ70, activators bind –30 to –100
For repressors, they overlap the RNAP binding site. -35,
-10. Physically stop the RNAP from binding.
4 functions of regulators
1. Bind specific DNA sequences.
2. Multimerize for increased stability on DNA.
3. Directly contact and recruit RNAP.
4. Their activity can be modief (by other proteins).
It turns out that most Tn factors are made up of modules,
& these modules appear in a number of different proteins. Lecture 7 4
1. DNA-binding domains –
A. Helix turn helix (HTH)
B. Zinc fingers (rare in prokaryotes)
C. Leucine zipper (rare)
2. Oligomerization – Dimers or tetramers (increases
the concentration of activating signals on DNA)
3. Activation domains – (interact with RNAP through
acidic amino acids).
Activator or Repressor?
Are there rules to determine whether a regulator functions Lecture 7 5
as a repressor or activator?
Site of Binding – Activators Prefer -30 or -90.
Repressors Are downstream of -30.
No Activators are known to bind downstream of -30 (would
interfere with RNAP binding).
Single Tn activator stimulates Tn initiation.
Can be studied easily in vitro
1. E. coli lac promoter+ CRP (cyclic AMP recognition
2. λ CI repressor and P RMpromoter
3. MerR activator of merT (Involved in resistance to
These studies show that activators accelerate the
formation of open promoter complexes that are similar to
those that form at activator-independent promoters. Lecture 7 6
Because of the competition for RNAP in the cell, small
differences in promoter efficiency will result in big