Chapter 11
Molecular
Mechanisms
of Gene
regulation
Jones and Bartlett Publishers © 2005
The 4 critical sequences in the lac operon bound by
CRP, RNA polymerase, repressor and the ribosome
See Figure 11.8.
lac operon
repression loop.
Binding of tryptophan (the co-repressor) activates
an inactive repressor into an active form
capable of binding to the trp operator site
The terminal region of the trp attenuator sequence.
Structure of the leader polypeptide
in the trp operon
The two tandem tryptophans in the leader peptide act as “stalling
sequences” in the absence of tryptophan in the cell
Alternative conformations that the trp leader RNA
can assume which are important in attenuation
Other operons with repeated amino acid sequences
that act as “stalling sequence” during attenuation
Other amino acid operons using attenuation: leucine and
isoleucine,
TRAP
• B. subtilis utilizes a different way of controlling
the expression of trp operon.
• trp operon is not repressible in B. subtilis.
• No aporepressor, no short leader polypeptide
sequence.
• TRAP: trp RNA-binding attenuation protein.
• Each monomer binds a tryptophan molecule.
• 11-mer radially-symmetric TRAP is active form.
• Binds to regions 1+2 in figure 11.13.
Riboswitches
• A 5’ untranslated leader mRNA can have a
small molecule bind to it to change its
conformation between two states:
• Antiterminator
• Terminator
• yitL gene in B. subtilis methionine
biosynthesis
Riboswitch regulation of
transcription termination
The genetic map of bacteriophage l
Control of transcription in bacteriophage l life cycle
by the anti-terminators N and Q proteins, the activator
CII protein and the repressor/activator CI protein
• cI and cro proteins function as a genetic
switch:
• cI turns on lysogeny.
• cro turns on the lytic cycle.
Transcriptional regulation in
eukaryotes
• Many housekeeping genes are expressed
constitutively.
• Levels of expression can be 2-10 fold
different in induced vs. uninduced genes.
• Galactose metabolism in yeast is a good case
study.
GAL pathway in yeast
• Extensively studied system.
• Eight genes of this pathway are involved in
galactose metabolism and regulation.
• Five genes (GAL2, MEL1, GAL4, GAL80,
and GAL5) are on different
chromosomes.
• Three genes are clustered on chromosome II
(GAL7, GAL10, GAL1).
The steps and enzymes involved in the utilization
of the sugar galactose in the yeast Saccharomyces
Function of the yeast GAL genes
GAL3, GAL4,
and GAL80 are
regulatory genes.
The transcriptional orientation of the 3 genes
coding for enzymes important in
galactose utilization in Saccharomyces
Their synthesis is regulated by the transcription activator
Gal4 protein.
Regulation of the yeast GAL genes
Transcriptional Activator Proteins
Two broad types of transcriptional activator
proteins:
helix-turn-helix motif, so it fits into
grooves of DNA
zinc finger motif, because zinc ions are
involved.
Transcriptional Activator Proteins
Transcriptional Enhancers
(Silencers – opposite of enhancers)
Deletion scanning
Transcriptional Enhancers
(Silencers – opposite of enhancers)
Deletion scanning
Transcriptional activation by
recruitment
Chromatin remodeling complexes
Alternative promoters