Chapter 21 (Part 2)
Transcriptional Regulation and
RNA Processing
Regulation of Gene Expression
RNA Processing
5’CAP
Active
enzyme
Post-translational
modification
mRNA
AAAAAA
RNA Degradation
Protein Degradation
Activators and Repressors
co-A
A
RNAP
+1
RNAP
co-R
R
+1
The Helix-Turn-Helix
Motif
• contain two alpha
helices separated by
a loop with a beta
turn
• The C-terminal helix
fits in major groove
of DNA; N-terminal
helix stabilizes by
hydrophobic
interactions with Cterminal helix
The Zn-Finger Motif
Zn fingers form a folded beta strand and an alpha helix that
fits into the DNA major groove.
The Leucine Zipper Motif
• Forms amphipathic
alpha helix and a
coiled-coil dimer
• Leucine zipper proteins
dimerize, either as
homo- or heterodimers
• The basic region is the
DNA-recognition site
• Basic region is often
modeled as a pair of
helices that can wrap
around the major
groove
Transcription Regulation in
Prokaryotes
Binding of some trans-factors is
regulated by allosteric modification
lac operon
Glucose is E. coli’s
primary carbon
source.
But.. it can grow
on different carbon
sources.
Diauxic growth of E. coli on a mixture of
lactose + glucose.
The lac I protein
Operator and RNA Polymerase Bind at Overlapping Sites
Inhibition of repression of lac
operon by inducer binding to lacI
Inducer : Allolactose,
produced by side
reaction of lacZ
Lehninger: Principles of Biochemistry, 3rd Ed.
IPTG is a Gratuitous Inducer
• Synthetic molecule
Repression of
the Tryptophan
operon:
A variation of
the theme
Catabolite Repression of lac Operon
(Positive regulation)
Molecular Cell Biology, 4th Edition, Lodish et. al. (2000)
Why does the Lac Operon
need an activator?
Lac promoter has lousy promoter!!!
tRNA Processing
rRNA Processing
Processing of Eukaryotic mRNA
Splicing of Pre-mRNA
Splicing of Pre-mRNA