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Chapter 21 (Part 2)
Transcriptional Regulation and
RNA Processing
Regulation of Gene Expression
AAAAAA5’CAPmRNA
RNA Processing
RNA Degradation
Protein DegradationPost-translational modification
Activeenzyme
Activators and Repressors
RNAP
RNAP
Aco-A
co-R
+1
+1
R
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 C-terminal 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 hetero-dimers
• 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
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