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