Lecture 39: Gene Expression Control

Question 1

Lac operon model

Answer 1

3 genes:
1. z = β-galactosidase
2. y = galactoside permease
3. a = thiogalactoside transacetylase
Regulatory regions:
- Operon = gene + promoter
- Promoter:
1. O = operator (RNA poly. binding)
2. CRP = cyclic AMP receptor protein bind. site
- i region (regulatory gene that binds repressor

Question 2

Lac operon - glucose condition

Answer 2

Repressor expression binds operator; blocks RNA poly binding to promoter

Question 3

Lac operon - lactose + glucose condition

Answer 3

Lactose enters cell via permease -> converted to allolactose
Allolactose binds + inhib. repressor
Low level transcription occurs

Question 4

Lac operon - low glucose, high lactose condition

Answer 4

[Glucose] inversely regulates [cAMP]
cAMP binds CRP which binds CRP enhancer on DNA, stimulating RNA poly binding. High level of transcription occurs.

Question 5

CRP recognition helix

Answer 5

Helix-turn-helix motif binds major groove; all transcription factors have these DNA binding regions

Question 6

Common transcription factor motifs

Answer 6

1. Helix-turn-helix
2. Zn finger
3. Leu zipper/bZIP
4. Basic helix-loop-helix

Question 7

Zn finger

Answer 7

Strength through repetitive 2 Cys + 2 His/Cys motif

Question 8

Leu zipper / bZIP

Answer 8

Hydrophobic zipper region dimerizes to form coiled coil
Basic DNA-binding region with alpha helices

Question 9

TF motif features

Answer 9

1. Name gives structure
2. Alpha helix binds major groove
3. Many weak contacts facilitate strong specific protein-DNA interaction
4. AA side chain/DNA base H-bond in major groove give recognition/specificity
5. Form heterodimers; allows interactions w/ other regulatory proteins

Question 10

DNA methylation control of gene expression

Answer 10

1. CpG islands block TFs in promoter
2. Recruitment of chromatin remodeling proteins e.g. HDAC

Question 11

Epigenetics

Answer 11

Heritable gene function change without changing DNA sequence e.g. methylation

Question 12

Normal eukaryotic methylation

Answer 12

5mC is the only normal methylation in eukaryotes

Question 13

Imprinting

Answer 13

“Parent of origin” gene expression; 1 copy of gene gets methylated aka imprinted aka not expressed. Which copy matters!

Question 14

Prader-Willi syndrome

Answer 14

Normally egg copy of gene is imprinted. In disease, sperm chromosome is truncated or trisomy w/ 2 egg 1 sperm copy and sperm copy is lost. Either way, functioning gene is lost

Question 15

Angelmann’s Syndrome

Answer 15

Normally sperm copy of gene is imprinted. In disease, egg gene is truncated or trisomy w/ 2 sperm 1 egg and egg copy is lost. Either way, functioning gene is lost.

Question 16

How do nucleosomes affect gene expression?

Answer 16

Expression req. loosening nucleosomes by adding positive superhelical tension or ATP driven chromatin remodeling or covalent mod. of histone tails. Loosening nucleosomes allows TF access to initiate transcription of genes

Question 17

Covalent histone modification

Answer 17

Several paths; most important = acetylation/deacetylation of Lys by HAT/HDAC.