L9

Question 1

Coregulators

Answer 1

• Do not directly bind DNA, but rather activation/repression domain of a transcription factor (TF)
• Increase or decrease transcription rate by binding or enzymatically modifying other TFs

Question 2

Yeast GAL system

Answer 2

• Yeast cells depend on GAL system to express genes that allow the breakdown of galactose as a carbon source
• GAL1, GAL2, GAL7, GAL10: structural genes
• GAL3, GAL4, GAL80: genes encoding regulatory proteins that modulate the expression of the structural genes
• structural genes are expressed when galactose is present and glucose absent

Question 3

GAL4

Answer 3

• key regulator of the GAL system (induces GAL gene expression)
• Each structural genes has 2 or more GAL4 binding sites
• is regulated by GAL80 and GAL3

Question 4

When no galactose:

Answer 4

• The corepressor GAL80 binds GAL4 and prevents its binding to other proteins
• The GAL80 gene is constitutively expressed = GAL4 is always repressed = no expression of the GAL genes

Question 5

In presence of galactose:

Answer 5

1. GAL3 (coactivator) binds galactose, then binds GAL80
2. Releasing of GAL80 from GAL4
3. GAL4 binds TFIID, allowing it to bind to the TATA box of the promoter
4. GAL4 binds the mediator/RNA polymerase complex to target it to the promoter
5. GAL4 recruits Swi/SNF ATPase for chromatin remolding

Question 6

Enhanceosome

Answer 6

For expression to occur:

1. All TFs must be bound to all required DNA regulatory sequences
2. All coregulator proteins must be in place for proper folding of the DNA strand

Question 7

Chromatin Structure

Answer 7

• structure prevents TFs from accessing DNA regulatory sequences -> heterochromatine (condensed)
• Chromatin alterations are necessary for transcription to occur -> euchromatin (loose)

Question 8

Chromatin remodelling

Answer 8

change in nucleosome position by sliding

Question 9

Chromatin modifications

Answer 9

Neutral acetylated histone proteins have less affinity for DNA = reduced chromatin compaction (more open chromatin)
- Acetylation creates a binding site for BD protein motif contained in many TFs

Question 10

Histone variants

Answer 10

Different types of histones are more vulnerable to disassembly = transcription

Question 11

Chromatin modifications and the GAL system

Answer 11

• The absence of glucose is essential for the expression of the GAL structural genes
  In presence of glucose:
  1. Mig1 is a general TF that binds the Mig1 site (proximal enhancer)
  2. Mig1 recruits the corepressor Tup1, an enzyme that deacetylates histone
  3. Condensed chromatin = no transcription of GAL genes
  No glucose:
• Removal of Mig1 from enhancer
• Expression of GAL genes