Eukaryotic transcription Flashcards
If the eukaryotic RNAP II holoenzyme isn’t sufficient to initiate transcription on its own, what else is required?
General transcription factors (GTFs)
How can GTFs be a point of gene regulation?
Many regulatory elements interact with them
What are the 7 GTFs needed for RNAP II to initiate transcription?
- TFIIA
- TFIIB
- TFIID/TBP
- TFIIE
- TFIIF
- TFIIH
- The mediator complex
What is the pre-initiation complex (PIC)?
RNAP II + all 6 GTFs at the promoter
What are the two models of the assembly of the pre-initiation complex?
- An assembly cascade where certain components are already binding to the promoter, and assemble in a certain order
- Partial assembly before promoter binding
What are the 4 functions of the GTFs?
- Recognize and bind to the core promoter elements
- Recruit RNAP II to the promoter and help it position itself
- Initial melting around the +1 nucleotide and promoter clearance
- Interact with regulatory factors
What does TFIIF do?
- Binds to free RNAP II and prevents it from binding to DNA by itself
- Interacts with TFIIB and stabilizes RNAP II at the promoter
What does TFIID do?
Recognize and bind to the core promoter elements - TATA boxes and other elements. It will be the first GTF to bind to the promoter
What is the difference between TFIID and TBP?
TBP is a subunit of the TFIID protein. It binds to a TATA box and the rest of the GTF will follow
Can TFIID still bind to a promoter that doesn’t have a TATA box?
Yes, it has other subunits that bind to other regulatory elements
What were the results of the Tijan et al. 1995 study on the role of TBP vs the entire TFIID protein?
When the TATA box was the only core promoter element, TBP alone worked as well as the whole protein to initiate transcription. When the TATA box was paired with other promoter elements, the whole TFIID protein worked better than TBP alone
What does TFIIA do?
- Initial promoter recognition in some promoters
- Interacts with TBP and stabilizes its binding to the TATA box
- Is an activator, but isn’t necessary for transcription to start
What does TFIIB do?
- Bind to DNA non-specifically
- Binds to TFIID and stabilize it
- Directly interacts with RNAP II, recruits it to the promoter, and positions it at the promoter
What does the mediator do?
- Transcriptional coactivator, multiprotein complex
- Associates with the GTFs and the C-term of RNAP II
- Bridges the PIC to other TFs besides the GTFs
- Required for transcription to occur in most RNAP II promoters
What does TFIIH do?
- DNA helicase activity
2. Kinase activity to phosphorylate RNAP II so it becomes elongation competent
What does TFIIE do?
- Increases the efficiency of TFIIH so that it can actually do something
- Helps to load TFIIH onto the PIC
Is the process of initiation the same across all eukaryotes?
It isn’t even the same across all genes
What is the sequence of events in transcription initiation in humans?
- RNAP II, TFIIA, TFIIB and TFIID/TBP bind to the promoter
- TFIIF binds and fixes the DNA in place
- TFIIE binds and interacts with TFIIF and RNAPII
- TFIIH binds and transcription begins
What is the sequence of events in transcription initiation in yeast?
- TFIIA, TFIID/TBP, TFIIE, and TFIIH bind to the promoter
- TFIIB binds, and a pre-assembled complex of RNAP II and TFIIF bind
- Transcription begins
What happens to the GTFs during elongation?
Most dissociate
Do eukaryotes do abortive transcription too?
Yes
How is transcription elongation another point of regulation in eukaryotes?
Depends on RNAP II pausing. First near the promoter and later on at certain DNA sequences. Pausing can also be influenced by other factors
What are elongation factors?
Proteins that influence RNAP II pausing events. Some will cause it and some will alleviate it
When does transcription terminate?
When the polyadenylation signal gets transcribed
How does transcription termination occur?
- RNAP II transcribes the polyadenylation signal
- Proteins get recruited and the transcript gets cleaved
- The 3’ polyA tail gets added to the cleaved transcript
- The cleavage allows RNAP to dissociate
