Transcription 2 Flashcards
When the open complex forms between RNA polymerase and DNA how many base pairs are unwound?
17bp of DNA are unwound at the start site.
What happens after promoter clearance?
The RNA chain is extended and the polymerase moves along the DNA.
How large is the transcription bubble?
Remains as 17bp of DNA unwound as the RNA polymerse moves along the DNA.
In RNA polymerase where is the active site?
In the cleft between the beta and beta’ subunits.
What happens to the DNA strands once they have been unwound?
The two strands go through separate channels in the RNA polymerase.
What are the two mechanisms of termination in E.coli?
ρ dependent and ρ independent.
What is the ρ dependent mechanism of termination?
Requires an additional protein factor, ρ, which moves through RNAP and binds CA-rich sequences in the RNA chain. It then uses ATP driven helicase to unwind the RNA/DNA helix within the transcription bubble.
What is the ρ independent mechanism of termination?
Hairpin structures followed by UUU in the RNA chain causes RNAP to pause and release the transcript due to weaker A-U bonds. This pauses transcription as the hairpin clogs the RNA exit channel.
Describe RNA polymerases in eukaryotes.
Large proteins (>500kDa) and multisubunit (>8)
What is RNA polymerase I used for and where is it found?
Makes a single pre-rRNA transcript that is processed to give 18S, 5.8S and 28S rRNAs. Found in the nucleolus.
What is RNA polymerase II used for?
Makes some small RNAs and mRNAs.
What is RNA polymerase III used for?
Makes tRNAs, 5S rRNA and other small RNAs such as those associated with the spliceosome.
Which of the eukaryotic RNA polymerases is most susceptible to inhibition by α-aminitin?
RNA polymerase II.
What are the main differences between eukaryotic and prokaryotic promoter sequences?
Eukaryotic promotes are larger and more complex. Prokaryotic promoters are all upstream, whereas RNAPIII in eukaryotes has downstream promoter sequences, in the region being transcribed. Eukaryotic promoters rely on the binding of basal transcription factors for initiation.
What are the features of RNAPII promoters?
- TATA box at -30
- Inr sequence near +1
- Upstream and downstream elements
- Enhancer sequences
