Prokaryotic transcription termination Flashcards
Why is transcription termination a critical point of gene regulation in prokaryotes?
A lot of transcripts are polycistronic. Termination can regulate expression of the later ORFs so they don’t get transcribed in the first place
What are the 2 types of terminators in E coli?
Intrinsic terminators and Rho dependent terminators
How many terminators in E coli are Rho dependent?
~20-30%
What are the two sequence elements in the DNA present in intrinsic terminators?
- A usually GC rich inverted repeat that folds into a hairpin
- A T-rich region in the non-template strand that comes right after the hairpin
What is the function of the T-rich region in an intrinsic terminator?
Forms a string of UA bp in the DNA:RNA hybrid, which weakens it
What are the 4 steps in intrinsic transcription termination?
- The hairpin and the string of UA base pairs get transcribed, and the stretch of UA bp is a weak interaction and causes RNAP to pause
- The pause lets the GC rich hairpin form
- The hairpin pulls on the weak UA bp and dislodges it from the elongation complex
- RNAP dissociates from the template and transcription is terminated
Where does Rho bind to an RNA transcript?
The Rut site (Rho utilization site)
What are the 6 steps in Rho dependent termination?
- Rho interacts with RNAP in the early stages of elongation
- The rut site is transcribed and RNAP binds to it
- RNA synthesis continues and Rho pulls the transcript through a hole in its centre
- RNAP pauses at the termination hairpin, but Rho keeps moving
- Rho catches up, pulls the transcript tight so that RNAP can’t keep transcribing
- Rho uses ATP hydrolysis and helicase activity to unwind the RNA:DNA hybrid and release the finished transcript
Even though Rho is following RNAP through elongation, how does it not stop transcription until the right time?
It can only catch up if RNAP pauses. As long as RNAP is moving, then there will always be a loop of RNA between Rho and RNAP
