Prokaryotic Transcription Flashcards
In prokaryotes, how many RNA polymerases are responsible for synthesis of all classes of RNA?
one
What is the subunit composition of the bacterial RNA polymerase core?
- sigma head
- α2ββ’ω
Although the core enzyme of bacterial polymerase is catalytically active, is it able to recognize specific promoter DNA sequences.?
- No
- σ subunit is added to the core RNA polymerase to form a holoenzyme
- σ subunit recognizes specific promoter DNA sequences
holo-RNA polymerase:
- = core RNA polymerase + σ subunit
- σ subunit recognizes specific DNA promoter sequences
- core RNA polymerase carries out transcription
Although there is only a single core RNA polymerase, bacteria have several different _______ that recognize (bind) different promoter sequences.
σ subunits
What is the active form of the bacterial RNA polymerase?
- holo-enzyme
- σ subunit binds to core RNA polymerase
As RNA polymerase transcribes DNA, what kind of supercoils are formed in the direction of transcription, and what kind are formed behind the RNA polymerase?
- ahead = positive supercoils
- DNA gyrase removes
- behind = negative supercoils
- topoisomerase I removes
What enzyme removes positive supercoils ahead of the RNA polymerase during transcription?
- DNA gyrase
What enzyme removes negative supercoils behind of the RNA polymerase during transcription?
- topoisomerase I
What are the three discrete steps in transcription?
- initiation
- elongation
- termination
During initiation in bacterial transcription, what occurs?
- sigma subunit of holo-RNA polymerase binds promoter DNA at:
- -10 (Pribnow Box) region AND
- -35 region
- transcription starts at +1
What are the promoter regions on the DNA in bacterial transcription?
Regions at:
- -10 (Pribnow Box)
- -35
Sigma-subunit of holo-RNA polymerase binds to both, transcription starts at +1.
Transcription is regulated primarily at the level of:
- initiation
Transcriptional activators:
- recruit RNA polymerase to the promoter
- stabilize RNA polymerase binding to promoter DNA
Transcriptional repressors:
- block the RNA polymerase-DNA interaction
Elongation begins when:
- RNA polymerase exits the promoter
- σ factor left behind and dissociates
- core enzyme continues on, catalyzing RNA synthesis
Transcription termination signals: what level are they present and what level do they function?
- present in genes (DNA)
- function on RNA level
Elongation process:
- Lengthens transcript by addition of nucleotides to 3ʼ end of RNA located in active site of RNA polymerase
What is initiation dependent on?
- sigma subunit
- promoter regions
What is the Pribnow box?
- a promoter sequence on DNA located at -10
- rich in T and A
Termination:
- RNA synthesis stops
- RNA transcript released
- RNAP dissociates from DNA template
- Depends of DNA-encoded, RNA signals.
The two classes of bacterial terminators:
- rho-dependent
- rho-independent
Rho-dependent termination:
- Rho protein binds a specific RNA sequence as a hexameric protein and subsequently contacts RNA polymerase.
- Rho/RNA polymerase interaction is a signal for the polymerase to terminate transcription and to dissociate from the DNA template
Rho-independent termination:
- RNA hairpin structures followed by U-rich sequence
- a specific structure (stem-and-loop) forms in the RNA transcript, signaling termination.
Rho-independent terminators are RNA hairpin structures generated by:
- palindromic sequences
- repeat sequences
Rifambcin:
- drug
- binds β subunit of bacterial RNA polymerase
- inhibits transcription initiation
- treats TB
