Prokaryotic Transcription

Question 1

In prokaryotes, how many RNA polymerases are responsible for synthesis of all classes of RNA?

Answer 1

one

Question 2

What is the subunit composition of the bacterial RNA polymerase core?

Answer 2

• sigma head
• α₂ββ’ω

Question 3

Although the core enzyme of bacterial polymerase is catalytically active, is it able to recognize specific promoter DNA sequences.?

Answer 3

• No
• σ subunit is added to the core RNA polymerase to form a holoenzyme
• σ subunit recognizes specific promoter DNA sequences

Question 4

holo-RNA polymerase:

Answer 4

• = core RNA polymerase + σ subunit
• σ subunit recognizes specific DNA promoter sequences
• core RNA polymerase carries out transcription

Question 5

Although there is only a single core RNA polymerase, bacteria have several different _______ that recognize (bind) different promoter sequences.

Answer 5

σ subunits

Question 6

What is the active form of the bacterial RNA polymerase?

Answer 6

• holo-enzyme
• σ subunit binds to core RNA polymerase

Question 7

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?

Answer 7

• ahead = positive supercoils
  
  • DNA gyrase removes
• behind = negative supercoils
  
  • topoisomerase I removes

Question 8

What enzyme removes positive supercoils ahead of the RNA polymerase during transcription?

Answer 8

• DNA gyrase

Question 9

What enzyme removes negative supercoils behind of the RNA polymerase during transcription?

Answer 9

• topoisomerase I

Question 10

What are the three discrete steps in transcription?

Answer 10

1. initiation
2. elongation
3. termination

Question 11

During initiation in bacterial transcription, what occurs?

Answer 11

• sigma subunit of holo-RNA polymerase binds promoter DNA at:
  
  • -10 (Pribnow Box) region AND
  • -35 region
  • transcription starts at +1

Question 12

What are the promoter regions on the DNA in bacterial transcription?

Answer 12

Regions at:

1. -10 (Pribnow Box)
2. -35

Sigma-subunit of holo-RNA polymerase binds to both, transcription starts at +1.

Question 13

Transcription is regulated primarily at the level of:

Answer 13

• initiation

Question 14

Transcriptional activators:

Answer 14

• recruit RNA polymerase to the promoter
• stabilize RNA polymerase binding to promoter DNA

Question 15

Transcriptional repressors:

Answer 15

• block the RNA polymerase-DNA interaction

Question 16

Elongation begins when:

Answer 16

• RNA polymerase exits the promoter
• σ factor left behind and dissociates
• core enzyme continues on, catalyzing RNA synthesis

Question 17

Transcription termination signals: what level are they present and what level do they function?

Answer 17

• present in genes (DNA)
• function on RNA level

Question 18

Elongation process:

Answer 18

• Lengthens transcript by addition of nucleotides to 3ʼ end of RNA located in active site of RNA polymerase

Question 19

What is initiation dependent on?

Answer 19

• sigma subunit
• promoter regions

Question 20

What is the Pribnow box?

Answer 20

• a promoter sequence on DNA located at -10
• rich in T and A

Question 21

Termination:

Answer 21

• RNA synthesis stops
• RNA transcript released
• RNAP dissociates from DNA template
• Depends of DNA-encoded, RNA signals.

Question 22

The two classes of bacterial terminators:

Answer 22

1. rho-dependent
2. rho-independent

Question 23

Rho-dependent termination:

Answer 23

1. Rho protein binds a specific RNA sequence as a hexameric protein and subsequently contacts RNA polymerase.
2. Rho/RNA polymerase interaction is a signal for the polymerase to terminate transcription and to dissociate from the DNA template

Question 24

Rho-independent termination:

Answer 24

1. RNA hairpin structures followed by U-rich sequence
2. a specific structure (stem-and-loop) forms in the RNA transcript, signaling termination.

Question 25

Rho-independent terminators are RNA hairpin structures generated by:

Answer 25

1. palindromic sequences
   
   • repeat sequences

Question 26

Rifambcin:

Answer 26

• drug
• binds β subunit of bacterial RNA polymerase
• inhibits transcription initiation
• treats TB