EC2: RNA polymerase, RNA processing, and antibiotics & RNA polymerase Flashcards
What are the different sections of bacterial RNAP, and how many of each are there?
- 2 x α (alpha)
- 2 x β (beta)
- 1 x ω (omega)
What is a holoenzyme?
What is RNAP’s holoenzyme?
A core enzyme in its complete functional form, plus additional components
For RNAP: core plus sigma (σ)
How do RNAP’s core and holoenzymes differ in function?
-
Core enzyme:
- Native DNA: random starts
- Foreign DNA: random starts
-
Holoenzyme:
- Native DNA: specific starts (highly selective)
- Foreign DNA: random starts
What does the sigma region do in RNAP’s holoenzyme?
It recognises specific promoters, but only from within the holoenzyme
How do RNAP’s holo- and core enzymes work together in transcription?
The holoenzyme recognises the promoter and initiates transcription
The core enzyme elongates and completes transcription
What are the functions of RNAP’s subunits?
- β and β’: active site processivity*
- the active site is shared between the β-subunits when they come together
- α: DNA binding (non-specific)
- ω: enzyme assembly (only recently confirmed)
- it is part of the glue that holds the holoenzyme together
- σ: promoter recognition
*processivity = enzyme’s ability to catalyze consecutive reactions without releasing its substrate
What is the role of sigma factors?
They regulate gene expression in bacteria:
- They get turned on and off
- The presence of sigma factors turns on blocks of genes when needed
- When they are destroyed, families of genes are turned off
- They are highly specific
Describe the different stages of bacterial transcription.
- Initiation: the holoenzyme opens the promoter – a region of about 2 helical turns – and forms a transcription bubble
- Elongation of transcription: the bubble advances along the RNA, with DNA being unwound ahead of it. Nascent RNA is made as a complementary copy of the template strand. DNA is rewound behind the bubble.
- Termination of transcription: (in bacteria) sometimes dependedent on rho factor (ρ), sometimes not. Uses energy from ATP hydrolysis
Do RNA polymerases need a primer?
No – they work de novo
What is at the 5’ end of the nascent RNA during the elongation of transcription?
A triphosphate (ppp)
What does the triphosphate at the 5’ end of nacent RNA act as?
A marker for the intact, 5’ end of nascent RNA
What is the angle fo the bend in DNA in the transcription bubble?
90° (right angle)
What is present in the RNAP core enzyme to keep the DNA strands apart once they have been unwound?
Knobs and valleys
How do the substrates get into RNAP core enzyme?
What ion does this use?
Through the NTP channel, which uses Mg2+
What is another name from post-transcriptional modification of RNA?
RNA processing
