Transcription Flashcards
What is the main difference between transcription in prokaryotes and eukaryotes
In prokaryotes transcription/translation is instant.
In eukaryotes mRNA needs to travel out of the nucleus to be translated
What is a transcription ‘bubble’
The place where the two strands of DNA are separate and being transcribed.
Bubble length ~12-14 bp
What are the three main steps in transcription
(Pre-initiation)
Initiation
Elongation
Termination
What is the structure of e.coli RNA polymerase
Core enzyme made up of two parts:
Alpha 1+2 subunits dimerise and bind to beta subunit (alpha2beta)
Beta’ subunit binds to omega subunit (beta’omega)
These two subunits combine to form the core enzyme (alpha2betabeta1omega)
Give an example of a conserved RNA polymerase binding site sequence
Thermus aquaticus:
-NADFDGD-
Forms an aspartic acid loop - highly conserved
What is the importance of a protein bridge in RNA polymerase
Changes the conformation. Restricts nucleotide entry - controls/regulates transcription.
What is a holoenzyme complex and why is it important
Holo = core enzyme + sigma factor
Core enzyme catalyses transcription; but sigma factor is needed for initiation.
How does sigma factor affect transcription initiation
Binds to ‘core’ polymerase enzyme. It reduces affinity to non-specific DNA; and increases affinity for promoters.
How does a holoenzyme find/bind to a promoter sequence
Rapidly binds to random DNA.
‘Slides’ or ‘hops’ along DNA until it finds a promoter - forms a tight complex.
1D diffusion - limits how far it can go.
What is the difference between an open and closed DNA complex
Closed - DNA is double stranded (normal DNA structure)
Open - DNA is separated; transcription bubble has formed.
What is abortive initiation
Lots of short sequences (<8 bp) are formed. Sigma factor is released when sequence reaches 8-9 bp; so transcription is stopped before then if sequence is not required.
Where is the transcription ‘bubble’ in relation the strand
Between ~ -10 and +1 bps.
How many different sigma factors are present in e.coli
6/7 - 6 unique factors.
What is the most common sigma factor in e.coli
sigma70; coded by rpoD. Used for general transcription.
What are the three components of a promoter consensus sequence
hexonucleotide at ~ -35
hexonucleotide at ~ -10 (TATA box)
Start point.
What sequences does sigma factor (70) recognise and direct RNA polymerase to.
Two hexonucleotides (6 bp sequence) One at ~-35; then TATA box at ~-10
What effect do down-mutations of consensus sequence have on promoter efficiency
Decreased efficiency.
Usually because decreased conformance to consensus sequence.
(Up-mutations do the opposite)
What specific effect do mutations in the -35 consensus sequence have
Affects initial binding of RNA polymerase
decreased - down-mutation; increased - up-mutation
What specific effect do mutations in the -10 consensus sequence have
Affects the melting reaction that converts a closed complex to an open complex.
(decreased - down-mutation; increased - up-mutation)
How can mutations in the consensus sequence be counteracted
Compensatory mutations in sigma factor.
What does ‘K2’ refer to
The measure of the activity of a promoter (promoter strength)
What effect does negative supercoiling have on transcription
Increases efficiency of certain promoters by assisting the melting reaction (bubble formation).
Where does supercoiling occur during transcription
Positive supercoiling (ahead of RNA pol) Negative supercoiling (behind RNA pol)
How is supercoiling (caused by transcription) rectified
Positive supercoiling - Gyrase introduces negative supercoils
Negative supercoiling - Topoisomerase relaxes negative supercoils