L16-17: Transcription in Bacteria Flashcards
What are the main drugs that target transcription in bacteria?
Rifampicin
Streptolydigin
Lipiarmycin
What are the 2 different strands of DNA known as during transcription?
Sense and antisense
Which DNA strand acts as a template in transcription?
The antisense strand
What are the 3 main regions needed during transcription?
1) Promotor
2) Transcribed region
3) Terminator
What is a main feature of bacteria during transcription?
It is polycistronic (encodes more than one protein, allows co-ordinated expression of a group of genes)
What are features of the sigma 70 promotor?
-upstream of txn start-site + binding site for RNA polymerase
-2 hexameric sequences at -35 and -10
-Stronger promotor means quicker initiation of txn
-Strength dictated by sequence
What is the consensus sequence?
Located at -35 and -10, closer match the promotor has to the consensus sequence the stronger the promotor is
What are features of RNA polymerase (RNAP)?
It is Mg2+ dependent and it has a multi subunit composed of:
2x alpha
1x beta
1x beta prime
1x omega
What are the differences between core and holoenzyme RNAP?
Core:
Catalyse process of transcription
Holoenzyme:
Recognises & binds to promotor
σ factor on core makes it holoenzyme
σ directs recognition of promotor sequence
σ70 is the main factor
What are the alternative sigma factors?
S- stationary phase
E- Envelope stress
F- Flagellar regulation
N- Nitrogen assimilation
H- Heat shock
FeCl- Iron metabolism
What happens during the formation of the txn complex for initiation and elongation?
Closed complex: holoenzyme binds so sense strand
Open complex: provides access to the template
Elongation
Which way does elongation take place?
5’ to 3’
What are features of elongation?
-Not as fast as replication
-RNAP has no proofreading mechanism (has backtracking ability)
-Error rate 1 in 10000
What are the 2 features of factor independent termination?
1) series of 4-10 consecutive AT base pairs
2) A G+C rich region with palindromic sequence that immediately precedes the series of AT base pairs
How does the G+C palindromic sequence cause termination?
Weakens the interaction as there is more contact with itself so it stops transcription
What is rho dependent termination?
It is a helicase that unwinds RNA-DNA and RNA-RNA duplexes which is powered by ATP
What are the 2 different ways to regulate transcription during initiation?
1) Repression
2) Activation
What are activators or repressors known as?
DNA binding proteins
What is the primary way to regulate transcription?
By regulating initiation
What is an operon?
A cluster of genes under control of a single operator
What is the lac operon?
Regulates the expression of genes to allow uptake of lactose to use it as an energy source
When does high transcription of lac genes take place?
When lactose is present and glucose is absent
What is the lac repressor?
It is the key to regulation of the lac operon
It is the product of the lacI gene
Forms a homotetramer
Binds to lac operon
What is the lack of operator and what are key features?
It is the binding site for the repressor
Has a 35 bp palindrome
Has a higher affinity for binding which is more efficient than other DNA
Why is the lac repressor a tetramer?
There are 3 different lac operator sites so the repressor forms a loop by binding the primary and 2 secondary operators
How does lactose control the lac operon?
Lactose combines with beta-galactosides (inducer) to form allolactose which binds to the repressor causing a conformational change meaning affinity for repressor is reduced
How does glucose influence the transcription of the lac operon?
Glucose inhibits the catalysis of ATP with adenylate cyclase to form cAMP so cAMP cannot bind to the CAP protein
What happens to levels of transcription as glucose levels drop?
cAMP accumulates ad binds to the CAP protein as it binds to the CAP binding site which enhances the ability of RNAP to bind so transcription is activated
What happens when glucose is present but there is no lactose?
No transcription, repressor binds blocking txn and CAP doesn’t bind
What happens when no glucose and no lactose is present?
No transcription, repressor blocks RNAP
What happens when glucose and lactose are both present?
Little transcription takes place as the repressor doesn’t bind and the CAP is not activated
What happens when there is no glucose and lactose is present?
High transcription, no repressor and CAP is activated
