L16-17: Transcription in Bacteria

Question 2

What are the main drugs that target transcription in bacteria?

Answer 2

Rifampicin
Streptolydigin
Lipiarmycin

Question 3

What are the 2 different strands of DNA known as during transcription?

Answer 3

Sense and antisense

Question 4

Which DNA strand acts as a template in transcription?

Answer 4

The antisense strand

Question 5

What are the 3 main regions needed during transcription?

Answer 5

1) Promotor
2) Transcribed region
3) Terminator

Question 6

What is a main feature of bacteria during transcription?

Answer 6

It is polycistronic (encodes more than one protein, allows co-ordinated expression of a group of genes)

Question 7

What are features of the sigma 70 promotor?

Answer 7

-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

Question 8

What is the consensus sequence?

Answer 8

Located at -35 and -10, closer match the promotor has to the consensus sequence the stronger the promotor is

Question 9

What are features of RNA polymerase (RNAP)?

Answer 9

It is Mg2+ dependent and it has a multi subunit composed of:
2x alpha
1x beta
1x beta prime
1x omega

Question 10

What are the differences between core and holoenzyme RNAP?

Answer 10

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

Question 11

What are the alternative sigma factors?

Answer 11

S- stationary phase
E- Envelope stress
F- Flagellar regulation
N- Nitrogen assimilation
H- Heat shock
FeCl- Iron metabolism

Question 12

What happens during the formation of the txn complex for initiation and elongation?

Answer 12

Closed complex: holoenzyme binds so sense strand
Open complex: provides access to the template
Elongation

Question 13

Which way does elongation take place?

Answer 13

5’ to 3’

Question 14

What are features of elongation?

Answer 14

-Not as fast as replication
-RNAP has no proofreading mechanism (has backtracking ability)
-Error rate 1 in 10000

Question 15

What are the 2 features of factor independent termination?

Answer 15

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

Question 16

How does the G+C palindromic sequence cause termination?

Answer 16

Weakens the interaction as there is more contact with itself so it stops transcription

Question 17

What is rho dependent termination?

Answer 17

It is a helicase that unwinds RNA-DNA and RNA-RNA duplexes which is powered by ATP

Question 18

What are the 2 different ways to regulate transcription during initiation?

Answer 18

1) Repression
2) Activation

Question 19

What are activators or repressors known as?

Answer 19

DNA binding proteins

Question 20

What is the primary way to regulate transcription?

Answer 20

By regulating initiation

Question 21

What is an operon?

Answer 21

A cluster of genes under control of a single operator

Question 22

What is the lac operon?

Answer 22

Regulates the expression of genes to allow uptake of lactose to use it as an energy source

Question 23

When does high transcription of lac genes take place?

Answer 23

When lactose is present and glucose is absent

Question 24

What is the lac repressor?

Answer 24

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

Question 25

What is the lack of operator and what are key features?

Answer 25

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

Question 26

Why is the lac repressor a tetramer?

Answer 26

There are 3 different lac operator sites so the repressor forms a loop by binding the primary and 2 secondary operators

Question 27

How does lactose control the lac operon?

Answer 27

Lactose combines with beta-galactosides (inducer) to form allolactose which binds to the repressor causing a conformational change meaning affinity for repressor is reduced

Question 28

How does glucose influence the transcription of the lac operon?

Answer 28

Glucose inhibits the catalysis of ATP with adenylate cyclase to form cAMP so cAMP cannot bind to the CAP protein

Question 29

What happens to levels of transcription as glucose levels drop?

Answer 29

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

Question 30

What happens when glucose is present but there is no lactose?

Answer 30

No transcription, repressor binds blocking txn and CAP doesn’t bind

Question 31

What happens when no glucose and no lactose is present?

Answer 31

No transcription, repressor blocks RNAP

Question 32

What happens when glucose and lactose are both present?

Answer 32

Little transcription takes place as the repressor doesn’t bind and the CAP is not activated

Question 33

What happens when there is no glucose and lactose is present?

Answer 33

High transcription, no repressor and CAP is activated