Bacteriophage lamdba II Flashcards Preview

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Flashcards in Bacteriophage lamdba II Deck (10)
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1
Q

How is lysogeny maintained ?

A

The lambda repressor, encoded by cI is required to maintain lysogeny
The repressor acts at OL and OR to block transcription of the immediate early genes
This repression prevents the lytic cycle from proceeding

2
Q

What is the structure of the lambda repressor ?

How is it regulated ?

A

The repressor monomer has two distinct domains
- N-terminal domain binds DNA
- C-terminal domain dimerizes
Binding to the operator requires the dimeric form
Cleavage of the repressor by proteases reduces the affinity for the operator and induces a lytic cyle

3
Q

During lysogeny, what happens at the :

  • immediate early stage ?
  • delayed early stage ?
  • middle stage ?
  • late stage ?
A
Immediate early : 
- N and cro transcribed
Delayed early : 
- N antiterminates
- cII + cIII are transcribed
Middle  :
- lysogenic establishment --> cII acts at Pre : cI is transcribed
Late : 
- lysogenic maintenance --> repressor binds at OL and OR, cI is trancribed from Prm
4
Q

What are the key point of lysogeny ?

A
  • cII and cIII cause repressor synthesis to be established and trigger inhibition of late gene transcription
  • Establishment of repressor turns off early gene expression
  • Repressor maintains its own synthesis
  • Lambda DNA is integrated in the final stage of establishing lysogeny
5
Q

What does lambda integration require ?

What does excision require ?

A

Integration : Int (integrase) + IHF (Integration Host Factor)
Excision : Int, Xis (Excisionase) + IHF

6
Q

How does lambda insertion into the bacterial genome occur ?

A
  • Phage integrates into the chromosome by recombination between the attP site on the phage and attB on the bacterial DNA –> phage and bacterial DNAs align and staggered cleavages lead to crosswise pairing
  • Excision occurs via recombination between sites at the end of the linear prophage
  • Lambda int codes for an integrase that catalyses the integration reaction.
  • xis is additionally required for excision
7
Q

How many sites does the operator have ?

How do cI and cro bind the operator regions ?

A

Operator has three sites, each composed of two half sites
Lambda repressor and cro each bind to these
Repressor has highest affinity for OR1 and OL1 and co-operative binding to the second sites
Cro has highest affinity for OL3 and OR3

8
Q

How is the lytic cycle initiated ?

A
Immediate early : 
- N and cro are transcribed
Delayed early :
- N antiterminates
- cII + cIII transcribed
Delayed early continuation : 
- cro binds to OL and OR
Late expression : 
- cro represses cI and all early genes; pQ activates late expression
9
Q

What are the key point of lytic cycle initiation ?

A
  • When Cro binds to OR3 it prevents RNA polymerase from binding to PRM and blocks the maintenance of repressor protein
  • When Cro binds to the other operator sites it prevents RNA polymerase from binding to PL and PR and turns off early gene expression
10
Q

What determines whether the the lytic or lysogenic cycle is initiated ?

A
  • The delayed early stage when both Cro and repressor are being expressed is common to both lysogeny and the lytic cycle
  • The critical event is whether cII causes sufficient synthesis of repressor to overcome the action of cro