temperate bacteriophage

TEMPERATE BACTERIOPHAGE

TEMPERATE BACTERIOPHAGE

• Stable relationship with the host• Phage DNA replicates together with the

host DNA• Late phase proteins are not

synthesized• Lytic or lysogenic cycles• No degradation of the host DNA • Modification of the host RNA

polymerase

• Cells carrying the temperate phage = lysogen

• Phage DNA + bacterial DNA = prophage• Lysogenic cycle lytic cycle –

spontaneously or when the host metabolism is disturbed

LYSOGENIC CYCLE1. Phage adsorption, injection of DNA2. DNA circularization3. Integration into the host DNA4. Replication of host and phage DNA5. Transcription of host and phage DNA6. Induction by stress, chemicals7. Excision of the phage DNA from the

prophage8. Degradation of host DNA, modification of

the polymerase9. Early and late phase transcription10. Assembly of phage particle11. Host lysis

• Excision from the prophage DNA during induction into the lytic cycle requires excionase, integrase

• Transcription is controlled by cro, cII proteins – determine whether lytic or lysogenic cycle is initiated

LYSOGENY

• Circularization of the phage chromosome – cohesive ends

Lygase

Closed Circle

Cohesive Ends

Linear Double Stranded Opened Circle

LYSOGENY• Site specific

recombination– Enzyme is

coded by the phage

• Suppression of phage genome expression

– repressor protein (cI)– Specific– Immunity to super-infection (second phage)

gal bio

gal bio

gal

bio

Map of Lambda Phage

STOPPING LYSOGENY• Induction

– Environment is not suitable

• Role of protease– recA Protein– Degradation of

repressor protein cI

•Excision•Lytic growth

gal

bio

gal bio

gal bio

gal bio

CHOICE BETWEEN LYTIC AND LYSOGENIC CYCLES

• PL, PR: Promoter to start a competition between both cycles – synthesis of croand N proteins

• N Protein : regulator protein• N binds to tL, tR1, tR2 - synthesizes cII, O,

P and Q• cII: activator protein• O,P: protein to start λ DNA synthesis

• Q: lytic cycle regulator protein• N binds to tL – synthesizes int, xis, cIII• int: integrase• xis: excisionase• cIII: protein to stablize cII• cIII, cII, O, P, Q: early phase genes

LYSOGENIC CYCLE• cII-cIII complex:• Binds to PRE – activates cI transcription (λ

repressor)• cI: binds to P1 – activates int transcription• λ repressor binds to OR, OL - prevents

transcription from PR, PL – PREVENTS THE LYTIC CYCLE

• cI is transcribed from PRM - synthesizes λrepressor

LYTIC CYCLE• Cro protein binds to OR, inhibits

transcription from PRM – no synthesis of λ repressor

• Transcription from PR produces O, P, Q proteins

• O,P, catalyze λ DNA replication• Q: instructs transcription from PR' –

synthesize structural proteins and lysozyme

λ BACTERIOPHAGE• Host = E. coli• Temperate phage• Sticky cos ends• Host DNA ligase circularizes phage DNA• Lytic or lysogenic cycle• Genes are arranged according to function • Host RNA polymerase is modified• Replication by rolling circle or theta• Sequential transcription• Integrates into the host genome by

recombinatian