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