Outline
Phage has to make a decision

How lytic and Lysogenic processes happened

How the choice is made

Phage Lambda
Starts as a linear DNA with cohesive ends
Can either
Replicate ->Lytic phage
Or integrate into host's genome ->Lysogenic phage

Figure 11.1

Three Classes of Genes
Immediate early genes

Delayed Early genes

Late genes

Figure 11.4

Lytic Cycle
Involves cascade
All three classes of genes are involved:
Immediate Early produce N and Cro protein:
N gene
Codes for N protein that recognize NUT sites (N Protein Utilizing sequence)
Antiterminator binds before terminator (cf. chap 9)
Transcribed to the left
Turns on the delayed early gene

(See Handout)

Lytic Cycle
Involves cascade
All three classes of genes are involved:
Cro gene
Codes for cro protein
Antirepressors
Turns off immediate early genes
Transcribed to the right

Lytic Cycle
Delayed early genes
Turn on CII/CIII
Q gene
Produces Q protein which is antiterminator
Recognizes QUT
Allows RNA Polymerase to go to late genes.

Lytic Cycle
Late Genes
Turned on with Q protein
Produce head and tail genes.
Lysis genes

Summary
All genes are turned on
Recombination
Regulation
Replication

Lysogenic Cycle
Lysogenic = the genetic material of the phage are integrated into the bacterial genome (prophage)
Integration into host genome occurs by recombination
Att = site of attachment by phage to host
Int = allows for integration of phage lambda
Xis = gene for excising DNA once phage wants to leave
cI gene
located between PL and PR
Transcribed from PRM and terminated at the left end of the gene
cI gene
Code for repressor proteins that bind to operators (OL and OR)
Sequence of each operator overlap with the promoter that it controls
The binding of repressors to operators prevent RNA polymerase from binding to promoter ¦ phage genome cannot enter lytic cycle
Mutation at repressor protein prevent lysogeny

Repressor
A dimer
The presence of repressor protein is necessary for its own synthesis
In order to transcibe cI, repressor has to bind to the operator (OR)
When phage first enter, there are no repressors - cI gene cannot be transcribed

Figure 11.15

How is repressor synthesis established?

When phage enters, N and cro genes are transcribed

N protein allows transcription to be extended to delayed early gene

Figure 11.24

Figure 11.14

How is repressor synthesis established?
In delayed early gene, cII and cIII are transcribed:
cII and cIII are positive regulators, whose products are needed to initiate the expression of cI
cII
unstable
transcribed to the right
Establishes repressor synthesis and integrase synthesis
cIII
transcribed to the left
Stabilizes cII

How is repressor synthesis established?
When cII binds to PRE, RNA polymerase recognizes the promoter
cI is translated into repressor protein
Repressors bind to operators and maintain lysogeny autogenous circuit

Lytic vs. Lysogeny
cro
Turns on lytic cycle
Prevent transcription via PRM and the synthesis of repressor
cI
Turns on lysogeny cycle
Prevent the synthesis of
cro - turns on lysis and turns off repressor
N - turns on Q - lysis

Lytic vs. Lysogeny

cro
Binds more readily to OR3
Non-cooperative binding
Binding to OR2and OR3 prevent RNA polymerase from to PRM

cI
Binds more readily to OR1
Cooperative binding
Binding to OR1and OR2 prevent the expression of cro

Lysis or Lysogeny ?
Bad environment:
cII more stable -> cI highly present ->¦ lysogeny
Good environment:
cII unstable -> cI low -> lysis

Conclusion

Lytic cycle - cascade

Overview of the Lytic Cycle

Figure 11.28

Conclusion
Lytic cycle - cascade

Lysogenic cycle

Overview of the Lysogeny Cycle

Figure 11.14

Conclusion
Lytic cycle - cascade

Lysogenic cycle

Choice between the two cycles