1. Bacteriophage lambda (l)
Transcriptional switches can regulate cellular decisions

2. Bacteriophage lambda can do either.
Lysis or Lysogeny
Lysis: Infection by phage produces many progeny and breaks open (lyses) the host bacterium
Lysogeny: After infection, the phage DNA integrates into the host genome and resides there passively
No progeny
No lysis of the host
Can subsequently lyse (lysogeny)
Bacteriophage lambda can do either.

3. UV Induction
Lysogeny
Lysis

4. Lysogeny: CII and CIII stimulate expression of cI to make repressor+ +
CIII
CII
CII oR
Pint oL PL
PRM PR
PRE
PR‘
tR3
tL1
tR1
tR2
t6S
att
int
xis
red
gam
cIII N cI
cro
cII O P Q S R
A…J
tint
PRE = promoter for
repression
establishment
Int CI
Repressor

5. Lysogeny: Repressor turns off transcriptionCI CI oR
Pint oL PL
PRM PR
PRE
PR‘
tR3
tL1
tR1
tR2
t6S
att
int
xis
red
gam
cIII N cI
cro
cII O P Q S R
A…J
PRM = promoter for
repression
maintenance CI
Repressor
Activated by Repressor
binding to oR1 & oR2

6. l operators overlap promoters
-35
-10
TTGACT
GATAAT N
cro
TTAGAT 5’
ATAGAT 5’
-10
-35
PRM

7. Repressor structure
l repressor is a dimer; monomer has 236 amino acids.
l repressor can bind cooperatively
to operator sub-sites.

8. l-lac hybrid genes Place l cI gene under lac control.
Use lacZ as a reporter.
lac p, o
l cI
l pR , OR
lacZ
321
Control amount of l repressor by [IPTG].
E. coli with lac repressor,
no lacZ.
See effect of l repressor
by b-galactosidase activity

9. Repressor stimulates transcription from PRM
lac p, o
l cI
l pRM , OR
lacZ
123
b-galactosidase
l repressor
[IPTG]
l repressor at oR1 and oR2 stimulates transcription from pRM.

10. Binding of repressor blocks transcription from pR but activates pRM
-35
-10
2 dimers of
Repressor,
bound
cooperatively
oR3
RNA Pol N
cro
oR2
oR1
-10
-35
PRM
= operator
= promoter
-35
-10

11. Events at initiation of transcription

12. Abortive initiation assay
Let R = RNA polymerase, P = promoter (closed), and Po= promoter (open)
R + P RP
RPo
ATP + UTP*
ApUp*U kf kr KB
Abortive transcripts
[ApUp*U]
time
lag

13. Measure kf and KB from lag time vs. 1/[R]
Lag time in abortive initiation assay is inversely proportional to [R].
KB kf 1 1
[R] kf 1
Lag time = x +
Lag time
KB kf 1
Slope = 1
[R] kf 1
Y-intercept =

14. Effect of wild-type and pc mutant λ repressors on activity of PR & PRM

15. Effect of Operator Mutations on Transcriptional Control of PR&PRM
OR1-OR2+OR3-
OR1+OR2-OR3+

16. Effect of λ-pc mutations on KB and k2

17. Architecture of λOR

18. Mutations in the Activating Region of λ Repressor
Glu
Glu
Glu

19. Mutations in the δ subunit of RNA polymerase that interfere w/λ repressor-mediated activation of PRM transcription

20. Effect of mutations in the δ subunit of RNA polymerase on activator-dependent and independent transcription of the lac promoter

21. A model for interaction of the δ subunit of RNA polymerase with λ repressor

22. The Awesome Power of Genetics