Download B1 Lecture-1 - Trinity College Dublin

Bacterial and Phage
Genetic Switches
Prof. C. J. Dorman
Department of Microbiology,
Moyne Institute of Preventive Medicine,
Trinity College, Dublin.
Lecture 1
The genetic switch controlling
the lytic-lysogen decision
in bacteriophage lambda
Switch types
active
Binary (On/Off)
Rheostat
(set over a spectrum of values)
inactive
active over a range of
activity levels
Transcription control: regulating the activity of RNA
polymerase
α-NTD
ββ'
α−CTD
σ
UP element
-61.5
-41.5
Activators
-35
-10
Repressors
+1
Coordinated control of transcription (I)
individual gene
operon
Stimulus
regulon
Common regulatory
protein
Coordinated control of transcription (II)
Stimulus 1
Regulatory
protein 1
Stimulus 2
Regulatory
protein 2
Effects of stimulus on the regulator
P
Fe
Covalent modification e.g. phosphorylation
Bind ligand (metal, sugar, etc)
oligomerize
Proteolytic degradation
Regulator-regulator interactions
Activator displaces repressor
inactive
active
Activator reinforces a second activator
active
highly active
Bacteriophage lambda, a temperate phage
Head
DNA
Tail
infection
LamB protein
Lambda cos sites: genome circularization
cos
cos
cos
cos
GGGCGGCGACCT
CCCGCCGCTGGA
GGGCGGCGACCT
CCCGCCGCTGGA
Summary of the lambda lifecycle: lytic
2. nucleic acid
injection
3. genome
circularization
1. adsorption
4. latent
period
6. lysis
5. maturation
Summary of the lambda lifecycle: lysogenic
1. adsorption
2. nucleic acid
injection
3. genome
circularization
4. recombination
LYSOGEN
INDUCTION
LYSIS
Lambda integration: site-specific recombination
lambda
attP
bacterial chromosome
gal
attB
bio
Integrase (int gene product)
+ Integration Host Factor
lambda prophage
gal
attL
attR
bio
Lambda excision: site-specific recombination
lambda prophage
attL
attR
bio
gal
integrase (int)
excisionase (xis)
+ Integration Host Factor
lambda
attP
gal
attB
bio
Lambda genome (48,502 bp)
DNA
replication
recombination
immunity
Head
Nu1
A W B
lysis
Tail
C Nu3 D E FI FII Z U V G T H M L K I lom 401 314 194 Ea47 Ea31 Ea59 att int xis Ea8.5 Ea22 exo bet gam kil cIII ssb ral git N rexBA cI cro cII O P ren 146 290 Q S R Rz
cos
cos
PL
PM
PE
PR
Transcription post infection and circularization - lytic cycle
tL1
cIII
PL
N
cI
cro
PR
Q
tR1
tR2
S
tR3
PR'
FIRST gene products to appear are N and Cro. EARLY GENES
N is an anti-terminator. It allows transcription to ignore terminators tL1, tR1, and tR2.
ANTI-TERMINATION allows expression of the DELAYED EARLY GENES
THEN
Q enables transcription from PR'. this transcript encodes proteins for phage head and tail assembly
(the LATE GENES).
MEANWHILE
Cro builds up and represses the PL and PR promoters when they are no longer needed.
Establishment of lysogeny
PE
PL
cIII
PM
N
cI
cro
cII
PR
• The cI repressor switches off the lytic cycle
• The cI gene is expressed from the promoter for repression establishment, PE.
• Activity of PE requires the cII activator, an unstable protein.
• The cIII protein stabilizes cII.
• Neither cII nor cIII can be expressed until the N anti-terminator is active.
• Production of cII and cIII allows expression of cI repressor.
Effects of cI repressor activity
ACTIVATION
REPRESSION
PE (LOSS OF ACTIVATION)
PL
cIII
PM
N
cI
cro
cII
PR
REPRESSION
•cI repressor binds to operator sites at the PL and PR promoters and shuts them off.
•This stops production of N and Cro regulatory proteins, blocking the lytic cycle.
•HOWEVER, it also shuts off cII and cIII production, needed for cI transcription.
•Repressor binding has a stimulatory effect on PM, the promoter for repressor maintenance.
•PM activity keeps cI repressor levels topped up.
Cro and cI are competitors
REPRESSION by Cro
ACTIVATION by cI
operator OR
PM
cIII
N
cI
cro
cII
PR
REPRESSION by Cro
REPRESSION by cI
•Cro and cI compete for binding to the operator sequence between promoters PM and PR.
•Each is a repressor of PR.
•But Cro represses PM too.
•Cro prevents cI synthesis in two ways:
1) by switching off the cII gene and
2) by inhibiting the promoter for cI maintenance.
Cro and cI interactions with the operator OR
PM
ACTIVE
cI
cI
OR3 OR2 OR1
REPRESSED
PR
PM
REPRESSED
Cro Cro Cro
REPRESSED
PR
The lytic-lysogenic decision
???
???
Lysis
Lysogeny
•Binding of cI to OR1 prevents the lytic cycle
•Binding of Cro to OR3 prevents the maintenance of lysogeny by repressing PM.
•(Remember, PE will only continue to function for as long as the cII protein survives.)
READING MATERIAL
"Phage Strategies"
(Chapter 27 in Lewin's Genes X by B Lewin [2011]
Edited by JE Krebs, ES Goldstein, ST Kilpatrick)
Article is available in the Microbiology Departmental Office
First floor
Moyne Institute of Preventive Medicine