Determination of l Phage Cycles
Upon Infection of E. coli
Mike Gleason
University of Nebraska Medical Center
Initial Infection
Cro
int
xis
cIII
N
cII
O
P
cI
Upon infection, here is what the injected l phage genome looks
like. All genes are turned off until host transcription machinery
binds to the phage DNA.
1
PR
Immediate Early Genes
STOP
Cro
int
xis
cIII
N
O
P
cI
PL
STOP
cII
Products: N, Cro
The host’s RNA polymerase has now started transcribing from
both the PL and PR promoters. This results in production of the
antiterminator (N) protein and some Cro protein. At this time,
both transcripts terminate early due to the presence of a rhoindependent terminator sequence.
2
PR
Early Genes
Cro
int
xis
cIII
N
cII
O
P
cI
PL
N
N
Products: N, cIII, xis, int, Cro, cII, O, P
The antiterminator (N) protein allows both PL and PR to be read
through, producing longer transcripts. Since prokaryotes have
polycistronic mRNAs, each longer transcript can now code for
multiple proteins.
3
PR
Decision Time
Cro
int
xis
cIII
N
cII
O
P
cI
PL
N
N
Products: N, cIII, xis, int, Cro, cII, O, P
The virus now uses the host’s relative health to decide whether to
enter the lytic phase or lysogeny. If there is ample glucose
available to the host, cAMP is exported and [cAMP] is low.
Supplies of cAMP determines whether it can bind and inactivate
the host proteolytic enzyme HflA.
4
PR
Lysis
Cro
xis
cIII
N
N
cII
O
P
cI
Cro
Cro
PR
int
N
PRM
PRM
PL
OR3
OR2
Cro
Cro
OR1
Products: Cro, cII, O, P
Glucose was in ample supply, so cAMP is exported and [cAMP] is low. cAMP does not bind HflA, so HflA is free
to degrade cII, and therefore not lead to cI.
The site between the PR and PRM is the operator OR. It has three similar cis elements, OR1, OR2, and OR3, each
of which can be bound by Cro or cI. There are slight differences in the sequences, so Cro has highest affinity for
OR3, followed by OR2 and OR1. cI has the opposite affinity; it prefers OR1, then OR2 and OR3. When cI binds
at OR1, it prevents RNA Pol from binding at PR. Similarly, Cro binding at OR3 blocks PRM. Therefore, Cro and cI
can compete for these binding sites.
At lower [cI], Cro is able to outcompete cI at OR and OL and stimulate lysis. At medium levels of [Cro], PL and
PRM are disabled, but PR remains active, producing O and P for lysis.
5
PR
Lysis 2
STOP
Cro
xis
cIII
N
STOP
O
P
cI
Cro
Cro
PR
int
cII
PRM
PRM
PL
OR3
OR2
OR1
Cro
Cro
Cro
Products: (None)
When [Cro] becomes high, all promoters are shut off.
6
PR
Decision Time
Cro
int
xis
cIII
N
cII
O
P
cI
PL
N
N
Products: N, cIII, xis, int, Cro, cII, O, P
Now let’s see what happens when lysogeny is chosen.
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PR
Lysogeny
Cro
int
xis
N
cII
O
P
cI
PRE
PRM
cIII
N
PL
PI
cII
cIII
N
cII
cIII
Products: N, cIII, xis, int, Cro, cII, O, P, cI.
Glucose was scarce, so cAMP was not exported and [cAMP] is high. cAMP binds HflA, so HflA does not degrade
cII. The cIII protein helps protect cII, and together they activate P I and PRE. This results in more int protein, and
the cI protein (The l Repressor).
The PRE promoter that leads to production of cI is a very weak promoter. cII is needed so that RNA Polymerase is
able to bind this promoter effectively. The transcript has a Shine-Dalgarno sequence, so it is efficiently translated
once it is produced. This helps accelerate initial production of cI.
The PRM promoter, which also produces cI, is not yet active. When it is active, it is not efficiently translated (about
8X less than PRE) because it lacks the Shine-Dalgarno sequence. This promoter is only needed for occasional
production of cI (in maintenance mode), so this reduced level of expression is desirable.
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Lysogeny 2
PR
cI
STOP
Cro
xis
cIII
N
P
cI
cI
PRM
PI
STOP
O
PR
int
cII
PL
PRM
OR3
OR2
OR1
cI
cI
Products: cI
At this point, enough proteins for integration into the host genome
have been produced, so lysogeny begins. When cI is produced, it
shuts off all promoters except PM and maintains lysogeny. It
upregulates PM, until [cI] becomes very high, where it then shuts
off PM also.
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Induction
RecA
PR
cI
STOP
Cro
xis
cIII
N
P
cI
PRM
STOP
O
cI
PRM
PL
RecA
PR
int
cII
Products: cI
OR3
OR2
OR1
cI
cI
RecA
RecA
When the host cell is in danger, its “S.O.S.” response activates the
host’s multi-purpose RecA protein. RecA interacts with cI, and cI
is cleaved.
10
PR
Induction 2
Cro
xis
cIII
N
N
cII
O
P
cI
Cro
Cro
PR
int
N
PRM
PRM
PL
OR3
OR2
Cro
Cro
OR1
Products: Cro, cII, O, P
With RecA degrading any cI that is produced, Cro is able to
outcompete cI and begin lysis.
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References
• Tapprich, W. University of Nebraska
Omaha. (2004) Virology notes.
• Cox, G. S. University of Nebraska Medical
Center. (2006) BRTP 822 handouts.
• Echols, H., Murialdo, H. (1978)
Microbiology Reviews 42:577-591.
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