Biomedical Engineering Reference
In-Depth Information
Head
Tail
xis
cro
Left
cohesive
end
Right
cohesive
end
AWB DEFZ
J
att int red
gam
N cI
OP
Q SR
Fig. 4.10
Map of the λ chromosome,
showing the physical position of some
genes on the full-length DNA of wild-
type bacteriophage λ. Clusters of
functionally related genes are indicated.
%
λ
1
0 0 0 0 0 0 0 0 0
0
%
λ
Non-essential region
Absorption
A
R
A
Cell envelope
Host lysis
R
Addition of
phage tails
Nicks sealed
by ligase
A
Cleavage
cos
cos
cos
R
A
Early
replication
(bidirectional
mode)
Lysogeny
cos
R
A
Origin
cos
R
cos
att
gal
x
att
bio
Recombination
E.coli
chromosome
cos
Integration
Excision
gal
N
att
R cos A
J
bio
Late replication
(rolling circle mode)
att
Replication along with
E.coli
chromosome
Fig. 4.11
Replication of phage-λ DNA in
lytic and lysogenic cycles.
the repressor gene (
cI
) (Fig. 4.12). This transcription
is subject to repression by the product of the
cI
gene
and in a lysogen this repression is the basis of
immunity to superinfecting
stage transcription by anti-termination. The
N
gene
product, expressed from P
L
, directs this switch. It
interacts with RNA polymerase and, antagonizing
the action of host termination protein
. Early in infection,
transcripts from P
L
and P
R
stop at termination sites t
L
and t
R1
. The site t
R2
stops any transcripts that escape
beyond t
R1
. Lambda switches from early- to middle-
λ
, permits it to
ignore the stop signals so that P
L
and P
R
transcripts
extend into genes such as
red
,
O
and
P
necessary for
the middle stage. The early and middle transcripts
ρ
