Download ϕX174 Program of T4 lytic cycle

ϕX174 rolling circle replication
Fig. 19.3
ϕX174
• + strand ssDNA
genome, 5386 nt
with 10 genes
• Icosahedral capsid,
attaches to LPS
• Genome contains
overlapping genes:
some regions of
polycistronic mRNA
can be translated in
>1 reading frame
Many phages use this
strategy to produce
progeny genomes
ϕX174 CisA nicks +
strand of RF I
dsDNA, remains
attached to 5ʼ end;
3ʼOH of nicked strand
primes synthesis (but
no “lagging strand
synthesis” for this
phage)
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

Fig. 19.4
Infection by T4 (T even phages)
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•
•
•
•
Fig. 10.10
T4 binds to LPS in
OM (recognition by
tail fibers)
169 kd linear dsDNA
is injected into cell
from head by tail
Capsid remains
outside cell
200 phage in 25ʼ lytic
cycle
Genomes of T2, T4,
and T6 are 85%
identical, with
differences
accounting for binding
different host
receptors
Program of T4 lytic cycle
Fig. 10.15
Early phage proteins subvert host RNA synthesis (modifying host
σ) and degrade host DNA, allowing phage to take over host
cell; Specific phage DNA synthesis proteins.
Late proteins: capsids assembled in 3 different paths.
1
Replication of T4 DNA, part I
By 4 min post-infection, viral DNA pol initiates viral DNA replication.
Viral DNA pol and recombination proteins produce complex
intermediates--linear repeats called concatemers, that
subsequently are processed into progeny genomes.
Processing T4
genomic DNA
• Replication &
recombination yields
long concatemers
• Phage endonuclease
processes DNA into
linear molecules of
specific length =105%
of genome (no cutting
at specific sequence)
• Terminal redundancy
of varying sequence
called circular
permutation
Fig. 10.13
λ: lysis or lysogeny
Phage λ
• Linear dsDNA
genome, 48.5Kb
• Simpler tail structure
than T even phages
• Binds to maltoporin
in OM of E. coli
• Temperate phage,
capable of 2 distinct
fates upon infection
Temperate phages
can proceed with
lytic infection or
can integrate into
genome and
maintain prolonged
association with
host.
Fig. 10.16
2
Forming a λ lysogen
• λ-encoded Integrase
catalyzes site-specific
recombination with
E. coli chromosome
between gal-bio
• Maintenance of
lysogeny requires
continued expression
of cI repressor
• Reversible process…
Fig. 10.18
3