Escherichia virus T4 is a species of bacteriophages that infect Escherichia coli bacteria. It is a member of virus subfamily Tevenvirinae (not to be confused with T-even bacteriophages, which is an alternate name of the species). T4 is capable of undergoing only a lytic lifecycle and not the lysogenic lifecycle.

1. BACTERIOPHAGE:
Dr. Dinesh C. Sharma,
Associate Professor & Head
Dept of Zoology
Km. Mayawati Govt. Girls P.G. College, Badalpur, Gb nagar

2. A bacteriophage, also known informally as a phage, is a virus that
infects and replicates within bacteria and archaea. The term was
derived from "bacteria" and the Greek, meaning "to devour".
Bacteriophages are composed
of proteins that encapsulate a DNA or RNA genome, and may have
structures that are either simple or elaborate. Their genomes may
encode as few as four genes (e.g. MS2) and as many as hundreds
of genes. Phages replicate within the bacterium following the
injection of their genome into its cytoplasm.
Bacteriophages are ubiquitous viruses, found wherever bacteria
exist. It is estimated there are more than 1031 bacteriophages on the
planet, more than every other organism on Earth, including bacteria,
combined and up to 70% of marine bacteria may be infected by
phages.
Phages have been used since the late 19th century as an alternative
to antibiotics in the former Soviet Union and Central Europe, as well
as in France. They are seen as a possible therapy against multi-drug-
resistant strains of many bacteria.

3. Phylum: incertae sedis
Class: incertae sedis
Order: Caudovirales
Family: Myoviridae
Genus: Tequatrovirus
Species: Escherichia virus T4
Virus classification
Escherichia virus T4 is a species of bacteriophages that
infect Escherichia coli bacteria. It is a member of virus
subfamily Tevenvirinae (not to be confused with T-even
bacteriophages, which is an alternate name of the species). T4 is
capable of undergoing only a lytic lifecycle and not the lysogenic
lifecycle.

4. The T4-type bacteriophages are
ubiquitously distributed in nature
and occupy environmental niches
ranging from mammalian gut to
soil, sewage, and oceans.
More than 130 such viruses that
show similar morphological
features as phage T4 have been
described; from the T4
superfamily ~1400 major capsid
protein sequences have been
correlated to its 3D structure.
The features include large
elongated (prolate) head,
contractile tail, and a complex
baseplate with six long, kinked
tail fibers radially emanating from
it.

6. Structure of phage T4 capsid
The overall architecture of the phage
T4 head determined by negative stain
electron microscopy of the procapsid,
capsid, and polyhead, including the
positions of the dispensable Hoc and
Soc proteins, has basically not changed
as a result of cryo-electron microscopic
structure determination of isometric
capsids. However, the dimensions of
the phage T4 capsid and its inferred
protein copy numbers have been
slightly altered on the basis of the
higher resolution cryo-electron
microscopy structure. The width and
length of the elongated prolate
icosahedron are Tend = 13 laevo and
Tmid = 20 (86 nm wide and 120 nm
long), and the copy numbers of gp23,
Hoc and Soc are 960, 155, and 870,
respectively
Structure of the bacteriophage T4 head. A) Cryo-EM reconstruction of
phage T4 capsid ; the square block shows enlarged view showing gp23
(yellow subunits), gp24 (purple subunits), Hoc (red subunits) and Soc
(white subunits); B) Structure of RB49 Soc; C) Structural model showing
one gp23 hexamer (blue) surrounded by six Soc trimers (red). Neighboring
gp23 hexamers are shown in green, black and magenta; D) Structure of
gp24 [6]; E) Structural model of gp24 pentameric vertex.

7. The bacteriophage T4 capsid is an elongated icosahedron, 120 nm long and 86 nm wide, and is built with three
essential proteins; gp23*, which forms the hexagonal capsid lattice, gp24*, which forms pentamers at eleven of
the twelve vertices, and gp20, which forms the unique dodecameric portal vertex through which DNA enters
during packaging and exits during infection.
The capsid also contains two non-essential outer capsid proteins, Hoc and Soc, which decorate the capsid surface.

8. Display on capsid
In addition to the essential capsid proteins, gp23, gp24, and
gp20, the T4 capsid is decorated with
two non-essential outer capsid proteins:
Hoc (h ighly antigenic o uter c apsid protein), a dumbbell
shaped monomer at the center of each gp23 hexon, up to 155
copies per capsid (39 kDa; red subunits); and
Soc (s mall o uter c apsid protein), a rod-shaped molecule that
binds between gp23 hexons, up to 870 copies per capsid (9 kDa;
white subunits). Both Hoc and Soc are dispensable, and bind to
the capsid after the completion of capsid assembly. Null (amber
or deletion) mutations in either or both the genes do not affect
phage production, viability, or infectivity.
An in vitro display system has been developed taking advantage of the
high affinity interactions between Hoc or Soc and the capsid
In this system, the pathogen antigen fused to Hoc or Soc with a hexa-
histidine tag was overexpressed in E. coli and purified
In vitro display of antigens on bacteriophage T4 capsid. Schematic representation of the T4 capsid decorated with large
antigens, PA (83 kDa) and LF (89 kDa), or hetero-oligomeric anthrax toxin complexes through either Hoc or Soc binding . The
insets show electron micrographs of T4 phage with the anthrax toxin complexes displayed through Soc (top) or Hoc (bottom).

9. Structure of the packaged
components of the phage T4 head
Packaged phage T4 DNA shares a
number of general features with
other tailed dsDNA phages: 2.5 nm
side to side packing of
predominantly B-form duplex DNA
condensed to ~500 mg/ml. The
discontinuous pattern of DNA such
as in the icosahedral-bend or
spiral-fold models.
The internal protein I* (IPI*) of
phage T4 is injected to protect the
DNA from a estriction
endonuclease of a pathogenic E.
coli that digests glucosylated
hydroxymethylcytosine DNA of T-
even phages Models of packaged DNA structure. a) T4 DNA is packed
longitudinally to the head-tail axis , unlike the transverse
packaging in T7 capsids (b). Other models shown include spiral
fold (c), liquid-crystal (d), and icosahedral-bend (e). Both
packaged T4 DNA ends are located in the portal .

10. DNA packaging
Two nonstructural terminase proteins, gp16 (18 kDa) and gp17 (70
kDa), link head assembly and genome processing.
These proteins are thought to form a hetero-oligomeric complex,
which recognizes the concatemeric DNA and makes an
endonucleolytic cut (hence the name "terminase"). The terminase-
DNA complex docks on the prohead through gp17 interactions with
the special portal vertex formed by the dodecameric gp20, thus
assembling a DNA packaging machine.
The T4 virus's double-stranded DNA genome is about
169 kbp long and encodes 289 proteins. The T4 genome
is terminally redundant and is first replicated as a unit, then
several genomic units are recombined end-to-end to form
a concatemer. When packaged, the concatemer is cut at
unspecific positions of the same length, leading to several
genomes that represent circular permutations of the original. The
T4 genome bears eukaryote-like intron sequences

11. Replication
Bacteriophages may have a lytic cycle or a lysogenic cycle.
With lytic phages such as the T4 phage, bacterial cells are broken open (lysed) and
destroyed after immediate replication of the virion. As soon as the cell is destroyed, the
phage progeny can find new hosts to infect. Lytic phages are more suitable for phage
therapy. Some lytic phages undergo a phenomenon known as lysis inhibition, where
completed phage progeny will not immediately lyse out of the cell if extracellular phage
concentrations are high.
In contrast, the lysogenic cycle does not result in immediate lysing of the host cell.
Those phages able to undergo lysogeny are known as temperate phages. Their viral
genome will integrate with host DNA and replicate along with it, relatively harmlessly, or
may even become established as a plasmid. The virus remains dormant until host
conditions deteriorate, perhaps due to depletion of nutrients, then,
the endogenous phages (known as prophages) become active. At this point they initiate
the reproductive cycle, resulting in lysis of the host cell. As the lysogenic cycle allows
the host cell to continue to survive and reproduce, the virus is replicated in all offspring
of the cell. An example of a bacteriophage known to follow the lysogenic cycle and the
lytic cycle is the phage lambda of E. coli.
T4 is capable of undergoing only a lytic lifecycle and not
the lysogenic lifecycle.

13. Infection process
The T4 virus initiates an Escherichia coli infection by binding
OmpC porin proteins and lipopolysaccharide (LPS) on the surface
of E. coli cells with its long tail fibers (LTF).
A recognition signal is sent through the LTFs to the baseplate.
This unravels the short tail fibers (STF) that bind irreversibly to
the E. coli cell surface. The baseplate changes conformation and
the tail sheath contracts, causing GP5 at the end of the tail tube to
puncture the outer membrane of the cell.
The lysozyme domain of GP5 is activated and degrades the
periplasmic peptidoglycan layer. The remaining part of the
membrane is degraded and then DNA from the head of the virus
can travel through the tail tube and enter the E. coli cell.
Morphogenesis

14. Reproduction
The lytic lifecycle (from entering a bacterium to its destruction)
takes approximately 30 minutes (at 37 °C) and consists of:
• Adsorption and penetration (starting immediately)
• Arrest of host gene expression (starting immediately)
• Enzyme synthesis (starting after 5 minutes)
• DNA replication (starting after 10 minutes)
• Formation of new virus particles (starting after 12 minutes)
After the life cycle is complete, the host cell bursts open and
ejects the newly built viruses into the environment, destroying
the host cell. T4 has a burst size of approximately 100-150 viral
particles per infected host. These Escherichia viruses infect a
host cell with their information and then blow up the host cell,
thereby propagating themselves.

15. Attachment and penetration
Bacterial cells are protected by a cell wall of polysaccharides, which are important
virulence factors protecting bacterial cells against both immune host defenses
and antibiotics. To enter a host cell, bacteriophages attach to specific receptors on the
surface of bacteria, including lipopolysaccharides, teichoic acids, proteins, or
even flagella. This specificity means a bacteriophage can infect only certain bacteria
bearing receptors to which they can bind, which in turn, determines the phage's host
range. Polysaccharide-degrading enzymes, like endolysins are virion-associated
proteins to enzymatically degrade the capsular outer layer of their hosts, at the initial
step of a tightly programmed phage infection process. Host growth conditions also
influence the ability of the phage to attach and invade them. As phage virions do not
move independently, they must rely on random encounters with the correct receptors
when in solution, such as blood, lymphatic circulation, irrigation, soil water, etc.
Myovirus bacteriophages use a hypodermic syringe-like motion to inject their genetic
material into the cell. After contacting the appropriate receptor, the tail fibers flex to
bring the base plate closer to the surface of the cell. This is known as reversible
binding. Once attached completely, irreversible binding is initiated and the tail
contracts, possibly with the help of ATP, present in the tail, injecting genetic material
through the bacterial membrane. The injection is accomplished through a sort of
bending motion in the shaft by going to the side, contracting closer to the cell and
pushing back up.

19. https://en.wikipedia.org/wiki/Escherichia_virus_T4
https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-7-356
https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-7-355