The document discusses the T7 bacteriophage, which infects E. coli bacteria. It has a lytic lifecycle where it uses the host cell's machinery to replicate itself, then causes the cell to burst releasing new phage particles. The T7 phage has a head and tail structure, with its genome being a 40kbp double stranded DNA. Upon infection, the phage genome enters the cell and hijacks the cellular replication processes to produce new phage particles over 17 minutes, before the mature phage particles exit by lysing the host cell membrane.

1. JANANI K R
III B.Sc., MICROBIOLOGY

2. INTRODUCTION
 A virus which infects the bacteria called bacteriophage.
 T7 bacteriophage infects most strains of Escherichia coli.
 T7 bacteriophage is a T-odd phage.
 Bacteriophage T7 has a lytic life cycle meaning that it
destroys the cell it infects.
 Bacteriophage T7 is also known as Entereobacteria phage.
 In 1945 study by Demerec and Fano, T7 was used to
describe one of the seven phage types (T1 to T7) that grow
lytically on E.coli.
 The genetic material of T7 bacteriophage is dsDNA.

3. STRUCTURE OF T7 BACTERIOPHAGE
 T7 bacteriophage is composed of icosahedral head, a
connector and flexible tail.
 The icosahedral head has a diameter of 60 to 61nm with a
capsid that is 2nm thick.
 The capsid is composed of two forms of the gene 10
protein.
 The head-tail connector is composed of 12 gp8 molecules.
 The connector has a 12-lobed wide domain inserted into
the head cavity and a narrower domain that interacts with
the tail.
 Inside the head, and attached to the connector in the
coaxial orientation, is a 26nmx21nm cylindrical structure
that is usually referred to as the internal core.

4.  The core has recently been shown to
exhibit 8-fold symmetry, it consists
of stacked rings and contains three
distinct proteins, the products of
genes 14,15,and 16.
 The tail is 23nm long, tapering from
a diameter of 21nm at the connector
to 9nm at its distal end, and is
known to consist of two major
proteins gp11 and gp12.
 Attached near the head proximal
end of the tail are six symmetrically
positioned tail fibers.
 Each fiber is composed of a trimer
of gp16 that forms a kinked
structure.

5. GENOME OF T7 BACTERIOPHAGE
 The genome of phage T7 was among the first completely
sequenced genomes and was published in 1983.
 The head of the phage particle contains the roughly 40kbp
dsDNA genome which encodes 55 proteins.
 The genome features numerous overlapping genes that were
partially removed through ‘refactoring’ the genome to
produce.

6. REPRODUCTION OF T7
BACTERIOPHAGE
 The T-phages, T1 through T7, are referred to as lytic
phages because they always bring about the lysis and death
of their host cell, the bacterium E.coli.
 Lytic cycle is known as reproductive cycle of bacteriophage.
 Lytic cycle is also called as Master-Slave action. In the life
cycle of phage T7 virus acts as master and host as slave.
 T7 bacteriophage that only use the lytic cycle so it is called
as virulent phage.
 T7 bacteriophage has a life cycle of 17 min at 37 degree
celsius.
 Lytic cycle has five steps which are adsorption, penetration,
synthesis, maturation and release.

7. Step 1: Adsorption
 Attachment sites on the bacteriophage adsorb to receptor
sites on the host bacterium, E.coli.
 T7 bacteriophage adsorb to the bacterial cell wall by its
tail fibers.
 This is the first step in lytic cycle known as adsorption
and also known as attachment.
Step 2: Penetration
 Following adsorption, the phage secrete certain enzyme,
to “drills” a hole in the bacterial cell wall .
 Then the bacteriophage injects its genome into the
bacterial cytoplasm.
 This process is called penetration.

8. Step 3: Synthesis
 Immediately after the injection of the viral DNA there is a
process initiated called synthesis.
 Synthesis of Early Proteins
 Synthesis of early proteins refers to the transcription and
translation of a section of phage DNA to make a set of
proteins that are needed to replicate the phage DNA.
 The early proteins produced a repair enzyme to repair the
hole in the bacterial cell wall.
 A DNAase enzyme that degrades the host DNA into
precursors of phage DNA and a virus specific DNA
polymerase that will copy and replicate phage DNA.

9.  The result is the synthesis of several copies of the phage
DNA.
 Synthesis of late proteins
 After the synthesis of early proteins, the next is the
synthesis of late proteins.
 Each of the several replicated copies of the phage DNA
can now be used for transcription and translation of a
second set of proteins called the late proteins.
 The late proteins are mainly structural proteins that
make up the capsomeres and the various components of
the tail assembly.
 Lysozyme is also a late protein that will be packaged in
the tail of the phage.

10.  Lysozyme used to escape from the host cell during the last
step of the replication process.
 Step 4: Maturation
 Following synthesis of proteins, maturation takes place.
 The proteins that make up the capsomeres assemble
themselves into the heads and “reel in” a copy of the phage
DNA.
 The tail and accessory structures assemble and incorporate a
bit of lysozyme in the tail fibers.
 The phage arrange their escape from the host cell during the
assembly process.

11.  Step 5: Release
 After the assembly of new phages, the phage secretes the
lysozyme enzyme that lysis the cell wall of host bacterium, E.coli
and the matured new phages release and reinfects the strains of
E.coli.
 Step 6: Reinfection
 After the release of the new phages, T7 phage is ready to
reinfect the another strain of E.coli.

12. REPRODUCTION OF T7
BACTERIOPHAGE

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