Bacteriophage Families with a
detailed description of Models
Phages
Myoviridae – Mu
Viro102:
Bacteriophages & Phage Therapy
3 Credit hours
NUST Centre of Virology & Immunology
Bacteriophage Families
Siphoviridae
Cystoviridae
Myoviridae
Leviviridae
Inoviridae
Rudiviridae
Podoviridae
Fuselloviridae
Microviridae
Tectiviridae
Corticoviridae
Lipothrixviridae
Plasmaviridae
Myoviridae Family
Group I viruses
Single molecule of ds linear DNA.
Non enveloped.
diameter of about 55-110 nm.
Genome size ranges from 33.6 – 170 kb.
The genome contains unusual bases, they are
5-hydroxy-methyl cytosine (instead of
cytosine). This helps in protecting the phage
from the host defence system i.e. Restriction
enzymes.
Mu: Discovery
 Discovered in E. coli by Larry Taylor (1963).
 Given the name Mu, for mutator because of its ability to
cause mutations. It is known to cause mutations at high
rate.
 The mutations proved to be insertions to Mu at random
sites in the host genome disrupting the functioning of
different genes.
Mu: An Introduction
 linear ds DNA genome of 40-Kb and more than 35
genes.
 Capable of both Lytic & lysogenic life cycle.
 Most important feature is its capability to ‘move’ within
host genome, a process referred to as transposition.
This phage replicates by transposition.
 Mu uses multiple rounds of replicate transposition to
amplify it during lytic growth.
 During lytic cycle Mu completes about 100 rounds of
transposition per hour, making it most efficient
transposition known.
 The head of Mu phage has the capability to carry 2 kb
extra genome. This is because of headful packaging
mechanism.
DNA Transposition
 Transposons are sequences of DNA that can move around
to different positions within the genome of a single cell, a
process called transposition. In the process, they can
cause mutations & change the amount of DNA in the
genome.
 Class I mobile genetic elements (aka retrotransposons)
copy themselves by first being transcribed to RNA, then
transcribed back to DNA by reverse transcriptase, & then
being inserted at another position in the genome (copy
paste mechanism).
 Class II mobile genetic elements (aka DNA transposons)
move directly from one position to another using a
transposase to "cut and paste" them.
Mu: Structure
Isometric, icosahedral
head
a knob like neck
a contractile tail
a baseplate
six short tail fibers
Figure 1. Electron micrograph of a
Mu virion, negatively stained with
uranyl acetate. Scale bar represents
50 nm.
Each Mu is packaged from a different site in
the host genome, so the host DNA on the
ends of Mu is unique in every different phage
head.
Mu: Genome
When Mu DNA is packaged into a phage head it includes
about 50-150 bp of host DNA at the left end and a variable
amount of host DNA (2kb) on the right end.
Cont’d
Gene C encodes the Repressor
Gene A encodes Transposase that is
responsible for integration, replication
transposition, and excision of Mu DNA
Gene B encodes enhancer of
transposition.
Mom gene is responsible for protecting
the virus against restriction
endonucleases.
Mu: Host Recognition
The Mu genome contains a region, called G,
that can invert. Its about 3000 bp reigon.
Each orientation of this DNA fragment
corresponds to the synthesis of different
proteins involved in the host specificity of the
viral particle.
The two different kinds of particles are called
Mu G(+)
Mu G(-)
A phage specific Gin( G inversion) protien
is resposible for switching, which occur
time to time.
Mu
G(+)
G(-)
Viral proteins
on tail fibers
Host cell surface receptors
Host range
S&U
Glucose linked to
polysaccharide with (1-4)
glycosidic linkage
E. coli K12,
Salmonella & various
strains of Serratia sp.
S’ & U’
Glucose linked to
polysaccharide with (1-6)
glycosidic linkage
E coli C & strains of
Citrobacter, Shigella,
Enterobacter &
Erwinia sp.
Mu: Integration
into host genome
Little is known about how this
occurs apart from the fact that
the bacterial sequences at either
end of the Mu genome are lost
in the process
Mu: Transposition
Transposition requires two phage encoded
proteins:
1. Transposase (encoded by gene A)
2. Transposition enhancer (encoded by gene B).
In bacterial cells, Mu transposition can be
Non Replicative: Initial insertion of the Mu
genome into host chromosome (Lysogeny).
Replicative: Mu phage makes copies of its own
genome while inside the host chromosome
(Lytic).
Mu uses target immunity to
avoid transposing into its
own DNA
Transposition in its own genome
causes disruption in its genes.
That is solved by target immunity.
Achieved by interplay between MuA
trasposase and MuB ATPase.
MuA inhibit MuB from binding to
nearby DNA sites. This inhibition
requires ATP hydrolysis
MuB helps MuA to find a target site
for transposition
For Mu sequence within
approximately 15 kb of an existing Mu
insertion are immune to new insertion.
Mu:
Different
Phases
Entry into host cell &
integration into host
genome
If repressor protein in high
concentration, lysogenic
cycle starts
Harsh conditions cause
induction of repressed
prophage
Lytic cycle starts. Mu uses
repeated transposition as
a mode of DNA synthesis.
Mu particals are
assembled and released.
These may be G(+) or G(-)
Lytic Cycle & Replicative
Transposition
Things to remember about Mu
Mu phage can tranpose.
Mu phage genome does not
concatamerize.
Mu phage replicates by semi conservative
replication in the host genome.
Mu phage has a diverse host range
because of G fragment in genome.
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