Bacteriophage Families with a detailed description of Models

PhagesMyoviridae – Mu

Viro102: Bacteriophages & Phage Therapy3 Credit hoursNUST Centre of Virology & Immunology

Bacteriophage Families

Siphoviridae Cystoviridae

Myoviridae Leviviridae

Inoviridae Rudiviridae

Podoviridae Fuselloviridae

Microviridae Tectiviridae

Corticoviridae Lipothrixviridae

Plasmaviridae

Myoviridae Family

Group I virusesSingle 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: StructureIsometric, 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 represents50 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 RepressorGene 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 RecognitionThe 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 calledMu G(+)Mu G(-)

A phage specific Gin( G inversion) protien is resposible for switching, which occur

time to time.

Mu Viral proteins on tail fibers

Host cell surface receptors Host range

G(+) S & UGlucose linked to

polysaccharide with (1-4) glycosidic linkage

E. coli K12, Salmonella & various strains of Serratia sp.

G(-) 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: TranspositionTransposition 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

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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