Introduction: Mu phage also refers as “Temperate and transposable phage” because Mu-phage causes transposition of the genes into the host cell, at the time of multiplication. It mainly attacks the members or species of enterobacteria family and that’s why also known as “Enterobacterial phage Mu”.
The transposition of Mu-phage occurs by either of the two mechanisms:
- Replicative transposition
- Non-replicative transposition
Host range: E.coli, Serratia marcescens, Citrobacter freundii, Erwinia carotovora etc.
- Group: Group-I (ds-DNA)
- Order: Caudovirales
- Family: Myoviridae
- Genus: Mulikevirus
Content: Mu Phage
Definition Of Mu Phage
Several discoveries have led to the study and discovery of Mu-phage.
|Year of discovery||Discoverer||Discovery|
|1950||Larry Taylor||Discovered the Mu-phage.|
|1972-75||Ahmad Bukhari||Demonstrated the insertion of Mu randomly throughout the bacterial genome.
Introduced the host gene by the excision of Mu.
|1979||Jim Shapiro||Introduced transposition model as “Shapiro- intermediate”.|
|1983||Kiyoshi Shapiro||Demonstrated in vitro transposition by using mini Mu-plasmids.|
|1994-2012||Montano||Created crystal structure of the Mu-bacteriophage transposome.|
Structure of Mu Phage
As we know the basic structure of the bacteriophage, that is comprised of head, neck and tail. Therefore, the structure of Mu phage is also divided into three parts i.e. head, neck and tail. To know more about the structure of Mu phage, let us look at the picture given below.
There are the following structural components of Mu phage which includes:
It is the terminal part of the Mu phage which carries the viral genome and surrounds by a protein layer refers as Capsid.
Symmetry: The head of Mu phage is having icosahedral symmetry.
Shape: The shape is like spheroid.
Diameter: The diameter is 54 nm.
Capsid: It is the layer which surrounds the hexagonal head. The capsid of Mu phage is composed of about 152 smaller protein subunits, known as “Capsomeres”.
It is the middle portion of the Mu phage, acts as the joining element which joins the two components i.e. Head and tail.
It is long, thick and contractile because of the presence of crossbands.
Size: The size of the tail is 183 X 16-20nm.
Sheath: It is the covering of the contractile tail, which is composed of stacked rings. At the time of contraction, the sheath becomes shorter and thicker. And the length of the sheath during contraction is up to 60-90nm.
Genetic map of Mu Phage
Mu phage consists of a ds-DNA which is linear and non-segmented. The genome of Mu phage consists of 37,611 base pairs. The guanine and cytosine content is 35%. The genome is non-permuted, consists of some unusual base pairs and terminally redundant sequences. Mu phage genome encodes 55 genes which perform different functions in its life cycle. The genome size is 40 Kb in length.
The functions of different genes are as follows:
Mu-phage consists of two transposase binding sites which bind to the host DNA and represented as attL and attR. These two ends sometimes refer as MuL and MuR.
A gene: It encodes all transposition events.
B gene: It encodes the all replicative transposition.
C gene: It represses the expression of the transposase gene.
Head and tail genes: These are the structural genes that help in reconstruction or biosynthesis of Mu phage.
Gin gene: It catalyzes the site-specific inversion reactions.
Mom gene: It encodes a DNA modification function by converting adenine to acetamide adenine.
Life cycle of Mu Phage
The life cycle of Mu phage completes by the following steps, that are given below:
First, the tail fibres attach to the receptor site of the host cell surface. By the binding of the tail fibre, there is the conformational change in the base plate of Mu phage. Due to the conformational change in the base plate, the sheath of the tail contracts.
By the contraction of the tail’s sheath, the rigid internal material gets into the host cell surface through the cell envelope. The N protein also injects along with the viral genome which refers to non-replicative protein.
The N-protein undergoes circularization as it binds with the viral genome.
After circularization, early transcription occurs which give rise to the Repc and Ner repressors and DDE recombinase A (Mu A). These genes help in the integration of the viral genome to the host cell genome. During this step, the variable ends are cut off from the viral genome.
After the non-replicative transposition, the ratio of Repc and Ner repressors decides whether the phage will enter to the lysogenic phage or lytic phage.
Repc: It represses the early promoter by establishing latency or lysogeny.
Ner: It represses the expression of Repc by promoting the expression of the early genes for the replication of Mu phage.
After the inactivation of Repc, there is an expression of MuA and MuB genes. MuA is the DDE recombinase-A enzyme and MuB is the target DNA activator B. MuA performs the transposition of viral genome ends and host DNA. The target DNA activator B helps in the replication of viral host DNA, leading to the formation of the two copies. This type of replication refers as “Replicative transposition”. This replication can lead to 100 viral genomes after successive rounds.
This phase carries out the expression of adenine modification enzyme which makes the viral DNA resistant to the host restriction enzymes by modifying the adenines in the viral DNA.
Biosynthesis and Assembly
The late gene synthesizes the structural genes of Mu phage which leads to the biosynthesis of virus particles. Then the virus particles like empty capsid, tail fibres etc. get assemble.
The packaging of virion
The bacterial DNA is first cut on the left of the integrated Mu genome for about 50-150bp. Then, a second cut occurs after the filling of phage head. The packaging of viral DNA also occurs on the right side of the Mu genome. Therefore, at different sites of the bacterial genome, there is the packaging of the Mu genome.
Cell lysis and release of virion
After packaging, the newly synthesized virions releases out of the host cell by the help of lye gene which encodes lytic enzymes (responsible for cell lysis).