P1 Phage- An Overview


Bacteriophages replicate themselves by two pathways which are the lytic and lysogenic cycles. Both cycles differ from each other in the fact that in the lytic phase bacteriophage assembles its particles and causes the rupturing of the host bacteria whereas, in the case of the lysogenic cycle, the phage integrates a segment of DNA into the host genome which is transmitted to other generations as the bacteria divides.

The lysogenic cycle is also known as the temperate phase and bacteriophage P1 is one such type of virus that shows lysogeny. In this segment, we will be discussing the structure, genome, and significance of the P1 phage.

P1 Phage- An Overview

Temperate phages have a unique ability to incorporate the segment of their genetic material into the host genome and by doing so the bacteriophage becomes a prophage and their recombinant genome is called lysogen. Under favorable conditions phages may switch on to the lytic cycle, releasing themselves by destroying the host cell.

Bacteriophage P1 or phage P1 is also a type of temperate phage that infects E. coli and certain other bacteria. What makes it different from other temperate phages is the feature that it does not incorporate its genetic material into the host genome like that of Bacteriophage λ but instead, it exists as a plasmid and replicates independent of the host genome.

Structure of P1 Phage

P1 phage has a structure that is similar to the T4 phage. It consists of three parts namely the head, collar, and tail.


It is made up of an icosahedral head that encloses the double-stranded DNA of the virus inside a protein coat called a capsid. On one of the sides, the head is attached to the tail.


The collar is attached to the base of the head and is made up of rings of contractile membranes. The collar helps in the attachment of the tail fibers during viral assembly.


P1 contains complete two sets of tail fibers. Each set recognizes different receptors on the bacterial cell surface. The tail helps in the attachment and penetration of the membrane of the host cell. This part of P1 is very crucial for the infection.

The Genome of Phage P1

  • When compared with other temperate bacteriophages like T4 and Lambda, P1 is of medium length which is close to 93 kilobases.

  • The genetic material of P1 phage is a double-stranded DNA which is linear when outside the host but once the phage enters the host cell it circularizes itself and assumes the shape of the plasmid.

  • Terminal Redundancy- Sometimes the length of viral DNA appears longer than it is due to the presence of a concatemeric DNA chain which makes the ends of the DNA molecule identical and 110 kilobases in length. This phenomenon is known as terminal redundancy.

  • The genome of the P1 phage can code for 5 untranslated genes and 112 proteins.

P1 Phage Life Cycle

Following are the steps involved in the life cycle of P1 phage

Attachment and Penetration

P1 attaches itself to the surface of the bacterial cell with the help of tail fibers. Then with the help of the contractile membrane on the tail fiber, it injects its genetic material into the host.

In the next step cre enzyme encoded by the viral DNA combines the terminally redundant bases and results in the circularization of the double-stranded linear DNA of phage P1.

The virus remains in a lytic cycle or enters the lysogenic phase depending on the physiological condition in which P1 is present.

Lysogenic Phase

The copy number of the P1 phage plasmid is comparatively low as the size of the plasmid is large due to terminal redundancy.

It is very important to keep the copy number low in order to reduce the metabolic burden on the lysogen.

Although the copy number of the plasmid is low which reduces the chances of its transmission to daughter bacterial cells, the virus has unique solutions to mitigate this problem by-

  • Killing the daughter cell that has lost the plasmid.

  • Untangling the coiled plasmid quickly with the help of Cre-lox recombination.

  • By regulating the replication process tightly which ensures that each time the host cell divides, the plasmid also divides along with it.

Lytic Phase

Replication of viral plasmid is bidirectional and takes place by rolling circle mechanism.

Once replication is finished and viral particles are assembled, the lytic phase is initiated which causes the destruction of a host cell and the release of viral particles, and the whole cycle is repeated.

Significance of Phage P1

  • Phage P1 is of great interest to scientists as it can be used to transfer DNA from one bacterium to another by the process of transduction.

  • Since it can carry large DNA segments, phage P1 can be used as cloning vectors in cloning experiments.

  • Cre recombinase is synthesized by phage P1 which is used in time specific and cell specific recombination procedures.


Bacteriophage P1 also known as phage P1 is a temperate or lysogenic phage. It is different from other phages of the same class in the fact that it does not incorporate its genetic material into the host genome. It circularizes its linear DNA to form a plasmid and it makes sure that it replicates itself as the host cell divides so that it can transmit itself to other generations.

Once all viral particles are assembled it switches to the lytic phase and infects other bacteria. P1 has been of great importance as it can carry DNA from one cell to another by transduction. Recently, they have been used as artificial cloning vectors also they have been of great research interest for other recombination procedures.

Updated on: 17-May-2023


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