A phagemid or phasmid is a

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

-based

cloning vector A cloning vector is a small piece of DNA that can be stably maintained in an organism, and into which a foreign DNA fragment can be inserted for cloning purposes. The cloning vector may be DNA taken from a virus, the cell of a higher organism, o ...

, which has both

bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...

and

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...

properties. These vectors carry, in addition to the origin of plasmid replication, an

origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by semi ...

derived from bacteriophage. Unlike commonly used plasmids, phagemid vectors differ by having the ability to be packaged into the

capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...

of a bacteriophage, due to their having a genetic sequence that signals for packaging. Phagemids are used in a variety of biotechnology applications; for example, they can be used in a molecular biology technique called "

phage display Phage display is a laboratory technique for the study of protein–protein, protein–peptide, and protein–DNA interactions that uses bacteriophages (viruses that infect bacteria) to connect proteins with the genetic information that encodes ...

". The term "phagemid" or "phagemids" was coined by a group of Soviet scientists, who discovered them, named them, and published the article in April 1984 in Gene magazine.

Properties of the cloning vector

A phagemid (plasmid + phage) is a

that contains an f1

from an f1 phage. It can be used as a type of

cloning Cloning is the process of producing individual organisms with identical genomes, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction; this reproduction of an organism by itself without ...

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

in combination with filamentous phage M13. A phagemid can be replicated as a plasmid, and also be packaged as single stranded

in viral particles. Phagemids contain an origin of replication (ori) for double stranded replication, as well as an f1 ori to enable single stranded replication and packaging into phage particles. Many commonly used plasmids contain an f1 ori and are thus phagemids. Similarly to a plasmid, a phagemid can be used to clone DNA fragments and be introduced into a bacterial host by a range of techniques, such as transformation and

electroporation Electroporation, also known as electropermeabilization, is a microbiological and biotechnological technique in which an electric field is applied to cells to briefly increase the permeability of the cell membrane. The application of a high-vo ...

. However, infection of a bacterial host containing a phagemid with a 'helper' phage, for example VCSM13 or M13K07, provides the necessary viral components to enable single stranded DNA replication and packaging of the phagemid DNA into phage particles. The 'helper' phage infects the bacterial host by first attaching to the host cell's pilus and then, after attachment, transporting the phage genome into the cytoplasm of the host cell. Inside the cell, the phage genome triggers production of single stranded phagemid DNA in the cytoplasm. This phagemid DNA is then packaged into phage particles. The phage particles containing ssDNA are released from the bacterial host cell into the extracellular environment. Filamentous phages retard bacterial growth but, contrasting with the

lambda phage Lambda phage (coliphage λ, scientific name ''Lambdavirus lambda'') is a bacterial virus, or bacteriophage, that infects the bacterial species ''Escherichia coli'' (''E. coli''). It was discovered by Esther Lederberg in 1950. The wild type of ...

and the T7 phage, are not generally

lytic The lytic cycle ( ) is one of the two cycles of viral reproduction (referring to bacterial viruses or bacteriophages), the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane. Bact ...

. Helper phages are usually engineered to package less efficiently (via a defective phage origin of replication) than the phagemid so that the resultant phage particles contain predominantly phagemid DNA. F1 Filamentous phage infection requires the presence of a

pilus A pilus (Latin for 'hair'; : pili) is a hair-like cell-surface appendage found on many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some researchers ...

so only bacterial hosts containing the F-plasmid or its derivatives can be used to generate phage particles. Prior to the development of cycle sequencing, phagemids were used to generate single stranded DNA template for sequencing purposes. Today phagemids are still useful for generating templates for

site-directed mutagenesis Site-directed mutagenesis is a molecular biology method that is used to make specific and intentional mutating changes to the DNA sequence of a gene and any gene products. Also called site-specific mutagenesis or oligonucleotide-directed mutagenes ...

. Detailed characterisation of the filamentous phage life cycle and structural features lead to the development of

technology, in which a range of peptides and proteins can be expressed as fusions to phage coat proteins and displayed on the viral surface. The displayed peptides and polypeptides are associated with the corresponding coding DNA within the phage particle and so this technique lends itself to the study of protein-protein interactions and other ligand/receptor combinations.

References

{{Organisms et al. Cloning Molecular biology