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Bacteriophage T7 (or the T7 phage) is a
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...
, a virus that infects bacteria. It infects most strains of ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'' and relies on these hosts to propagate. Bacteriophage T7 has a lytic life cycle, meaning that it destroys the cell it infects. It also possesses several properties that make it an ideal phage for experimentation: its purification and concentration have produced consistent values in chemical analyses; it can be rendered noninfectious by exposure to UV light; and it can be used in
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 encode ...
to clone RNA binding proteins.


Discovery

In a 1945 study by Demerec and Fano, T7 was used to describe one of the seven phage types (T1 to T7) that grow lytically on ''Escherichia coli;'' although all seven phages were numbered arbitrarily, phages with odd numbers, or T-odd phages, were later discovered to share morphological and biochemical features that distinguish them from T-even phages. Before being physically referred to as T7, the phage was used in prior experiments. German-American biophysicist
Max Delbrück Max Ludwig Henning Delbrück (; September 4, 1906 – March 9, 1981) was a German–American biophysicist who participated in launching the molecular biology research program in the late 1930s. He stimulated physical scientists' interest int ...
worked with the same virus in the late 1930s, calling it phage δ, and French-Canadian microbiologist Félix d'Herelle likely studied its close relative in the 1920s.d’Herelle, F. (1926). The Bacteriophage and Its Behavior. Baltimore, MD: Williams & Wilkins


Hosts

T7 grows on rough strains of ''Escherichia coli'' (i.e. those without full-length O-antigen
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
on their surface) and some other
enteric bacteria Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut m ...
, but close relatives also infect smooth and even capsulated strains.Molineux, I. J. (2006). Chapter 20: The T7 group. In: The Bacteriophages (R. Calendar, ed.), pp. 277. Oxford University Press, Oxford. E. coli is more resistant to T7 than to some other similar phages.


Virion structure

The virus has complex structural symmetry, with a
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 the phage that is
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetric ...
(twenty faces) with an inner diameter of 55  nm and a tail 19 nm in diameter and 28.5 nm long attached to the capsid. The ejection of proteins from the capsid upon infection causes the virus to change structure when it enters the cell.


Genome

The
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
of phage T7 was among the first completely sequenced genomes and was published in 1983. The head of the phage particle contains the roughly 40
kbp KBP can mean: * The ISO 639-3 code for the Kabye language * KBP Knowledge Based Processor, is used for processing packets in computer networks * Kilo-base pair (''kb'' or ''kbp''), a unit of measurement of DNA or RNA length used in genetics, equa ...
dsDNA genome which encodes 55 proteins. The genome features numerous overlapping genes that were partially removed through 'refactoring' the genome to produce T7.1.


Life cycle

T7 has a life cycle of 17 min at 37˚C, i.e. the time from infection to the lysis of the host cell when new phage are released. Due to the short latent period, most physiological studies are conducted at 30˚C where infected cells lyse after 30 min. However, high-fitness strains of T7 have been isolated with a latent period of only ~11 min at 37˚C growing under optimal conditions in rich media results. This adapted phage can undergo an effective expansion of its population by more than 1013 in one hour of growth.


Infection of host bacteria

The T7 phage recognizes certain receptors on the surface of ''E.coli'' cells, and binds to the cell surface by its viral tail fibers. In some strains of T7, the tail fibers are replaced with tail-spikes that degrade the O- or K-antigens on the cell surface by way of
enzymatic activity Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition. Enzyme units The quantity or concentration of an enzyme can be expressed in molar amounts, as with any ...
. The adsorbtion and penetration process use
lysozyme Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
s to create an opening within the peptidoglycan layer of the bacterial cell wall, allowing transfer of the viral DNA into the bacterium. The short, stubby tail of the T7-like phage is too short to span the cell envelope and, in order to eject the phage genome into the cell at the initiation of infection, virion proteins must first make a channel from the tip of the tail into the cell cytoplasm. The phage also releases five proteins needed to begin replication of the viral genome and cleave the host genome. T7 bacteriophage has been evolved to override several of the host bacteria's defenses including the peptidoglycan cell wall and the CRISPR system. Once the T7 phage has inserted the viral genome, the process of DNA replication of the host genome is halted and replication of viral genome begins. Under optimal conditions, the T7 phage can complete the lytic process within 25 minutes, leading to the death of the ''E. coli'' host cell. At the time of lysis, the virus can produce over 100 progeny.


Components

Gp5 (encoded by gene ''gp5'') is T7 phage's
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...
. T7 polymerase uses
E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Es ...
's endogenous
thioredoxin Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. In humans, thioredoxins are encoded by ''TXN'' and '' TXN2'' genes. Loss-of- ...
, a REDOX protein, as a sliding DNA clamp during phage
DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritance ...
(though thioredoxin normally has a different function). The sliding clamp functions to hold the polymerase onto the DNA, which increases the rate of synthesis.


DNA replication and repair

Phage T7 has the simplest known DNA
replisome The replisome is a complex molecular machine that carries out replication of DNA. The replisome first unwinds double stranded DNA into two single strands. For each of the resulting single strands, a new complementary sequence of DNA is synthe ...
, consisting of a
helicase Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating ...
and
primase DNA primase is an enzyme involved in the replication of DNA and is a type of RNA polymerase. Primase catalyzes the synthesis of a short RNA (or DNA in some living organisms) segment called a primer complementary to a ssDNA (single-stranded ...
that reside in a single
polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...
chain that forms a hexamer in the presence of DNA and ATP or dTTP. T7
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...
, assisted by ''
E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Es ...
''
thioredoxin Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. In humans, thioredoxins are encoded by ''TXN'' and '' TXN2'' genes. Loss-of- ...
, performs both leading and lagging-strand DNA synthesis. In phage T7, DNA double-strand breaks are likely repaired by insertion of a patch of donor DNA into a gap at the break site. This repair of double-strand breaks is facilitated by the
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ...
2.5 protein that promotes the annealing of homologous complementary strands of DNA.


Replicative intermediates

The replicating intracellular DNA of phage T7, when stretched out after cell lysis, is usually longer than the mature phage chromosome (11 to 15 µM) and can occur in the form of highly concatenated linear strands up to 66 times the length of the mature phage chromosome.Bernstein C, Bernstein H. Coiled rings of DNA released from cells infected with bacteriophages T7 or T4 or from uninfected Escherichia coli. J Virol. 1974 Jun;13(6):1346-55. doi: 10.1128/JVI.13.6.1346-1355.1974. PMID 4598784; PMCID: PMC355455. The replicating DNA can also be seen in the form of coiled ring structures that appear to correspond to multiply looped DNA configurations in which superhelical twists, necessary for compaction of the DNA, were relieved by strand nicking upon cell lysis.


Applications in molecular biology

The T7 promoter sequence is used extensively in
molecular biology Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and phys ...
due to its extremely high affinity for
T7 RNA polymerase T7 RNA Polymerase is an RNA polymerase from the T7 bacteriophage that catalyzes the formation of RNA from DNA in the 5'→ 3' direction. Activity T7 polymerase is extremely promoter-specific and transcribes only DNA downstream of a T7 promo ...
and thus high level of expression. T7 has been used as a model in
synthetic biology Synthetic biology (SynBio) is a multidisciplinary area of research that seeks to create new biological parts, devices, and systems, or to redesign systems that are already found in nature. It is a branch of science that encompasses a broad ran ...
. Chan ''et al.'' (2005) "
refactored In computer programming and software design, code refactoring is the process of restructuring existing computer code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structur ...
" the genome of T7, replacing approximately 12
kbp KBP can mean: * The ISO 639-3 code for the Kabye language * KBP Knowledge Based Processor, is used for processing packets in computer networks * Kilo-base pair (''kb'' or ''kbp''), a unit of measurement of DNA or RNA length used in genetics, equa ...
of its genome with engineered DNA. The engineered DNA was designed to be easier to work with in a number of ways: individual functional elements were separated by
restriction endonuclease A restriction enzyme, restriction endonuclease, REase, ENase or'' restrictase '' is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class ...
sites for simple modification, and overlapping protein coding domains were separated and, where necessary, modified by single base pair
silent mutations Silent mutations are mutations in DNA that do not have an observable effect on the organism's phenotype. They are a specific type of neutral mutation. The phrase ''silent mutation'' is often used interchangeably with the phrase '' synonymous mutat ...
. T7 has been tested on human
osteosarcoma An osteosarcoma (OS) or osteogenic sarcoma (OGS) (or simply bone cancer) is a cancerous tumor in a bone. Specifically, it is an aggressive malignant neoplasm that arises from primitive transformed cells of mesenchymal origin (and thus a sarcoma) ...
to treat tumor cells.


References


External links

*

ViralZone
New Details about T7-host interactions
Microbe Magazine
T7 reference proteome
in
Uniprot UniProt is a freely accessible database of protein sequence and functional information, many entries being derived from genome sequencing projects. It contains a large amount of information about the biological function of proteins derived fro ...
{{DEFAULTSORT:T7 Phage Podoviridae