Class of viruses
Caudoviricetes
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Order Caudovirales. Structures of T Bacteriophages representing the seven T types of Escherichia coli phages described by Max Delbruck in the 1940s. T4 of the Myoviridae family, T5 of the Siphoviridae family, and T7 of the Podoviridae family. The structures were built from individual protein data bank (pdb) files in the UCSF Chimera software, which were updated to the year 2024 and at real scale.
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Virus classification
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(unranked):
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Virus
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Realm
:
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Duplodnaviria
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Kingdom:
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Heunggongvirae
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Phylum:
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Uroviricota
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Class:
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Caudoviricetes
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Subdivisions
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See text
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Caudoviricetes
is a class of
viruses
known as the
tailed
bacteriophages
(
cauda
is Latin for "tail").
[1]
Under the
Baltimore classification
scheme, the
Caudoviricetes
are group I viruses as they have double stranded
DNA
(dsDNA) genomes, which can be anywhere from 18,000 base pairs to 500,000 base pairs in length.
[2]
The virus particles have a distinct shape; each virion has an
icosahedral
head that contains the viral genome, and is attached to a flexible tail by a connector protein.
[2]
The order encompasses a wide range of viruses, many containing genes of similar nucleotide sequence and function. However, some tailed bacteriophage genomes can vary quite significantly in nucleotide sequence, even among the same genus. Due to their characteristic structure and possession of potentially
homologous
genes, it is believed these bacteriophages possess a common origin.
[2]
There are 4 orders, 47 families, 98 subfamilies, 1197 genera, 3601 species in the class. This makes
Caudoviricetes
the most populous class among viruses, accounting for approximately 30% of all recognized virus species and nearly half of all virus genera.
[3]
Infection
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Upon encountering a host bacterium, the tail section of the virion binds to
receptors
on the cell surface and delivers the DNA into the cell by use of an injectisome-like mechanism (an injectisome is a nanomachine that evolved for the delivery of proteins by
type III secretion
). The tail section of the virus punches a hole through the
bacterial cell wall and plasma membrane
and the genome passes down the tail into the cell. Once inside the genes are expressed from transcripts made by the host machinery, using host
ribosomes
. Typically, the genome is replicated by use of
concatemers
, in which overlapping segments of DNA are made, and then put together to form the whole genome.
[2]
Assembly and maturation
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Viral
capsid
proteins come together to form a precursor
prohead
, into which the genome enters. Once this has occurred, the prohead undergoes maturation by cleavage of capsid subunits to form an icosahedral
phage
head with 5-fold symmetry. After the head maturation, the tail is joined in one of two ways: Either the tail is constructed separately, and joined with the connector, or the tail is constructed directly onto the phage head. The tails consist of helix based proteins with 6-fold symmetry. After maturation of virus particles, the cell is lysed by
lysins
, holins, or a combination of the two.
[2]
Taxonomy
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History
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For most of virological history,
Caudoviricetes
which was known as the order
Caudovirales
, had lower taxa defined via morphology and contractile ability of their "tails". The
Myoviridae
had long tails that were contractile; the
Podoviridae
had short noncontractile tails; and the
Siphoviridae
had long noncontractile tails.
[4]
Siphoviridae
constitute the majority of the known tailed viruses.
[2]
[5]
Bradley referred to what was known as the
Myoviridae
as type A,
Siphoviridae
as type B, and the
Podoviridae
as type C. He also divided his groups on the basis of head morphology: Within group A, A1 have small isometric heads; A2 have prolate heads; and A3 have elongated heads. Within groups B and C, numbers were similarly assigned: B1 and C1 have small isometric heads; B2 and C2 have prolate heads; and B3 and C3 have elongated heads.
[
citation needed
]
Because the "families"
Myoviridae
,
Podoviridae
and
Siphoviridae
were abolished for being polyphyletic, there are now many free-floating families, subfamilies, and genera in the class without any preceding taxa before
Caudoviricetes
. There are currently 7 orders, 63 families, 109 subfamilies, 1360 genera, and 4079 species in the class. This article lists all official and proposed taxa of
Caudoviricetes
. (Note: quotation marks mean that the taxon is proposed and has not yet been ratified by the ICTV.)
Assigned taxa
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Unassigned taxa
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There are many unassigned taxa in
Caudoviricetes
as of ICTV (2022).
[3]
Unassigned genus in the order
Crassvirales
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Unassigned families
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Unassigned genus in the family
Chaseviridae
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Unassigned genera in the family
Demerecviridae
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Unassigned genera in the family
Drexlerviridae
[
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Unassigned genera in the family
Guelinviridae
[
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Unassigned genera in the family
Herelleviridae
[
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Unassigned genera in the family
Pootjesviridae
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Unassigned genera in the family
Salasmaviridae
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Unassigned genera in the family
Schitoviridae
[
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Unassigned genera in the family
Straboviridae
[
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Unassigned genera in the family
Vilmaviridae
[
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Unassigned genera in the family
Zobellviridae
[
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Unassigned subfamilies
[
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Unassigned genera
[
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Etymology
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Crassvirales
infects bacteria and is named after the computer program crAss, which was used to identify the first member of the
Crassvirales
order.
[6]
[7]
The rest (including the proposed orders) infect
archaea
.
Kirjokansivirales
,
Thumleimavirales
, and
Nakonvirales
are named after mythological objects or deities;
Methanobavirales
and
Magrovirales
are named after archaic synonyms of the archaea they infect.
Bacteriophage evolution
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Bacteriophages occur in over 1100 bacterial or archaeal genera.
[3]
Over 6300 bacteriophages have been examined in the electron microscope since 1959. Of these, more than 96 percent have tails. Of the tailed phages, about 57 percent have long, noncontractile tails (
"Siphoviridae"
). Tailed phages appear to be monophyletic and are the oldest known virus group.
[5]
[8]
See also
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References
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Further reading
[
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