Caudovirales
Encyclopedia
The Caudovirales are an order of virus
es also known as the tailed bacteriophage
s. Under the Baltimore classification
scheme, the Caudovirales 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. The virus particles have a distinct shape; each virion has an icosohedral head that contains the viral genome, and is attached to a flexible tail by a connector protein. The order encompasses a wide range of viruses, many of which contain genes of similar nucleotide sequence and function. Some tailed bacteriophage genomes can vary quite significantly in nucleotide sequence, however, even amongst the same genus. Due to their characteristic structure and posession of potentially homologous
genes, it is believed these bacteriophages share a common origin.
There are at least 350 recognised species in this order.
). 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 over lapping segments of DNA are made, and then put together to form the whole genome.
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 icosohedral 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.
. This classification scheme was originated by Bradley in 1969 and has been extended since.
All viruses in this order have icosahedral or oblate heads but differ in the length and contractile abilities of their tails. The Myoviridae
have long tails that are contractile; the Podoviridae
have short non contractile tails; and the Siphoviridae
have long non contractile tails.
Bradley referred to the what is now known as the Myoviridae as type A, Siphonviridae 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.
Xu J, et al (2004) Conserved translational frameshift in dsDNA bacteriophage tail assembly genes. Mol Cell, 16, 1
Casjens SR (2005) Comparative genomics and evolution of the tailed-bacteriophages. Cur Opin Micro 8 (4) 451-458
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es also known as the tailed bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
s. Under the Baltimore classification
Baltimore classification
The Baltimore classification, developed by David Baltimore, is a virus classification system that groups viruses into families, depending on their type of genome and their method of replication....
scheme, the Caudovirales are group I viruses as they have double stranded DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
(dsDNA) genomes, which can be anywhere from 18,000 base pairs to 500,000 base pairs in length. The virus particles have a distinct shape; each virion has an icosohedral head that contains the viral genome, and is attached to a flexible tail by a connector protein. The order encompasses a wide range of viruses, many of which contain genes of similar nucleotide sequence and function. Some tailed bacteriophage genomes can vary quite significantly in nucleotide sequence, however, even amongst the same genus. Due to their characteristic structure and posession of potentially homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
genes, it is believed these bacteriophages share a common origin.
There are at least 350 recognised species in this order.
Infection
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 secretionType three secretion system
Type three secretion system is a protein appendage found in several Gram-negative bacteria....
). The tail section of the virus punches a hole through the bacterial cell wall and plasma membrane
Cell envelope
The cell envelope is the cell membrane and cell wall plus an outer membrane, if one is present.Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative. These are differentiated by their Gram staining characteristics....
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 over lapping segments of DNA are made, and then put together to form the whole genome.
Assembly and Maturation
Viral capsidCapsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
proteins come together to form a precursor prohead
Prohead
A prohead or procapsid is an immature viral capsid structure formed in the early stages of self-assembly of some bacteriophages, including the Caudovirales or tailed bacteriophages. Production and assembly of stable proheads is an essential precursor to bacteriophage genome packaging; this...
, into which the genome enters. Once this has occurred, the prohead undergoes maturation by cleavage of capsid subunits to form an icosohedral 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.
Taxonomy
Because of the lack of homology between the amino acid and DNA sequences of these viruses precludes these being used as taxonomic markers as is common for other organisms, the three families here are defined on the basis of morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
. This classification scheme was originated by Bradley in 1969 and has been extended since.
All viruses in this order have icosahedral or oblate heads but differ in the length and contractile abilities of their tails. The Myoviridae
Myoviridae
The Myoviridae is a family of bacteriophages. It has been divided into three subfamiles and a number of genera not yet assigned to a subfamily. There are at least 130 species in this family.-Virology:...
have long tails that are contractile; the Podoviridae
Podoviridae
The Podoviridae are a family of bacteriophages. Unlike the other families of tailed viruses this family has short tails that are non contractile.There are at least 74 species in this family.-Virology:...
have short non contractile tails; and the Siphoviridae
Siphoviridae
Siphoviridae are a family of double-stranded DNA viruses infecting only bacteria. The characteristic features of this family is the presence of a head and non contractile tail.There are at least 256 species in this family.-Virology:...
have long non contractile tails.
Bradley referred to the what is now known as the Myoviridae as type A, Siphonviridae 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.
Bacteriophage evolution
Bacteriophages occur in over 140 bacterial or archaeal genera. They arose repeatedly in different hosts and there are at least 11 separate lines of descent. Over 5100 bacteriophages have been examined in the electron microscope since 1959. Of these at least 4950 phages (96%) have tails. Of the tailed phages 61% have long, noncontractile tails (Siphoviridae). Tailed phages appear to be monophyletic and are the oldest known virus group.Additional reading
Maniloffi J and Ackermann HW (1998) Taxonomy of bacterial viruses: establishment of tailed virus genera and the order Caudovirales. Achieve Virol. 143, 10Xu J, et al (2004) Conserved translational frameshift in dsDNA bacteriophage tail assembly genes. Mol Cell, 16, 1
Casjens SR (2005) Comparative genomics and evolution of the tailed-bacteriophages. Cur Opin Micro 8 (4) 451-458