Caudoviricetes

Caudoviricetes
	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.
Virus classification
(unranked):	Virus
Realm:	Duplodnaviria
Kingdom:	Heunggongvirae
Phylum:	Uroviricota
Class:	Caudoviricetes
Subdivisions
	See text

Caudoviricetes is a class of viruses known as tailed viruses and head-tail viruses (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 viruses possess a common origin.^[2]

Infection

Upon encountering a host cell, 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 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

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

History

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.^[3] Siphoviridae constitute the majority of the known tailed viruses.^[2]^[4]

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. The class contains 11 orders and a large number of unassigned taxa, listed hereafter.^[5]

Orders

Unassigned taxa

Unassigned families

The following families are unassigned to an order:^[5]

Unassigned subfamilies

The following subfamilies are unassigned to a family and order:^[5]

Unassigned genera

The following genera are unassigned to a family, subfamily, and order:^[5]

Bacteriophage evolution

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.^[4]^[6]

References

^ Ackermann HW (1998). "Tailed bacteriophages: the order caudovirales". Advances in Virus Research. 51: 135–201. doi:10.1016/S0065-3527(08)60785-X. ISBN 9780120398515. PMC 7173057. PMID 9891587.
^ ^a ^b ^c ^d ^e ^f "Double-Stranded DNA Bacteriophages". Boundless. 2017-11-11. Archived from the original on 2013-06-28.
^ Maniloff J, Ackermann HW (1998). "Taxonomy of bacterial viruses: establishment of tailed virus genera and the order Caudovirales". Archives of Virology. 143 (10): 2051–63. doi:10.1007/s007050050442. PMID 9856093. S2CID 34921877.
^ ^a ^b Ackermann HW (May 2003). "Bacteriophage observations and evolution". Research in Microbiology. 154 (4): 245–51. doi:10.1016/S0923-2508(03)00067-6. PMID 12798228.
^ ^a ^b ^c ^d "Virus Taxonomy: 2024 Release". International Committee on Taxonomy of Viruses. Retrieved 5 March 2025.
^ Ackermann HW, Prangishvili D (October 2012). "Prokaryote viruses studied by electron microscopy". Archives of Virology. 157 (10): 1843–9. doi:10.1007/s00705-012-1383-y. PMID 22752841. S2CID 16699662.