| Caudoviricetes | |
|---|---|
 | |
| Structures of T Bacteriophages representing the seven T types of Escherichia coli phages described by Max Delbruck in the 1940s. T4 of theMyoviridae family, T5 of theSiphoviridae family, and T7 of thePodoviridae 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 | |
Caudoviricetes is a class ofviruses known astailed viruses andhead-tail viruses (cauda is Latin for "tail").[1] It is the sole representative of its own phylum,Uroviricota (fromouros (ουρος), a Greek word for "tailed" +-viricota). Under theBaltimore classification scheme, theCaudoviricetes are group I viruses as they have double strandedDNA (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 anicosahedral 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 potentiallyhomologous genes, it is believed these viruses possess a common origin.[2]
Upon encountering a host cell, the tail section of the virion binds toreceptors 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 bytype III secretion). The tail section of the virus punches a hole through thecell 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 hostribosomes. Typically, the genome is replicated by use ofconcatemers, in which overlapping segments of DNA are made, and then put together to form the whole genome.[2]
Viralcapsid proteins come together to form a precursorprohead, into which the genome enters. Once this has occurred, the prohead undergoes maturation by cleavage of capsid subunits to form an icosahedralphage 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 bylysins, holins, or a combination of the two.[2]
For most of virological history,Caudoviricetes which was known as the orderCaudovirales, had lower taxa defined via morphology and contractile ability of their "tails". TheMyoviridae had long tails that were contractile; thePodoviridae had short noncontractile tails; and theSiphoviridae had long noncontractile tails.[3]Siphoviridae constitute the majority of the known tailed viruses.[2][4]
Bradley referred to what was known as theMyoviridae as type A,Siphoviridae as type B, and thePodoviridae 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]
In 2021 the "families"Myoviridae,Podoviridae andSiphoviridae were abolished for being polyphyletic. The same happened toCaudovirales, which has accumulated into somewhat of a wastebasket taxon. As a result, there are now many free-floating families, subfamilies, and genera in the class without any preceding taxa beforeCaudoviricetes. The ICTV report associated with the specific change is: 2021.001BAbolish the order Caudovirales and the families Myoviridae, Siphoviridae and Podoviridae (Caudoviricetes).[5]
As of ICTV 2024 release the class contains 11 orders and a large number of unassigned taxa, listed hereafter.[6]
The following families are unassigned to an order:[6]
The following subfamilies are unassigned to a family and order:[6]
The following genera are unassigned to a family, subfamily, and order:[6]
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][7]
