Higher order taxa
Virus; Caudovirales (order); Myoviridae; T4-like viruses, P1-like viruses, P2-like viruses, μ-like viruses, SP01-like viruses, φH-like viruses, or PBS1-like viruses
Bacteriophage T4 (example)
Description and Significance
Myoviruses are a family of bacteriophages (from "bacteria" and the Greek phagein, "to eat"), or viruses that infect bacteria. Myoviruses, along with several other bacteriophages, have a "head and tail" morphology that is not found in other groups of viruses. In myoviruses, the tail contracts; this is related to the virus' mode of penetration of the host cell. Bacteriophage T4, a myovirus, infects E. coli.
The myovirus genome is non-segmented and contains a molecule of linear, double-stranded DNA. The complete genome is 33600-170000 nucleotides long, and has terminally redundant sequences. Guanine + cytosine content is 35%.
Virion Structure of a Myovirus
Myoviruses are not enveloped and consist of a head and a tail, which are separated by a neck. The head has icosahedral symmetry, while the tail is tubular and has helical symmetry. The capsid that constitutes the head is made up of 152 capsomers. The head has a diameter of 50-110nm; the tail is 16-20nm in diameter. The tail consists of a central tube, a contractile sheath, a collar, a base plate, six tail pins and six long fibers. Tail structure is similar to tectiviridae, but differs in the fact that a myovirus' tail is permanent. Contractions of the tail require ATP. When the sheath is contracted, it measures 10-15nm in length.
Reproductive Cycle of a Myovirus in a Host Cell
After a myovirus attaches to a host cell, it uses its contractile sheath to function like a syringe, piercing the cell wall with its central tube and injecting its genetic material into the host. The myovirus' genetic information takes over the host cell's mechanisms for transcription and translation and begins making new viruses. Once the cell has created as many myoviruses as it can, lysis occurs and the new viruses escape from the dead host cell.
Viral Ecology & Pathology
Myoviruses, being bacteriophages, infect bacteria. The most commonly infected bacteria is Escherichia coli. Myoviruses are virulent phages, meaning they do not integrate their genetic material with their host cell's, and they usually kill their host cell. It is difficult for myoviruses, like other phages, to create a successful infection. Because phages are so common, bacteria have developed specific mechanisms to fight off phage infections.