Virophages

Q; What are virophages?

Krishna: Viruses are not living, so how  can they make other viruses their hosts?

Virophages are small, double-stranded DNA viral phages that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for their own replication.

Sputnik, which was first discovered in the water-cooling tower in Paris in 2008, was the first virophage ever identified. A second virophage known as Mavirus was identified in 2011. Organic Lake Virophage, or OLV is the third one found in  the same year.

Virophages are small, double-stranded DNA viral phages that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for their own replication. One of the characteristics of virophages is that they have a parasitic relationship with the co-infecting virus. Their dependence upon the giant virus for replication often results in the deactivation of the giant viruses. The virophage may improve the recovery and survival of the host organism.

Virophages, which are known as virus eaters, attack other viruses, as is the case with the first virophage, Sputnik. Unable to multiply within a host, virophages rely on hosts infected with other viruses. In the case of Sputnik, it was an amoeba infected with a mamavirus. Sputnik would essentially take over the replication process of the mamavirus. Because of this takeover, the mamavirus is unable to produce properly, thus reducing its ability to infect the amoeba.

Unlike satellite viruses, virophages have a parasitic effect on their co-infecting virus. Virophages have been observed to render a giant virus inactive and thereby improve the condition of the host organism.

For instance, mimivirus-dependent virus Sputnik does not infect mimiviruses.

Instead, what it does is infects cells already infected with mimivirus. A virus contains genetic instructions that tell a cell to stop what it’s doing and start manufacturing viruses instead. So it will typically contain instructions to make all the parts of a virus: the viral genome, the viral capsid proteins, and so on.

Mimivirus-dependent virus Sputnik does not have all the instructions to make copies of itself. Somewhere along the line, it lost some genes it needs to force a cell to manufacture crucial parts of itself. So if Mimivirus-dependent virus Sputnik infects a cell, it will take over that cell, but it will be unable to make copies of itself.

The mimivirus makes the missing bits that Mimivirus-dependent virus Sputnik requires to reproduce. So if Mimivirus-dependent virus Sputnik takes over a cell that is already infected with mimivirus, it will instruct the cell to stop making mimivirus (except for the parts of the mimivirus that are also used for making Mimivirus-dependent virus Sputnik), and instruct the cell to make itself instead, using the bits of mimivirus to make up for the bits of itself it has lost the ability to make.

Mimivirus-dependent virus Sputnik does not infect mimiviruses.

Mimivirus-dependent virus Sputnik infects cells. Like all viruses, it must infect a cell. However, once it infects a cell, it disrupts the cell but does not create more copies of itself unless the cell is also infected by mimivirus.

The only way Mimivirus-dependent virus Sputnik can reproduce is if it infects a cell that has previously been infected with mimivirus. When it does this, it shuts down mimivirus production and instructs the cell to make more Mimivirus-dependent virus Sputnik instead.

 Viruses like Sputnik that depend on co-infection of the host cell by helper viruses are known as satellite viruses.

Mimiviruses are themselves infected by parasites known as virophages, the first of which was named “Sputnik.” Technically, virophages are satellite viruses; that is, they are defective viruses that need a helper virus to provide missing functions (see Section 24.5.1). Typically, satellite viruses infect and hence damage the same host cell as their helper virus. However, virophages differ in a major respect. Instead of replicating separately in the host cell they invade and infect the virus assembly compartment where mimivirus replicates. Consequently, they cripple the replication of mimivirus. This, paradoxically, benefits the host cell since the virophages are tiny relative to the colossal mimivirus (50 nm versus 750 nm; 20 genes versus 1000). Virophages also infect the phycodnaviruses that infect algae. They reduce algal death from phycodnaviruses and overall promote growth of the algae in the marine environment.

Sources: 

Sheree Yau et al., Virophage control of antarctic algal host–virus dynamics, Published online before print March 28, 2011, Proceedings of the National Academies of Sciences doi:10.1073/pnas.1018221108

https://www.sciencedirect.com/topics/biochemistry-genetics-and-mole...