Virophages

Virophages are a group of small double-stranded DNA viruses that parasitize giant viruses, most commonly during coinfection of a unicellular eukaryotic host such as an amoeba. They depend on the replication machinery of the giant virus and are only able to replicate in cells already infected by that giant virus.

Virophages were first described in association with mimiviruses; the best known example is Sputnik virophage, discovered in a mimivirus-infected amoeba and named after the Soviet satellite. Sputnik’s genome is approximately 18 kilobases and encodes about twenty genes. Since then other virophages have been described, including Zamilon and Mavirus.

Sputnik and Zamilon differ in their host range among mimiviruses; Zamilon does not replicate with all mimiviruses, suggesting compatibility limitations. Mavirus is notable for its interaction with Cafeteria roenbergensis and the giant virus CroV; it can integrate into the host genome and, upon CroV infection, can produce virophage particles that interfere with CroV replication, thereby helping protect the host cell.

Genomes of virophages are typically small, ranging from roughly 17 to 40 kb, with a handful to couple dozen open reading frames; they sometimes carry genes related to capsid formation and replication enzymes but rely on the giant virus replication compartments. They are packaged in virions that assemble in a manner similar to other small dsDNA viruses.

Ecology and significance: Virophages broaden understanding of viral interactions; they can influence dynamics of giant-virus infections, possibly affect microbial communities, and contribute to horizontal gene transfer in viral evolution. They are detected in various environments by sequencing approaches, with occurrences in marine, freshwater, and soil systems known.