Virology: A parasite's parasite saves host's neighbours.

Abstract

Viruses can be attacked by parasitic viruses, which compete with them for cellular resources. It emerges that one such parasitic virus can defend a host-cell population from a viral attack. See Letter p.288 Matthias Fischer and Thomas Hackl show here that endogenous viral elements found in a marine protozoan have a function in defence against infection by giant viruses. Virophages are a recently discovered group of DNA viruses that are obligate parasites of protist-infecting giant DNA viruses such as mimivirus. Fischer and Hackl find mavirus, a virophage sharing an evolutionary origin with a class of self-synthesizing DNA transposons called Maverick/Polinton elements, integrates into the genome of the marine protozoan Cafeteria roenbergensis. Superinfection of the protozoan in vitro with the giant virus Cafeteria roenbergensis virus (CroV) induces the production of infectious mavirus particles, which are released upon host cell lysis and can then suppress CroV replication in other CroV-infected flagellate populations.