Virus infects virus: Mimivirus paradigm

A virus is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. A complete virus particle, known as a virion, consists of genetic material (DNA or RNA) surrounded by a protective coat of protein called a capsid.Viruses infect all cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected. Viruses that infect bacteria are known as bacateriophage or simply, phage. Once a phage infects a bacteria, it may have either a lytic cycle or a lysogenic cycle, inside the bacterial cell. A phage that follows a lytic cycle is called lytic phage (virulent phage) and that follow a lysogenic cycle is called lysogenic phage (temperate phage). However, a few viruses are capable of carrying out both lytic and lysogenic cycles, for example, lambda phage. In lytic cycle, the phage (eg. T4 phage) upon entering entering in to the bacterial cell, replicate and lyse (destroy) the bacterial cell. In contrast, in lysogenic cycle the viral genome integrates (or lysogenize) with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid and retain inside the bacterial cell. Integrated lysogenic phages are also called propahge. Sometimes prophages may provide benefits to the host bacterium while they are dormant by adding new functions to the bacterial genome in a phenomenon called lysogenic conversion. A famous example is the conversion of a harmless strain of Vibrio cholerae by a phage (CTXØ) into a highly virulent one, which causes cholera.

So far this was all about virus that infects bacteria.

What about virus that infects another virus? There are viruses that infect giant viruses. Marine virologists have reported such small viruses occurring with larger ones in marine protist populations. Small viruses or so called “satellite viruses” require other larger viruses, especially for their reproduction, since they lack essential functions for multiplication. Usually satellite viruses are harmless to their giant virus host. Most recently, La Scola et al. report in Nature (2008) that a small virus, named Sputnik infects a giant DNA virus named Mimivirus (Acanthamoeba polyphaga mimivirus, or APM) and the infection decreases the yield of APM virions and generates APM virions with aberrant morphologies in amoeba cells. Since, unlike usual satellite viruses, Sputnik uses its host’s (Mimivirus) virus machinery and impairs its fitness authors call Sputnik a “virophage.” Sputnik (named after “traveling companion” in Russian) is an icosahedral virus with a DNA genome encoding 21 genes and was isolated in a cooling tower in Paris. Mimivirus once thought to be a bacterium since it contains more number of genes than many bacteria and bear properties like cellular organism. It is five to ten times larger than any other known virus It was La Scola et al. who classified it as a virus in 2003. Discovery of giant Mimivirus and its relation to Sputnik are expected to provide new clues to the virus-virus interaction in virosphere and its impact in the evolution of viruses.