Sputnik virophage

The Sputnik virus is a yet unspecified classified virus that can replicate in the amoeba Acanthamoeba castellanii only in the presence of another virus of the genus Mimivirus. In 2008 it was discovered as a passenger virus and suggested to him as a possible member of a putative new class of agents, the term " virophage ". In fact, the Sputnik virus but does not multiply in the virion of Mimivirus, but uses the refactored by the Mimivirus protein synthesis machinery of the cell ( Viroplasma ) and is dependent on the replication enzymes of Mimivirus. Thus, it is very similar in its propagation behavior of the hepatitis D virus ( Virusoid ) and adeno -associated viruses ( genus Dependovirus ) in animal viruses and the satellite virus of some plant viruses.

Genome

Sputnik the viral genome consists of a double-stranded DNA, which is a closed ring. It is 18,343 kb in size and features similar to Mimiviren by a very low GC content of 27% off. The genome contains 21 potential open reading frame ( ORF), which overlap to some extent. By comparing the protein sequences derived from them with known proteins, a probable homologous secondary structure and thus a possible function could be derived in some gene products. In virion Sputnik virus three proteins by polyacrylamide gel electrophoresis identified as structural proteins, which could be associated with the ORFs 8, 19 and 20 according to MALDI-TOF data. The protein sequences of ORF 6 and 12 have a certain similarity with proteins of Acanthamoeba polyphaga Mimivirus - APMV ( MIMI MIMI R196 and R546 ). The Sputnik virus gene sequences thus consist of the next previously unknown viral sequences from two other units, on the one hand the Mimivirus genome and on the other known sequences from viruses are similar, have the archaea or bacteria to the host. During replication, the Sputnik virus DNA may lead to exactly replace the APMV.

Morphology

The Sputnik virus virions are about 50 nm in diameter and consist of a non-enveloped, icosahedral capsid. The capsid is probably formed exclusively by a 595 amino acid major capsid protein (ORF 20); two other structural proteins (ORF 8 and 19 ) are found only once in the virion. In infected amoebae are found closely packed aggregates of Sputnik capsids in the area of ​​Viroplasmas. As the virus affects the packaging and morphogenesis of APMV, mitverpackte Sputnik virus can be found in the interior of deformed, defective APMV virions.

Biological Significance

The Sputnik virus is a first example of a dependent reproduction and genetic exchange between two marine species of virus. This had been described so far only in plants, viruses, bacteriophages and the animal viruses ( virusoids ). Although it is known that viruses ( bacteriophages in particular ) represent Femto or Virioplankton the largest biomass and probably the most diverse group in the oceans and other water bodies, they are just little research in their environmental and genetic context. The newly discovered Mimivirus and him similar genetic elements are regularly detected in marine ecosystems. Since the taxonomic and phylogenetic position of Mimivirus itself is still unclear, this also applies to the ratio of Mimivirus to Sputnik virus. Large gene segments of the Mimivirus genes are bacterial (plasmid -like) and eukaryotic genes of parasitized alga very similar to what might be based on a horizontal gene transfer between virus and host, as well as to a possible bacterial origin of evolution of Mimivirus. On this basis, it can be postulated that the Sputnik virus an original bacteriophage origin that would constitute bacterium, developed from the Mimivirus by the loss of its own synthesis apparatus.

Swell

• B. La Scola et al.: The virophage as a unique parasite of the giant mimivirus. Nature (2008) 455 ( 7209 ): pp. 100-104 PMID 18690211
• H. Ogata H, Claverie JM: Microbiology. How to infect a Mimivirus. Science ( 2008) SEP 5; 321 (5894): pp. 1305-1306 PMID 18772426