CORRESPONDENCE LINK TO ORIGINAL ARTICLE LINK TO AUTHOR’S REPLY located in front of 12 out of 21 Sputnik coding sequences and all 20 Cafeteria roenbergensis Virophages question the virus coding sequences (both promoters being associated with the late expression of genes) existence of satellites imply that virophage gene expression is gov- erned by the transcription machinery of the Christelle Desnues and Didier Raoult host virus during the late stages of infection. Another point concerns the effect of the virophage on the host virus. It has been argued In a recent Comment (Virophages or satel- genome sequences of other known viruses that the effect of Sputnik or Mavirus on the lite viruses? Nature Rev. Microbiol. 9, 762– indicates that the virophages probably belong host is similar to that observed for STNV and 763 (2011))1, Mart Krupovic and Virginija to a new viral family. This is further supported its helper virus1. However, in some cases the Cvirkaite-Krupovic argued that the recently by structural analysis of Sputnik, which infectivity of TNV is greater when inoculated described virophages, Sputnik and Mavirus, showed that MCP probably adopts a double- along with STNV (or its nucleic acid) than should be classified as satellite viruses. In a jelly-roll fold, although there is no sequence when inoculated alone, suggesting that STNV response2, to which Krupovic and Cvirkaite- similarity between the virophage MCPs and makes cells more susceptible to TNV11. Such Krupovic replied3, Matthias Fisher presented those of other members of the bacteriophage an effect has never been observed for Sputnik two points supporting the concept of the PRD1–adenovirus lineage9. With the size of or Mavirus, for which a negative effect on the virophage: first, Sputnik and Mavirus are not the Sputnik particle, the nature and com- host virus is always obtained. In addition, a defective particles but fully functional viruses, plexity of its genome, and the presence of a satellite virus has never been shown to pro- and second, they have been shown (at least ready-to-use set of viral RNAs in the virion10, duce a ‘diseased’ form of its helper virus or in silico) to divert the machinery of the host Sputnik challenges the current definition of (even though rare with Sputnik) to be fully virus, rather than that of the host cell, for satellite viruses, which relies on the idea of a encapsidated within it. genome replication and transcription. defective particle or subviral agent. Here, we want to go beyond semantics A virophage has ecological and evolutionary and look at the problem another way. We A virophage is a parasite of another virus. To significance. By predating on giant viruses propose a remodelled classification based on illustrate the similarity between the depend- such as members of the families Mimiviridae our previously suggested bipartite system of ence of a satellite virus on the helper virus or Phycodnaviridae, virophages regulate viral capsid-encoding organisms (CEOs) and ribo- and the dependence of a virophage on its viral population dynamics and probably influence some-encoding organisms (REOs)4, which, host, Krupovic and Cvirkaite-Krupovic used the whole microbial food web. In a recent in addition to the existing viruses of Bacteria the example of the satellite tobacco necro- study, Yau et al. used a Lotka–Volterra simula- (bacteriophages), Eukarya and Archaea, sis virus (STNV), the single-stranded RNA tion to model the effect of a virophage on the would incorporate a new group called ‘viruses genome of which is replicated by the phycodnavirus–green alga interaction8. The of Viruses’, encompassing virophages and RNA-dependent RNA polymerase (RdRp) of addition of a virophage to this system led to traditional satellite RNA viruses. the helper virus, TNV1. We believe that this an increase in the survival of the host algal cell is equivalent to comparing cows and apples. population and a deviation of the microbial What is a virophage? As the single-stranded RNA genomes serve loop towards secondary production. A virophage is a viral parasite of giant viruses, directly as mRNA, the replication cycle of the The presence of genes from different viral thriving by exploiting the machinery of the STNV–TNV couple does not have a transcrip- origins in the Sputnik genome has stressed viral host for genome replication and tran- tion stage. In addition, the RdRp is a typical the role of Sputnik as a probable vehicle for scription, impairing production of the viral viral enzyme that is not found in eukaryotic, gene transfer among viruses5. In addition, host and causing the generation of abnormal bacterial or archaeal genomes, so to repli- on the basis of the similarities between the and degenerated forms of the viral host5. cate, STNV has no choice but to rely on this Mavirus genome and transposable elements enzyme. As an alternative (although still of the Maverick–Polinton family, it has even A virophage is a virus. Sputnik (the first imperfect) comparison, a parallel could have been suggested that large DNA transposons virophage described)5 and the recently dis- been made with the adeno-associated virus found in eukaryotic genomes emerged from covered Mavirus6 are particles of about (AAV)–adenovirus system, as both are DNA a virophage ancestor6. 50 nm diameter, with double-stranded DNA viruses. Following the AAV example, and as The presence of numerous giant virus genomes of 18,343 bp and 19,063 bp cod- is the case for DNA viruses of comparable and virophage signatures in environmental ing for 21 and 20 ORFs, respectively. Both size, the Sputnik genome would be expected metagenomic data sets suggests that, as for possess structural (for example, major cap- to transit into the nucleus, where it would be bacteriophages in the ocean12,13, virophages sid protein (MCP)) and non structural (for replicated and transcribed by the host cell are common in the environment and play an example, polymerase and genome-packaging machinery. However, recent in situ hybridi- unrecognized part in regulating virus–host ATPase) proteins. As the Sputnik and Mavirus zation experiments have shown that the interactions and contributing to global gene genomes harbour their own capsid-encoding Sputnik infection cycle has no nuclear phase flow in the virosphere. genes, they are consistent with the defini- and that genome replication takes place inside tion of viruses as CEOs4,7. The absence of the viral factory, with a maximum of activity What is a satellite virus? significant similarities between the genome at 6–7 hours post-infection (C.D. and D.R., The International Committee on Taxonomy sequences of Sputnik, Mavirus and Organic unpublished observations). In addition, the of Viruses (ICTV) describes satellite viruses Lake virophage (OLV; a virophage sequence previously described polyadenylation signals as “subviral agents lacking genes that could identified from metagenomic data)8 and and the AT‑rich conserved promoter motifs encode functions needed for replication and NATURE REVIEWS | MICROBIOLOGY www.nature.com/reviews/micro © 2012 Macmillan Publishers Limited. All rights reserved CORRESPONDENCE Christelle Desnues and Didier Raoult are at the depending on the co-infection of a host cell CEOs with a helper virus for their multiplication” Unité de recherche sur les maladies infectieuses et tropicales émergentes, Unité mixte de recherche (REFS 14,15) Viruses of Bacteria . Unlike satellite nucleic acids, Centre national de la recherche scientifique 6236, satellite viruses also encode a structural pro- Viruses of Eukarya L’Institut de recherche pour le développement 198, tein that encapsidates their genome. Until the Aix-Marseille Université, Faculté de Médecine, 27 Bd Jean Moulin, 13005 Marseille, France. release of Virus Taxonomy. Ninth Report of the Viruses of Archaea Viruses of Viruses International Committee on Taxonomy of Viruses Correspondence to D.R. (on 8 November 2011)15, the satellite viruses e‑mail: [email protected] were classified in a unique category, itself sub- 1. Krupovic, M. & Cvirkaite-Krupovic, V. Virophages or satellite viruses? Nature Rev. Microbiol. 9, 762–763 divided into two subgroups: subgroup 1, with (2011). the chronic bee-paralysis virus-associated sat- Other Other 2. Fischer, M. G. Sputnik and Mavirus: more than just satellite viruses. Nature Rev. Microbiol. 10, 78 (2012). ellite virus as representative, and subgroup 2, 3. Krupovic, M. & Cvirkaite-Krupovic, V. Sputnik and with satellites that resemble STNV. Thus, Mavirus: not more than satellite viruses. Nature Rev. Microbiol. 10, 78 (2012). when we described Sputnik (which means 4. Raoult, D. & Forterre, P. Redefining viruses: lessons from Mimivirus. Nature Rev. Microbiol. 6, 315–319 satellite in Russian) the category of satellite Bacteria Eukarya (2008). viruses included only small particles of 15 5. La Scola, B. et al. The virophage as a unique parasite to 17 nm diameter encapsidating a single- of the giant mimivirus. Nature 455, 100–104 (2008). Archaea 6. Fischer, M. G. & Suttle, C. A. A virophage at the origin stranded RNA molecule of 796 to 1,239 bp of large DNA transposons. Science 332, 231–234 and displaying only one to two ORFs. (2011). REOs 7. Forterre, P. & Prangishvili, D. The great billion-year The AAVs, which are commonly called sat- war between ribosome- and capsid-encoding organisms (cells and viruses) as the major source of ellite viruses in the literature and were cited as evolutionary novelties. Ann. NY Acad. Sci. 1178, a representative of satellite viruses by Krupovic Figure 1 | The capsid-encoding organisms 65–77 (2009). and Cvirkaite-Krupovic1, were previously (CEOs), includingNature the Reviewstraditional | Microbiology viruses of 8. Yau, S. et al. Virophage control of antarctic algal Bacteria, viruses of Archaea and viruses host–virus dynamics. Proc. Natl Acad. Sci. USA 108, not included in the satellite virus category 6163–6168 (2011). of Eukarya plus the new ‘viruses of Viruses’ 9. Sun, S. et al. Structural studies of the Sputnik of the ICTV but instead were classified with domain.