''Mimivirus-dependent virus Zamilon'', or Zamilon, is a

virophage Virophages are small, double-stranded DNA viral phages that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for ...

, a group of small

DNA virus A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and ...

es that infect

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

s and require a

helper virus A helper virus is a virus that allows an otherwise-deficient coinfecting virus to replicate. These can be naturally occurring as with Hepatitis D virus, which requires Hepatitis B virus to coinfect cells in order to replicate. Helper viruses are a ...

to replicate; they are a type of

satellite virus A satellite is a subviral agent that depends on the coinfection of a host cell with a helper virus for its replication. Satellites can be divided into two major classes: satellite viruses and satellite nucleic acids. Satellite viruses, which a ...

. Discovered in 2013 in Tunisia, infecting '' Acanthamoeba polyphaga''

amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudo ...

e, Zamilon most closely resembles

Sputnik Sputnik 1 (; see § Etymology) was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to Earth for t ...

, the first virophage to be discovered. The name is

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walte ...

for "the neighbour". Its

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ...

particle is 50–60 nm in diameter, and contains a circular double-stranded DNA

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...

of around 17 kb, which is predicted to encode 20

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides ...

s. A related strain, Zamilon 2, has been identified in North America. All known virophages are associated with helpers in the giant DNA virus family ''

Mimiviridae ''Mimiviridae'' is a family of viruses. Amoeba and other protists serve as natural hosts. The family is divided in up to 4 subfamilies., UCPMS ID: 1889607PDF/ref> Fig. 4 and §Discussion: "Considering that tupanviruses comprise a sister g ...

''. Zamilon is restricted in its range of helper viruses; it can be supported by viruses from ''

Mimivirus ''Mimivirus'' is a genus of giant viruses, in the family '' Mimiviridae''. Amoeba serve as their natural hosts. This genus contains a single identified species named ''Acanthamoeba polyphaga mimivirus'' (APMV). It also refers to a group of ph ...

''-like ''Mimiviridae'' lineages B and C, but not from lineage A. This appears to be a consequence of a rudimentary immune system of the helper virus, termed MIMIVIRE (mimivirus virophage resistance element), akin to the

CRISPR-Cas CRISPR () (an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragments of bact ...

pathway. Unlike the Sputnik virophage, Zamilon does not appear to impair the replication of its helper virus.

Discovery and related virophages

Zamilon was discovered in 2013, in '' Acanthamoeba polyphaga''

e co-infected with the

giant virus A giant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria. All known giant viruses belong to the phylum '' Nucleocytoviricota''. Description While the exact criteria as defined in the ...

Mont1, isolated from a Tunisian soil sample. As of 2015, Zamilon is one of three

s to have been isolated physically, the others being

and Mavirus; several other virophage DNAs have been discovered using

metagenomics Metagenomics is the study of genetic material recovered directly from environmental or clinical samples by a method called sequencing. The broad field may also be referred to as environmental genomics, ecogenomics, community genomics or micr ...

but have not been characterised physically. A related strain, named Zamilon 2, was discovered by metagenomic analysis of a North American poplar wood chip

bioreactor A bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances ...

in 2015. Another virophage, Rio Negro, is also closely related to Sputnik.

Taxonomy

Zamilon virophage has been classified by the

International Committee on Taxonomy of Viruses The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclatures for viruses. The ICTV has developed a universal taxonomic scheme for viruses, and thus has the means to ap ...

into the species ''Mimivirus-dependent virus Zamilon'' within the genus ''Sputnikvirus'' in the family ''

Lavidaviridae Virophages are small, double-stranded DNA viral phages that require the co-infection of another virus. The co-infecting viruses are typically giant viruses. Virophages rely on the viral replication factory of the co-infecting giant virus for ...

''.

Virology

A Zamilon virion is

with a diameter of 50–60 nm, and is similar in appearance to those of Sputnik and Mavirus. Its circular double-stranded DNA

is 17,276 base pairs in length. Virophages typically have particles whose diameter is in the range 40–80 nm, with genomes in the range 17–30 kb. The virophage most closely related to Zamilon is Sputnik, with which it shares 76% sequence identity, although part of the Zamilon sequence is reversed compared with Sputnik. Zamilon's DNA is rich in

adenine Adenine () (symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivativ ...

and

thymine Thymine () (symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine n ...

bases; the proportion of guanine and cytosine bases is 29.7%.

Open reading frames

The Zamilon genome is predicted to contain 20

open reading frame In molecular biology, open reading frames (ORFs) are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a Prokaryote, prokaryotic DNA sequence, where only one of the #Six-fra ...

s (ORFs), of between 222 bases and 2337 bases in length. Of the 20 predicted products, 15 are similar to those of Sputnik, and three are similar to ''

'': two to ''

Megavirus chilensis Megavirus is a viral genus containing a single identified species named ''Megavirus chilensis'', phylogenetically related to Acanthamoeba polyphaga Mimivirus (APMV). In colloquial speech, Megavirus chilensis is more commonly referred to as jus ...

'' and another to the transpoviron of ''Moumouvirus ''. One Zamilon ORF additionally shows some similarity to Mavirus, and another to Organic Lake virophage and a ''

Phaeocystis globosa ''Phaeocystis'' is a genus of algae belonging to the Prymnesiophyte class and to the larger division of Haptophyta. It is a widespread marine phytoplankton and can function at a wide range of temperatures ( eurythermal) and salinities (euryhali ...

'' virophage, which are both associated with

algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...

rather than amoebae. The remaining two predicted products show limited similarity to other known proteins. Putative functions of the products include

transposase A transposase is any of a class of enzymes capable of binding to the end of a transposon and catalysing its movement to another part of a genome, typically by a cut-and-paste mechanism or a replicative mechanism, in a process known as transposition ...

helicase Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separat ...

integrase Retroviral integrase (IN) is an enzyme produced by a retrovirus (such as HIV) that integrates—forms covalent links between—its genetic information into that of the host cell it infects. Retroviral INs are not to be confused with phage i ...

cysteine protease Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal ...

, DNA primase–

polymerase A polymerase is an enzyme ( EC 2.7.7.6/7/19/48/49) that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using b ...

and DNA-packaging

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

enzymes, major and minor

capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may ...

proteins, a structural protein and a collagen-like protein. ORF6 is very similar to the Sputnik major capsid protein, which has a double " jelly-roll" fold.

Life cycle and helper virus

Like all other virophages, Zamilon replicates in the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

, within the

virus factory A viroplasm, sometimes called "virus factory" or "virus inclusion", is an inclusion body in a cell where viral replication and assembly occurs. They may be thought of as viral factories in the cell. There are many viroplasms in one infected cell, ...

of its helper, which acts as its host. Zamilon was first isolated in association with Mont1, a ''Mimivirus''-like ''Mimiviridae'' classified in lineage C by its polymerase B gene sequence. The virophage has subsequently been shown to be capable of replicating in association with Moumouvirus and Monve, two ''Mimiviridae'' from lineage B, as well as with Terra1 and Courdo11 from lineage C; however, it cannot replicate in association with either ''

'' or ''

Mamavirus Mamavirus is a large and complex virus in the Group I family ''Mimiviridae''. The virus is exceptionally large, and larger than many bacteria. Mamavirus and other mimiviridae belong to nucleocytoplasmic large DNA virus (NCLDVs) family. Mamaviru ...

'', classified as lineage A. This is unlike Sputnik, which can replicate in association with any ''Mimivirus''-like member of ''Mimiviridae''. Zamilon does not appear to inhibit the ability of its helper virus to replicate significantly, nor to lyse its host amoebae cells. Although the helper virus formed a high proportion of abnormal

virion A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky ...

s in the presence of Zamilon, these were also observed at a comparable level in the virophage's absence. This is again unlike Sputnik, which reduces its helper virus's infectivity, inhibits its lysis of amoeba, and is associated with the generation of an increased proportion of abnormal ''Mimiviridae'' virions. Bernard La Scola and colleagues, who isolated both Sputnik and Zamilon, state that, if confirmed, this "would question the concept of virophage", which has been considered to be differentiated from most other satellite viruses in having a deleterious effect on its helper virus. File:Zamilon and abnormal Mont1 (Gaia et al. Fig 3a).jpg,

Electron micrograph A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnify, magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken ...

of a

in an

co-infected with ''Mimivirus-dependent virus Zamilon'' (small particles) and Mont1. Arrows show abnormal Mont1 particles (scale bar: 0.1 μm) File:Zamilon virophage with Mont1 (Gaia et al Fig1e).jpg, Electron micrograph of virus factory in an amoeba co-infected with Zamilon and Mont1 (scale bar: 0.1 μm)

References

External links

Genbank accession no. NC_022990.1
{{Taxonbar, from=Q24809338 DNA viruses Virophages