All currently known archaeus viruses have genomes represented by DNA : single-stranded or double-stranded, or linear. Recently, however, in the hot springs of Yellowstone National Park , which are inhabited almost exclusively by archaea , using metagenomics , a viral RNA gene was found, remotely resembling eukaryotic , therefore, it is possible that RNA containing archaea viruses [1] .
Virus group | |
---|---|
Title | |
Archaeus viruses | |
Title Status | |
not determined | |
Parent taxon | |
Domain Viruses | |
Representatives | |
As of 2019, archaea viruses are represented by 17 families . It is worth noting that archaeal viruses make up two very different groups. The first group includes viruses that are structurally and genetically similar to the viruses of bacteria and eukaryotes , and the second group includes viruses that are unique to archaea and have little resemblance to viruses of other cellular forms. Practically all archaean-specific viruses infect representatives of the Crenarchaeota type , while viruses close to bacteriophages and eukaryotic viruses most often parasitize on archaeas of the Euryarchaeota type [1] .
Virion structure
Viruses that are specific to Archaea often are virions unusual shape. Thus, representatives of the family have virions in the form of champagne bottles, with the lipid membrane over the capsid proteins. For members of the family, the virions have the form of spirals. Such an unusual form of virions in viruses of these two families is associated with a special way of packaging the genome using capsid proteins [1] .
Some archaeal-specific viruses have fusiform capsids. Among them, the family of has a bundle of protein filaments at one of the pointed ends of the capsid, and the members of the family have single appendages as tails at one or two ends of the capsid. The morphology of the virions is no less unusual: these viruses also have virions, but their one end is rounded and has a drop-like shape [1] .
Many archaeal-specific viruses have filamentous virions that can carry special appendages designed to recognize archaeal cells. Sometimes, as in viruses of the Tristromaviridae family, the capsid is formed not by one, but by three types of proteins [1] .
Some archaeal-specific viruses have spherical virions, and sometimes there is a lipid membrane and another layer of proteins on top of the capsid, due to which the particles assume an icosahedral form [1] .
The virions are structurally similar to the vesicles that many archaea form: naked genomic DNA is located inside the membrane vesicle, which is laced with two types of proteins. Such bubbles can contain both single-stranded and double-stranded DNA of linear or circular form [1] .
Archaeal viruses related to bacteriophages or eukaryotic viruses have a more familiar appearance and consist of an icosahedral head, equipped with protein appendages ("tails"). Sometimes the "tails" have the ability to reduce, as in viruses of the family Myoviridae . The proteins that make up the icosahedral capsid often contain the structural motif , which is present in the capsid proteins of many bacteriophages and eukaryotic viruses [1] .
The virions of some archaeal viruses were not only examined under an electron microscope , but also studied in detail using cryoelectron microscopy . Thus, it was found that the genomes of some archaeal viruses in capsids are in A-form . This is the first known case in which A-form DNA is present in living organisms under normal conditions. One of the viruses with the A-DNA genome, AFV1, has a very thin lipid envelope with an unusual chemical composition - its main component is the lipid glyceroldibibitanylglyceroltetraether (GDGT-0), which has an unusual U-shaped configuration. In the membrane consisting of such lipids, their hydrophilic heads are turned outwards, and the hydrophobic arches are turned inwards. Along with the phospholipid bilayer and archaeal monolayer, such a structure can be considered the third known type of biological membrane [1] .
Genomes
All Archaean viruses currently isolated have DNA genomes (although, as noted above, it was possible to find the RNA genome of a possible Archaea virus in Yellowstone hot springs using metagenomics). In most cases, the genome is represented by a double-stranded DNA molecule, and only in members of the Spiraviridae and Pleolipoviridae families the genomes consist of single-stranded DNA. The sizes of genomes of archaeal viruses vary from 5,300 base pairs (p.o.) in the APBV1 virus (this is one of the smallest known genomes of DNA-containing viruses ) to 143,800 bp. in myovirus HGTV-1. As a rule, archaeal-specific viruses have smaller genomes than archaeal viruses related to bacteriophages and eukaryotic viruses [2] .
The mechanisms of replication of the genomes of archaeal viruses have been experimentally studied only for a small number of viruses. It is known that the DNA of representatives of the Caudovirales order (which, by the way, have the largest genomes among archaeal viruses) encodes part or even all the components of the DNA replication apparatus: DNA polymerases , sliding clamp proteins ( ), primases and helicases . Archaean viruses, which have more modest genome sizes, usually encode proteins that are necessary for attracting the host cell replication apparatus. However, it should be noted that in the genomes of many viruses specific for archaea, it was not possible to find proteins associated with DNA replication, so that they are either completely dependent on the host cell replication apparatus, or use unique, not yet studied DNA replication mechanisms . For example, it was possible to show that both the initiation and termination of the replication of the AFV1 lipotriksvirus genome are associated with recombination processes [1] .
The mechanisms of genome packaging in the capsid in archaeal viruses have not been studied in detail. However, it is known that members of the order of Caudovirales have homologues of the termination, which packs genomic DNA into an empty capsid. The mechanism of the formation of virions using terminations is also used by viruses of bacteria and eukaryotes. It can be assumed that the packaging of the genome into the capsid of archaeal viruses proceeds in the same way as that of bacteriophages and eukaryotic viruses, but in terms of DNA replication, archaeal viruses either depend entirely on the host cell or use unique, as yet unexplored mechanisms [1] .
Host Cell Interaction
Due to the diversity of morphology of virions, archaeal viruses use various methods of penetration into the cell. Many interact with the cell using protein appendages. Viruses that do not have appendages, such as fusiform, apparently, can penetrate into the cell due to interaction with receptors on its surface. The mechanisms underlying the specificity of archaeal viruses with respect to hosts are poorly understood. It is known that the ΟCh1 virus genome contains a specific region that can be cut and inserted at the same place in the opposite orientation. The structure of this site includes genes encoding the appendage proteins, and βinversionβ of these genes can lead to the formation of appendage proteins with different specificity with respect to the host cell [1] .
The emergence of mature virions from an archaeal cell is in many cases reminiscent of budding from a eukaryotic host cell in the influenza virus , HIV and the Ebola virus . When the virion leaves the archaeal cell, it takes with it a fragment of its membrane , which becomes an additional shell lying over the capsid. Some viruses archaea final stages of virion maturation occur after the exit from the cell, when a capsid undergoes morphological reconstruction [1] .
In some viruses (representatives of the families and ), all stages of virion maturation take place in the cytoplasm of the cell. New virus particles leave the cell through special structures with semi-axial symmetry on their surface, which are called virus-associated pyramids (VAP). VAPs form on the inner surface of the membrane of an infected cell, pass through its surface S-layer and open at the final stages of the infection , allowing virions to leave the cell [1] .
The release of some archaeal viruses from the cell is accompanied by its lysis . These include viruses of the Tristromaviridae family, which, although developed entirely in the cytoplasm, somehow acquire the lipid membrane. It is noteworthy that the ΟM2 virus encodes the pseudomorein endoisopeptidase enzyme , which destroys the archaea cell wall [1] .
Interestingly, the vast majority of archaeal hyperthermophiles have CRISPR -Cas systems for protection against viruses, while, according to the latest estimates, less than 40% of bacteria possess such systems. The reasons for the widespread distribution of CRISPR-Cas among hyperthermophilic archaea are not known for certain. It is possible that the viruses inhabiting hot springs mutate relatively slowly, so protection using the archaea of ββthe new spacers into the genome works longer than in the case of βordinaryβ viruses. In addition, the low diversity of genome sequences of hyperthermophilic viruses may be due to the fact that archaeal populations in hot springs are isolated, in other words, archaea become resistant to the viruses that inhabit the same hot spring, but not to the viruses from the neighboring source [1] .
Evolution and Family Relationships
Archaeal-specific viruses, as a rule, infect only representatives of the Crenarchaeota type. They differ from all other viruses not only by the unusual morphology of virions, but also genetically: about 90% of their genes have no homologs in the existing databases. In the genomes of some archaeal viruses, it was not possible to find a single protein for which a functionally characterized protein-homolog would exist [1] .
Sometimes, if homology cannot be established over nucleotide or amino acid sequences, spatial structures come to the rescue. Indeed, archaea obtained spatial structures for various proteins of viruses, but the situation did not become clearer from this: it turned out that many of them contain completely unique structural motifs. Moreover, the functions of many genes of archaea viruses are completely incomprehensible: for example, it turned out that the SSV1 virus survives without half of its genes. It can be assumed that such uncharacterized orphan genes encode proteins involved in the interaction of the virus with the archaeal cell, for example, counteracting CRISPR-Cas systems [1] .
However, many archaeal viruses are related to some bacteriophages and eukaryotic viruses. However, archaeal-specific viruses stand apart from all DNA-containing viruses. Moreover, the various groups of archaeus-specific viruses are unrelated to each other and evolve independently of each other. It is suggested that some groups of archaeal-specific viruses appeared at the dawn of the evolution of cell life and were subsequently lost by bacteria and eukaryotes. Other groups of specific archaeal viruses could appear at the time of isolation of the domain of archaea or even later, in separate groups of archaea [1] .
Of scientific interest is the affinity of some archaeal viruses - and mobile capsid free genetic elements (for example, plasmids ). These viruses, as well as mobile genetic elements, have related genes of the main proteins of the replicative apparatus [1] [2] .
Notes
- β 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Prangishvili David , Bamford Dennis H. , Forterre Patrick , Iranzo Jaime , Koonin Eugene V. , Krupovic Mart. The enigmatic archaeal virosphere (English) // Nature Reviews Microbiology. - 2017. - 10 November ( vol. 15 , no. 12 ). - P. 724-739 . - ISSN 1740-1526 . - DOI : 10.1038 / nrmicro.2017.125 .
- 2 1 2 Krupovic Mart , Cvirkaite-Krupovic Virginija , Iranzo Jaime , Prangishvili David , Koonin Eugene V. Viruses of archaea: Structural, functional, environmental and evolutionary genomics (English) // Virus Research. - 2018. - January ( vol. 244 ). - P. 181-193 . - ISSN 0168-1702 . - DOI : 10.1016 / j.virusres.2017.11.025 .
Links
- Elizabeth Minin. Mysterious archaeal viruses. Article on Biomolecula.ru (March 15, 2018). The appeal date is April 1, 2019.