Medusavirus

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"Medusavirus"
Systematics
Classification : Viruses
Area : Varidnaviria
Empire : Bamfordvirae
Phylum : Nucleocytoviricota ?
Class : unclassified
Order : unclassified
Family : "Medusaviridae"
Genre : "Medusavirus"
Taxonomic characteristics
Genome : dsDNA linear
Baltimore : Group 1
Symmetry : icosahedral
Cover : available
Scientific name
"Medusavirus"

" Medusavirus " is a proposed genus of large DNA viruses isolated from a Japanese hot spring . The icosahedral capsid of the virus particles (virions) has T = 277 symmetry and has appendages with spherical heads on the surface . Its genome is 381  kb . The virus can harden unprotected amoeba into stone-like cysts . Under laboratory conditions, the amoeba Acanthamoeba castellanii can serve as a host. This is the first virus to be found in a hot (43.4 ° C) environment. Morphologically and phylogenetically , " Medusavirus " differs greatly from other giant viruses of the phylum Nucleocytoviricota (outdated Nucleocytoplasmic large DNA viruses , NCLDV), which is why it was proposed to assign it to its own taxonomic family , the " Medusaviridae ". In the lineage of the NCLDVs, these seem to represent a basal branch or to be located close to " Mollivirus ".

The proposed taxon is named after Medusa , the monster in Greek mythology whose gaze turns people to stone. Official registration by the International Committee on Taxonomy of Viruses (ICTV) is currently pending (as of April 2020).

description

As shown by cryoelectron microscopy , the virus particles (virions) of the Medusaviruses have a very unusual morphology. The icosahedral protein kapsid (with T = 277 symmetry ) has a diameter of about 260  nm and a thickness of about 8 nm. It is covered with appendages (' spikes ') that are approximately 14 nm long and have spherical tips end up. There is exactly one such appendage on each capsomer. In “ Medusavirus ” - as in other large nucleocytoplasmic DNA viruses (NCLDVs) - there is a 6 nm thick inner membrane under the protein capsid

The genome of " Medusavirus " is a linear double-stranded DNA molecule with a length of 381277  bp . Its GC content is 61.7%; According to this indicator, “ Medusavirus ” ranks second among the NCLDVs after the genus “ Pandoravirus ”, which has also only been proposed so far . Presumably, the gemnome 461 contains protein- coding genes and various sequences that presumably stand for tRNA . In the protein- coding DNA, 115 genes that are homologous to eukaryotic genes, that is 39%, were found by comparison of genes in a database . 86% of these come from the genome of the amoeboid host. which indicates a horizontal gene transfer (HGT, also called lateral gene transfer, LGT) between the amoeba and the virus. Interestingly, there are no genes in the genome of “ Medusavirus ” that code for DNA-dependent RNA polymerase , the capping enzyme and topoisomerase II , as can otherwise be found in the genomes of all other large NCLDVs. Therefore, the replication of the Medusavirus genome is carried out by proteins of the host cell. Instead, the genes of all five histones (H1, H2A, H2B, H3 and H4) as well as a few other proteins not characteristic of viruses, such as a homologue of cyclin B , which can regulate the cell cycle of the amoeba host, and a putative metacaspase were found involved in the regulation of programmed cell death in amoebas . In addition, Medusavirus DNA polymerase is related to eukaryotic DNA polymerase δ . After these initial investigations, the " Medusavirus " may have acquired many eukaryotic genes in its tribal history as a result of horizontal gene transfer (HGT) from the amoeba host. The horizontal transfer apparently took place in several stages and was bidirectional. Of the 57 gene candidates for HGT, 13 suspected to be transmission from the virus to the amoeba (VtoA) and 12 in the opposite direction from the amoeba to the virus (AtoV). However, around half of the genes transferred to the amoeba do not appear to be active there (or these genes have been deactivated by the amoeba).

Life cycle

The DNA of " Medusavirus " is found in the amoeba nucleus 1 hour after infection. After a further 1 to 3 hours, most of the viral DNA can be detected on the periphery of the cell nucleus, see above. This means that the replication of the Medusavirus genome takes place in the core of the host cell. 8 hours after infection, the concentration of viral DNA reaches its highest value and the viral DNA is distributed throughout the cell nucleus. 8 to 10 hours after infection, numerous capsids can be detected in the cytoplasm of the infected cell and 14 hours after infection the viral DNA can also be detected in the cytoplasm. The release of new virus particles from the host cell begins 14 hours after infection.

evolution

A phylogenetic analysis based on DNA polymerase gene sequences and major capsid protein (MCP) did not reveal any indication that “ Medusavirus ” should be included in an existing group or family of NCLDVs. The analysis of the proteome and the gene composition showed that the lineage of " Medusavirus " branches off basal from the root of this phylum (including " Mollivirus " and " Pandoravirus "). Therefore it was suggested to group “ Medusavirus ” in a separate family “ Medusaviridae ”.

" Medusavirus " shows the traces of evolutionary interactions between virus and eukaryotic host, which were caused by the shared use of the DNA replication compartment and an evolutionarily long-lasting relationship between virus and host. In April 2019, biologists reported that the very large " Medusavirus " or a relative may have been at least partially responsible for the evolutionary emergence of complex eukaryotic cells from simpler prokaryotic precursors.

Individual evidence

  1. ↑ a b c ICTV: ICTV Master Species List 2019.v1 , New MSL including all taxa updates since the 2018b release, March 2020 (MSL # 35)
  2. ↑ a b c d e f g h i j Genki Yoshikawa, Romain Blanc-Mathieu, Chihong Song, Yoko Kayama, Tomohiro Mochizuki, Kazuyoshi Murata, Hiroyuki Ogata, Masaharu Takemura: Medusavirus, a novel large DNA virus discovered from hot spring water . In: Journal of Virology, ISSN 0022-538X . 93, No. 8, 2019. doi : 10.1128 / JVI.02130-18 . PMID 30728258 . Retrieved July 2, 2019. PDF
  3. ↑ a b c The giant Medusavirus turns defenceless cells to 'stone' . In: Nature . 566, No. 7745, 2019, p. 429. bibcode : 2019Natur.566R.429. . doi : 10.1038 / d41586-019-00591-2 . Retrieved July 2, 2019.
  4. ↑ Center national de la recherche scientifique: List of the main “giant” viruses known as of today , Université Aix Marseille, April 18, 2018. According to Masaharu Takemura et al. : Giant Viruses Isolated from Japanese Aquatic Environments, Tokyo University of Science, 3rd Ringberg Symposium on Giant Virus Biology , 19. – 22. November 2017 (unpublished)
  5. ↑ a b Tokyo University of Science : New giant virus may help scientists better understand the emergence of complex life - Large DNA virus that helps scientists understand the origins of DNA replication and the evolution of complex life . In: EurekAlert! , April 30, 2019. Retrieved July 2, 2019. 

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