Medusavirus: Newly-Discovered Giant Virus Turns Its Hosts into ‘Stone’

A team of researchers has isolated a new giant virus from hot spring water in Japan. Named medusavirus, the virus infects a species of amoeba called Acanthamoeba castellanii and can turn its host into a stone-like cyst.

Cryo-EM image of a DNA-filled medusavirus particle viewed from a 3-fold axis; spike, capsid, and membrane are labeled. Scale bar - 100 nm. Image credit: Yoshikawa et al, doi: 10.1128/JVI.02130-18.

Cryo-EM image of a DNA-filled medusavirus particle viewed from a 3-fold axis; spike, capsid, and membrane are labeled. Scale bar – 100 nm. Image credit: Yoshikawa et al, doi: 10.1128/JVI.02130-18.

“Viruses are classified based on their genetic characteristics, that is, by how they generate mRNA to produce proteins and genetic material,” said Professor Masaharu Takemura, a virologist at the Tokyo University of Science, and colleagues.

“Medusavirus is a nucleocytoplasmic large DNA virus that belongs to a group of recently discovered eukaryotic viruses with large and complex double-stranded DNA genomes.”

Medusavirus, with a diameter of 260 nm, has a icosahedral capsid with unique spherical-headed spikes on its surface.

It is the first giant virus isolated from a heated environment (110 degrees Fahrenheit, or 43.4 degrees Celsius), and it shows several unique features in its replication cycle and particle morphology.

Based on the dissimilarities with other known giant viruses, Professor Takemura and co-authors proposed that medusavirus represents a new viral family, Medusaviridae.

Medusavirus may help scientists better understand the origins of DNA replication and the evolution of complex life. Image credit: Tokyo University of Science.

Medusavirus may help scientists better understand the origins of DNA replication and the evolution of complex life. Image credit: Tokyo University of Science.

“Unlike most viruses, medusavirus contains genes that encode for proteins involved in DNA packaging,” the scientists said.

“The virus has a full set of histones, which are proteins that have evolved to keep the DNA folded inside the nucleus and regulate gene expression.”

“This is particularly strange, as we consider that viruses have no nucleus. This could mean that during the co-evolution, the virus might have acquired the genes that encode these histones.”

 

When medusavirus petrifies the Acanthamoeba castellanii amoeba, it does so by hijacking the cell directly from its nucleus.

The virus transfers its DNA to initiate replication and uses its own DNA polymerase (enzyme that synthesizes DNA) and histones, but overall, it relies on the host to complete the process.

The results of an evolutionary analysis done by the researchers suggest that in the evolution tree, the medusavirus DNA polymerase lies at the origin of the DNA polymerase found in eukaryotes.

“This could mean that our DNA polymerase probably originated from medusavirus or one of its relatives,” said Dr. Genki Yoshikwa, a scientist at Kyoto University.

The discovery is reported in a paper in the Journal of Virology.

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G. Yoshikawa et al. 2019. Medusavirus, a novel large DNA virus discovered from hot spring water. J Virol 93: e02130-18; doi: 10.1128/JVI.02130-18

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