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Multiple Discoveries of Giant Viruses in Japan Advance Understanding of Viral Evolution and Replication

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Giant Virus Discoveries Shed Light on Evolution and Host Hijacking

Researchers in Japan have announced the discovery of two new giant viruses, each with unique biological characteristics, while separate studies have detailed how a known giant virus hijacks host cell machinery. These findings contribute to the ongoing study of giant viruses, their replication strategies, and their potential role in the evolution of eukaryotic cells.

Discovery of Furtivovirus

Scientists from the Tokyo University of Science identified a new giant virus in the Inasegawa River in Kamakura City, Japan. The virus has been named furtivovirus, derived from the Latin word "furtivus" meaning "hidden" or "stealthy," reflecting difficulties encountered during its isolation.

Key Characteristics:

  • The virus is reported to bridge two related groups of giant viruses that possess significantly different genome sizes.
  • Researchers have proposed a new viral family, Manesviridae, to classify furtivovirus and similar viruses.
  • Furtivovirus exhibits a replication strategy not previously observed: it first breaks down the host cell's nucleus and then replicates within the remaining nucleoplasm. This contrasts with other giant viruses that either replicate inside an intact host nucleus or destroy the nuclear membrane and replicate outside.

According to virologist Masaharu Takemura, the finding may provide insights into the evolutionary relationship between giant viruses and host cells. The study supports hypotheses that giant viruses may have played a role in the evolution of the cell nucleus, a defining feature of eukaryotic cells.

The findings were published in the Journal of Virology.

Discovery of Ushikuvirus

A separate team of scientists in Japan identified a previously unknown giant virus, named "ushikuvirus," after its discovery infecting an amoeba in the Ushiku-numa pond near Tokyo.

Key Characteristics:

  • Ushikuvirus infects vermamoeba, the same host as clandestinovirus, and shares structural resemblances with medusaviruses.
  • It induces host cells to grow abnormally large.
  • Its capsid spikes have unique caps and fibrous structures.
  • Unlike clandestinovirus and medusaviruses, ushikuvirus forms a viral factory and destroys the host's nuclear membrane rather than preserving it.

Researchers, including Masaharu Takemura, stated that these similarities and differences are important for understanding the evolutionary history and diversification of giant viruses.

The discovery is expected to enhance knowledge regarding the Mamonoviridae family's evolution and phylogeny. The study was published in the Journal of Virology.

Hijacking of Host Protein Production by Mimivirus

In a separate study not directly related to the new discoveries, scientists reported experimental evidence that Acanthamoeba polyphaga mimivirus actively hijacks its host's protein-making machinery. The process involves the virus producing a complex of three proteins that directs the host's protein-production system to generate viral proteins.

This discovery confirms a long-standing hypothesis among virologists that viruses could co-opt such a system, which is typically associated with cellular life. The findings were published in Cell on February 17.

Acanthamoeba polyphaga mimivirus possesses a genome approximately five times larger than that of poxviruses, which have the largest genomes among human-infecting viruses. Its physical size allows it to be observed under a light microscope.

How the Hijacking Works

To investigate the virus's effect on host protein assembly, researchers isolated viral proteins that interact with ribosomes, the cellular structures responsible for translating RNA into proteins. They identified three viral proteins believed to be involved in this hijacking. When genetically engineered viruses lacked any one of these three proteins, their multiplication rate was observed to be 1,000 to 100,000 times slower than that of viruses possessing these proteins.

Broader Scientific Context

Giant viruses, once mistaken for bacteria due to their size, have gained recognition in recent decades as abundant biological entities. They are reported to play a role in the evolution of life, including influencing host cell evolution and facilitating horizontal gene transfer.

A theory known as viral eukaryogenesis proposes that viruses may have contributed to the evolutionary leap from single-celled prokaryotes to multicellular eukaryotes, which are characterized by a membrane-bound nucleus.

Molecular biologist Masaharu Takemura first proposed this idea in 2001, suggesting that a large DNA virus might have infected a prehistoric prokaryote and gradually evolved into the cell's nucleus. The discovery of DNA-containing giant viruses in 2003, which form membrane-enclosed "virus factories" within host cells that resemble eukaryotic nuclei, has been cited as evidence supporting this theory.