A team of researchers at the University of British Columbia in Canada has isolated and characterized the Bodo saltans virus, a giant virus that infects an ecologically important microbe commonly found in aquatic environments.
Mature BsV virion: DNA containing core is surrounded by two membranous layers; the capsid consists of at least two proteinaceous layers; the bright halo hints to the presence of short fibers; the top vertex of the virion contains a possible stargate structure. Scale bar – 100 nm. Image credit: Deeg et al, doi: 10.7554/eLife.33014.
In oceans, rivers and lakes, there are about a million viruses in every milliliter of water. Most of them are tiny, often 10 or 100 times smaller than bacteria. However, a few reach a similar size and complexity to bacteria, and so stand out as relative giants.
Relative to other viruses, giant viruses have much more DNA in their genome, which in turn provides the genetic template to produce the proteins that allow viruses to reproduce largely independently of its host.
Typically, more than half of the genes encoded by giant viruses have no evident similarity to genes from other viruses or cellular life.
Sequencing DNA from ocean water suggests that these viruses are abundant and ecologically important; yet, few have been isolated from the microbes that they infect.
Without being able to study giant viruses in the laboratory, little can be known about their biology, the way they infect their hosts, and their broader influence on aquatic life.
In an effort to isolate giant viruses that infect ecologically relevant organisms, University of British Columbia researcher Curtis Suttle and colleagues isolated microzooplankton from a variety of habitats and screened them against their associated virus assemblages.
One such screen using water collected from a temperate eutrophic pond in southern British Columbia yielded the Bodo saltans virus (BsV), a giant virus that infects the ecologically important microzooplankton called Bodo saltans, a member of the phylum Euglenazoa.
BsV is the first isolate of a klosneuvirus, a subgroup of the family Mimiviridae proposed from metagenomic data.
“BsV is one of the few giants we’ve characterized that infects a common and ecologically important host,” Dr. Suttle said.
“It’s representative of the most abundant giant viruses in the sea, yet until BsV was discovered, there was no way to investigate these viruses in the lab.”
BsV is one of the largest giant viruses ever isolated and the largest known to infect zooplankton.
Transmission electron microscopy revealed that BsV is an icosahedral particle approximately 300 nm in diameter. The particle consists of at least six layers akin to observations of Acanthamoeba polyphaga mimivirus.
The BsV genome is made up of 1.39 million bases of DNA, according to the team.
Because BsV has to compete with a multitude of other viruses to infect its plentiful host it comes armed to the hilt.
It possesses an arsenal of toxins and DNA cutting enzymes, which likely interfere with other viruses trying to replicate inside the host.
During infection, BsV maneuvers towards the rear of the host cell and releases its viral genome. It appears that as BsV evolved it stole genetic machinery from the host to help in the infection process.
BsV doesn’t carry transfer RNA (tRNA), part of the replication machinery all other giant viruses carry. It does, however, carry tRNA repair genes, making it likely that the virus uses the host’s own tRNA during infection. Again, these genes appear to have been coopted by the virus directly from the host.
More than 10% of BsV’s genome encodes the same group of proteins that are likely involved in combating the host’s antiviral system.
“These genes are actively being duplicated in an accordion-like mechanism in the periphery of the viral genome,” said Christoph Deeg, a graduate student at the University of British Columbia.
“This suggests that the virus is engaged in an evolutionary arms race with its host, and could offer on explanation of how the genomes of giant viruses could reach their impressive complexity.”
Giant viruses have much more DNA than typical viruses, enabling them to produce a multitude of proteins that allows them to replicate largely independently of their host.
Although researchers knew BsV-like viruses existed based on sequenced environmental DNA, only now has a representative virus been captured and studied in the lab.
The researchers described the Bodo saltans virus and its genome in a paper published in the journal eLife.
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Christoph M. Deeg et al. 2018. The kinetoplastid-infecting Bodo saltans virus (BsV), a window into the most abundant giant viruses in the sea. eLife 7: e33014; doi: 10.7554/eLife.33014
