The very first giant virus was discovered in a water-cooling tower in 2003. As the name suggests, giant viruses are unusually large and their genomes unusually complex, all of which went against the prevailing idea of viruses as small, simple, and primitive. Then one baffling giant virus became many, as scientists kept discovering different types: in water off the Chilean coast, in Siberian permafrost, in an Austrian sewage plant, and now in mud from a Japanese hot spring.
The newest giant virus is Medusavirus, so named because of the way it infects amoebas, single-celled organisms that commonly live in water. When Masaharu Takemura, a virologist at Tokyo University of Science, first grew microbes from the hot-spring mud in his lab, he noticed that some amoebas would die in the presence of the giant virus. The dead amoeba cells burst open. But others would shrivel and harden, which amoebas sometimes do when guarding against bacteria that also prey on them. (It’s a dangerous life out there for amoebas.)
Takemura told me in an email that he had long been fascinated with the myth of Medusa, who turned men who looked at her to stone. His computer background is Peter Paul Rubens’s famous painting of Medusa. Thus inspired, he named the new giant virus Medusavirus.
Takemura and his colleagues then analyzed the new virus more closely. They put it under an electron microscope and found that it resembles a 20-sided die, covered in 2,660 round-tipped spikes. It’s unclear exactly what the purpose of the spikes are, but they have been found in other giant viruses. The team sequenced the virus’s DNA, which revealed the most interesting information of all.
Medusavirus has a full set of genes for histones, the proteins around which long strands of DNA wind themselves. But viruses aren’t supposed to have histones. “Histones are a way to supercoil the DNA and to organize it,” says Gilbert Greub, a microbiologist at the University of Lausanne. Think of a set of headphone cords, or garden hoses, or long lengths of rope. “If you don’t have the rope arranged, it will be a disaster,” Greub says. Same with DNA. But viruses have too little DNA to need this sort of organization. Even bacteria don’t have histones. Humans and plants, or other complex life forms with long, complicated genomes, have histones.
How could Medusavirus have acquired histones—and not just one, but all five types that are present in complex cells? One theory is the virus acquired the genes from complex cells, such as the amoeba it infects. (Although unicellular, amoebas have complex-enough cells to be categorized as complex life. They have histones.) By comparing the genome of the Medusavirus with the amoeba it infects, the team found 57 examples of gene transfer. “Many people tend to believe viruses steal genes from hosts for their own purposes. This is true,” says Hiroyuki Ogata, a bioinformatician at Kyoto University and another member of the scientific team that discovered Medusavirus. But out of 57 likely gene transfers, only 12 seemed to be cases of the virus taking genes from the amoeba. In 13 cases, the virus seemed to have given the gene to the amoeba. The direction of transfer was unclear in the rest.
What’s more, the Medusavirus had a gene coding for DNA polymerase, an enzyme necessary to synthesize DNA. Its version of DNA polymerase was similar to those in complex life, but it didn’t exactly look like any particular animal or plant’s version—meaning it likely wasn’t stolen directly from a complex cell. Instead, the Medusavirus’s DNA polymerase seemed ancient in origin.
As more and more giant viruses have been discovered, scientists have started to wonder whether some genes in living organisms actually came from ancient viruses. Viruses are not traditionally considered alive, because they lack the cellular machinery to replicate on their own. But scientists have found giant viruses with genes for building proteins, which are necessary for replication. The Lausannevirus that Greub discovered also has genes for histones. Other viruses have yet other genes important to life.
If living organisms got genes from viruses, that would be a radical inversion of previous hypotheses about their origin. Scientists have suggested that viruses might be degenerate versions of living cells that lost most of their cellular machinery, retaining only their protein capsule and genetic material. Or that viruses might be mere fragments of cells that broke off and are unable to replicate on their own.
But if histones and enzymes for synthesizing DNA originated in viruses, then they might have been present when life first began in primordial soup. They might be one reason life on this planet exists at all.
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