Giant Viruses with Huge Genomes Found in Brazil Rewrite What We Know about These Bizarre Life Forms

tupanvirus closeup
This giant virus was found in a Brazilian lake. J. Abrahão et al./Nature Communications

Without a host, viruses wouldn't stand a chance. They need other organisms' cells to make the proteins they need to survive. But two new types found in Brazil are pushing the limits of what it means to be a virus. The discovery was reported Tuesday in Nature Communications.

These viruses, called tupanviruses, are likely related to another type of giant viruses known as mimiviruses. But even among giant viruses, these ones are weird. Not only are their genomes very large, but also the viruses themselves are so large they can be seen under a normal light microscope. Unlike previously identified mimiviruses, these new giant viruses have an even more complete set of the machinery that might be required to create proteins.

When those giant viruses were discovered in 2003, they "changed the field of virology," according to one paper. (That paper was written by the people who discovered them, so they may be a bit biased.) But those prior researchers aren't the only ones who think learning more about these viruses could be important.

Proteins are important—basically everything a cell does is based on proteins. These newly-discovered giant viruses, which have a very long tail and look a little fuzzy, could theoretically produce about 1,425 proteins from their genetic code. But viruses can't produce proteins. If they could, they wouldn't be a virus any more.

That's where the puzzle of these viruses lies: why such a big genome?

"The fact that this encodes so much of the machinery is interesting, because it makes you wonder how [those genes] got there," Vincent Racaniello, a virologist at Columbia University, told Newsweek. Racaniello wasn't involved in this research, but has followed the field for more than a decade. Understanding how viruses evolved could teach us more about how they cause disease, he said. "We need to know the driving evolutionary forces that led to the current crop of viruses that are on the planet today."

There are two main theories for how viruses came about. One theory holds that viruses started as a cell and moved away from being cell-like; the other sees them as evolving to become more like cells. Tupanviruses, Racaniello noted, could support either option.

giant viruses
No, this paper is about a different kind of giant, fuzzy virus. The viruses known as Tupanviruses have the most complete set of stuff needed to create proteins of any virus and are so big they can be seen with a regular, light-based microscope. This is a cold virus in the form of a plush toy. Hillary/Flickr

These tupanviruses aren't completely self-sufficient. "A virus requires so much from the cell," Racaniello said. For one thing, they're missing an important part of the protein production process called a ribosome. Without a ribosome, they're still reliant on host cells to produce proteins. And these viruses also don't have the genes required to produce the energy cells run on; they depend on a host for that, too.

"Viruses work because they're highly efficient parasites," Racaniello said. "They have less than what is needed to grow but they're able to get what they need by going into another cell." (These viruses don't infect human cells.)

Tupanvirus genomes are big, but don't worry—ours are bigger. The tupanvirus genome is about 1.5 million base pairs, and those are the biggest genomes of any giant viruses. Humans have about 3 billion.