A few giant viruses appear to generate their own energy, which viruses aren鈥檛 supposed to be able to do. The finding will fuel an already fierce debate about whether giant viruses really are viruses, and if they are alive or not.
鈥淚t is really incredible to have energy in a virus,鈥� says Bernard La Scola at Aix-Marseille University in France. Why any virus needs to produce its own energy remains a聽mystery, he says.
Up until 2003, all known viruses consisted of nothing more than RNA or DNA wrapped in a protein coat or membrane. These have no working machinery inside them and are reliant on cells they infect to copy themselves. Under many definitions of life, they aren鈥檛 alive.
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But in 2003, La Scola reported the discovery of the first giant virus, called mimivirus. Since then, hundreds more giant viruses have been discovered and the division between viruses and living cells has become blurred.
Some giant viruses are bigger than some bacterial cells, and have large genomes with lots of genes. They have some machinery to copy DNA into RNA on their own, which is unusual for viruses. They can get attacked by smaller viruses and have a kind of immune system. 鈥淭wenty years after the discovery of the mimivirus, all the definitions of a virus are no longer true,鈥� says La Scola.
He and his colleagues have now found that some giant viruses called pandoraviruses generate a membrane potential 鈥� an electrical gradient 鈥� across their outer membrane. It takes energy to generate a membrane potential, and since these are present in isolated viruses as well as in those inside cells, that energy must come from the virus itself, says La Scola.
Why they have membrane potentials is still unclear. In most cells, these drive the production of a molecule called ATP, but the viruses don鈥檛 make ATP.
The researchers also found that a virus called Pandoravirus massiliensis has many genes that code for enzymes resembling those needed to generate energy. They confirmed that at least one of these enzymes has this function by transferring it to a bacterium.
鈥淭his suggests an active energy metabolism in viral particles, similar to that of cells,鈥� says Gustavo Caetano-Anoll茅s at the聽University of Illinois at Urbana-Champaign.
But David Wessner at Davidson College in North Carolina isn鈥檛 convinced. The team looked only聽at viruses that had just been released from cells, he says, and not all had membrane potentials.
La Scola thinks the discovery adds to the evidence that giant viruses should be regarded as a group separate from both normal viruses and from prokaryotes 鈥� organisms with simple cells.
Even if they generate energy, they are still viruses, says Grieg Steward at the University of Hawai鈥檌 at Manoa. 鈥淧andoraviruses are viruses because they replicate by an assembly process inside of聽a聽host cell,鈥� he says.
La Scola previously saw viruses as living inside cells. But if a pandoravirus makes energy outside cells, it聽is聽even 鈥渕ore living鈥�, he says. 鈥淪o yes, it is alive, I think.鈥�
Frank Aylward at Virginia Tech recently reported finding genes involved in energy production in giant virus genomes, which may be used to manipulate a host鈥檚 metabolism. Giant viruses are turning out to be widespread, which suggests they have a huge impact on the planet.
鈥淚t does not matter whether they are alive or not, they are out there and doing all these important things,鈥� says Aylward.
Reference: bioRxiv, DOI:
