Two newly discovered species of giant virus both have the largest assembly of protein-coding genes ever found in the virus realm.
The two new species, note a team of scientists led by Bernard La Scola from Aix-Marseille University, have a protein shell, known as a capsid, around 450 nanometres long and a cylindrical tail roughly the same length. The tail is the largest ever discovered.
By comparison, a polio virus is typically just 70 nanometres long. A smallpox virus tops out at about 300 nanometres.
The two closely related species were discovered in, or near, Brazil, one in a soda lake, and the other in sediments at the bottom of the deep ocean – both extreme environments known to support ancient lifeforms.
La Scola and colleagues assign them to a new genus, known as Tupanvirus. The name was derived from a mythical figure called Tupan and Tupa – a god of thunder sacred to the South American Guarani people.
The Tupanviruses are related to another giant virus, Acanthamoeba polyphaga mimivirus (APMV), first discovered in 1992. All three species infect amoebas.
Apart from their size, the Tupanviruses are remarkable because they possess a quality absent in all other viruses except their APMV cousin. One of the classically defining features of viruses is their lack of translational abilities. That is, they are unable to produce their own peptides from the RNA instructions they carry and need, thus, to co-opt the protein-synthesising equipment of their hosts.
Tupanviruses, it seems, have no such shortcomings. La Scola’s team found that they contained the largest set of protein-coding genes ever found in a virus, with the grunt to produce up to 1425 proteins.
Looking at the genomes, the scientists found genes similar to those found in other types of virus, and in bacteria and archaea. However, about 30% of the genes identified do not have equivalents in other forms of life.
The discovery of the giant viruses seems set to ignite debate on their possible evolutionary emergence, as the authors acknowledge in the paper.
It is possible, they suggest, that they are descended from other, even larger viruses and have lost a few genes along the way. The other possibility is that they began with “a simpler ancestor” that acquired extra genes over multiple generations, benefitting from becoming “more resourceful”.