The DNA of the world's largest known virus contains a surprising collection of genes previously thought to exist only in bacteria and other living cells. This discovery further blurs the line between complex molecules and living organisms, the researchers report. Published online 14 October in Science, the study may also help to answer questions about how complex organisms first evolved on Earth.
The discovery of the Mimivirus in 2003 by Didier Raoult and Jean-Michel Claverie made headlines because of the virus' remarkable size--comparable to that of many small bacteria. The pair's new analysis of the Mimivirus genome reveals even more surprises. The DNA contains some 1260 genes, 50 of which code for functions never before seen in viruses, including DNA repair and translating mRNA into protein.
Viruses are generally considered nonliving because of their small size, simplicity, and inability to replicate outside of a host cell. They're just small packets of genetic material surrounded by a protein coating. But Claverie, a geneticist at Le Centre National de la Recherche Scientifique (CNRS) Structural and Genomic Information Laboratory in Marseille, France, believes that the Mimivirus's complexity should make researchers rethink their characterization of viruses as nonliving. The Mimivirus may once have replicated on its own but later lost that ability, Claverie says. But the virus still contains much of the machinery for self replication, a fact that Claverie believes makes them a type of living organism.
The Mimivirus genome may also help explain how complex life emerged. After comparing the sequence to those of a variety of other organisms, the researchers concluded that the Mimivirus lineage dates back some 3.3 billion years to the separation of early life into three major divisions: archaea, bacteria, and the more complex eukaryotes. Other scientists, noting the similarities between large DNA viruses and the cell nucleus that houses the genetic material in eukaryotes, have hypothesized that the nucleus may have originated when simple cells merged with viruses. Claverie says the findings fit with the idea that a large virus, perhaps similar to the Mimivirus, may have been the first to jump into a cell: The timing seems to fit, and the similarity between Mimivirus and eukaryotic genes involved in making proteins bolsters the hypothesis, he says.
The study is a "milestone in biology" and a "fabulous discovery," says Elodie Ghedin, a virologist and parasitologist at The Institute for Genomic Research in Rockville, Maryland. "My first thought was that they've found a missing link without us even realizing there was one," she says.
Science article on the discovery of the Mimivirus
Science article on the Mimivirus sequence
Science news article on the evolutionary origins of the nucleus
Primer on Genome Sequencing from the Human Genome Project