Editorial

Since the 1960s, scientists have argued About whether viruses are living organisms or just a bundle of very large molecules. Viruses are usually much smaller and simpler than bacteria, consisting simply of genetic material surrounded by a protein coat. A virus has to hijack another organism's biological machinery to replicate, which it does by inserting its DNA into a host. Bacteria, on the other hand, carry ail that they need to reproduce independently, and thus qualify as alive. Mimivirus, the biggest virus in the world has its genome sequenced, and scientists say that, unlike its fellow viruses, it may truly be called 'alive'. Officially, the virus got its name because it mimics bacteria.

The virus's genetic sequence also holds clues that may explain the evolution of the very first cells possessing a nucleus of DNA. Although it shows all the trademark features of a virus, the mimivirus is much more complex, says Jean-Michel Claverie, a biologist from the Institute of Structural Biology and Microbiology in Marseilles, France, who worked on the sequencing effort.

Mimi carries about 50 genes that do things never seen before in a virus. It can make about 150 of its own proteins, along with chemical chaperones to help the proteins to fold in the right way. It can even repair its own DNA if it gets damaged, unlike normal viruses. And although viruses can use either DNA or RNA to carry their genetic information, Mimi has both. Mimi was discovered in 1992, nestling inside an amoeba found inside a cooling tower in Bradford, UK, that was being investigated as the source of an influenza outbreak. Later research showed that it was a real monster, measuring about 800'nanometres across, more than four times as big as a smallpox virus. The new study shows that its genome contains 1.2 million bases, which is more than many bacteria contain and makes it several times bigger than the largest DNA viruses. The bases make up 1,260 genes, which makes it as complex as some bacteria, the scientists say. What's more, viral DNA often contains lots of 'junk' sequences, genetic material that does not seem to serve any useful function. Mimi, on the other hand, is lean and mean: more than 90% of its DNA does something specific. As Mimi carries some genes involved with replication, this could have helped it to spread faster than other viruses.

Although biologists sometimes divide life into three categories, the scientists say that Mimi is sufficiently different that it deserves a fourth branch of life all to itself. Bacteria are the simplest branch, because they lack a nucleus to gather their genetic material together. Archaea are very similar, but are thought to have evolved separately because of their unusual cell membranes. Every other living thing is a eukaryote, that is, an organism that groups its genetic material into a nucleus inside its cells. But Mimi carries seven genes that are common to all cellular life, putting it on a par with the other life-forms.