'Giant Virus' may be new lifeform

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The Bradford bug that may be a new life form
Reports by Roger Highfield, Science Editor
(Filed: 15/10/2004)

A strange life form has been identified in Bradford.

Genetic analysis reveals that the organism is so bizarre and unlike anything else seen by scientists that perhaps it should be placed in its own category of living things.


The mimivirus at 200,000 times' magnification
The creature, first discovered in a small industrial cooling tower on the outskirts of the city, could qualify for a new "domain" in the tree of life - where a domain is a bigger category than a kingdom or a phylum.

The "giant virus", dubbed the Mimivirus, or "mimicking microbe", because it was first mistaken for a bacterium, inhabits amoebae and is more than twice as big as any other virus so far found. At about half a millionth of a metre across - around the size of a small bacterium - it is one of the few that can be seen under a light microscope.

Two research teams in the Marseille School of Medicine, led by Prof Didier Raoult and Prof Jean-Michel Claverie, have "read" the genetic code of the organism and found a number of genes previously thought to belong only to more complex life forms.

The size and complexity of the Mimivirus genetic code - which is 1.2 million "letters" long, at least 10 times larger than the code of a typical virus - "challenges the established frontier between viruses and parasitic cellular organisms", they report today in the journal Science.

One of the defining characteristics of a virus is that it is unable to make proteins independently, instead relying on the cells it infects to manufacture its proteins and thus reproduce. But the Mimivirus contains a number of genes for protein translation.

It also contains genes for DNA repair enzymes and other proteins, all typically thought to be trademarks of cellular organisms.

The Mimivirus - which so far has only been found in Bradford - appears to represent a new family of "nucleocytoplasmic" large DNA viruses that emerged with the first life on Earth some four billion years ago, said Prof Raoult. After much debate among his team, "for the first time we have enough genetic information to conclude that there is a fourth domain of life", he said. "If this is true, this is revolutionary."

The other three domains of life are the eukaryotes, which have cells that contain a nucleus, and the prokaryotes, unicellular organisms that are divided into the bacteria and archaea.

The family tree drawn up by Prof Claverie shows that the Mimivirus is no more related to the eukaryotes as it is to bacteria. 'This organism is as old as all the rest of living organisms," he said.

However, Dr Dave Roberts, head of microbiology at the Natural History Museum, London, was "deeply sceptical" that the Mimivirus deserves to be placed in its own domain, though he agreed that it did mark a new family.

"There are a lots of odd things turning up in the microbial world all the time," he said.

"It is a fascinating paper and very exciting. The virus seems to link to a group prior to the appearance of the three domains we currently recognise. But we are not convinced that the tree of life is still a branching structure when you get that deep."

The giant virus has not so far been linked with disease.

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Different article, but same topic, in SLAM.