'Fourth Domain Of Life' Discovered?

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Bizarre New Organism May

Be 'Fourth Domain Of Life'

By Roger Highfield

Science Editor

The Telegraph - UK

10-15-4

 

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 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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