A team of researchers from Purdue University, the University of California at Irvine and the University of the Mediterranean in Marseilles, France, have uncovered new details about the largest known virus, Acanthamoeba polyphaga mimivirus (APMV) - or more commonly the mimivirus.

The mimivirus, which has the largest capsid diameter of all known viruses (400 nm), was accidentally discovered in 1992 by French scientists investigating the infectious disease Legionellosis - and was first classified as a gram-positive bacterium. In 2003, researchers at the Université de la Méditerranée in Marseille, France published a paper in Science identifying the micro-organism as a virus.

The mimivirus genome is a linear, double-stranded molecule of DNA roughly 1.2 million base pairs in length - making it the largest viral genome in scientific knowledge, eclipsing the next-largest virus genome of the myovirus Bacillus phage G by a little more than double.

The virus infects amoebas, but it is thought to possibly be a human pathogen because antibodies to the virus have been discovered in pneumonia patients. However, many details about the virus remain unknown, said Michael Rossmann, Purdue University's Hanley Distinguished Professor of Biological Sciences.

The team of scientists have determined the basic design of the virus' outer shell, or capsid, and also of the hundreds of smaller units called capsomeres making up this outer shell. Findings also confirmed the existence of a starfish-shaped structure that covers a "special vertex," an opening in the capsid where genetic material leaves the virus to infect its host, and an indentation in the virus's genetic material itself is positioned opposite this opening, Rossmann said.

"The findings are important in terms of studying the evolution of cells, bacteria and viruses," said Siyang Sun, a postdoctoral research associate working in Rossmann's lab. "The mimivirus is like an intermediate between a cell and a virus. We usually think of cells as being alive and a virus is thought of as being dead because it needs a host cell to complete its life cycle. The mimivirus straddles a middle ground between viruses and living cells, perhaps redefining what a virus is.”

The findings are detailed in a research paper that will appear online April 28 in the journal PLoS Biology:

http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.13...

For additional details see the official release: http://news.uns.purdue.edu/x/2009a/090428RossmannMimivirus.html