A virus so large and strange that it’s redefined the very concept of a virus has been photographed for the first time. It’s even weirder than expected.

Known as Acanthamoeba polyphaga, the virus was originally discovered infecting amoebas inside a Parisian water tower in 1992. It was orders of magnitude bigger than any other virus — so large, in fact, that researchers figured it was a microbe.

It took 11 years for A. polyphaga to be officially defined as a virus, though the definition didn’t quite fit. In addition to its enormity, many of A. polyphaga’s genes came from bacteria. It was finally deemed a member of a brand-new genus called mimivirus, of which it was the sole and charter member, but not all researchers were happy. Some researchers called it a “missing link” that blurred the boundaries between viruses and living cells, between living and dead.

Despite all this attention the mimivirus’ physical structure remained a blur. Like other viruses, it was made from DNA surrounded by a protein coat called a capsid, but long fibers on the capsid’s surface made it difficult to see the mimivirus’ underlying structure.

To get a clearer picture, French and American biochemists dissolved mimivirus fibers with enzymes, then used an electron microscope to take thousands of pictures that were eventually be combined into a three-dimensional structure. The results, published recently in Public Library of Science Biology, provide further evidence that mimiviruses straddle the boundary between virus and bacteria.

Whereas the DNA of other viruses are tightly wrapped, there’s a large gap between the mimivirus genome and its capsid. In some ways, this less resembles the structure of a virus than of a living cell, in which DNA is contained in a nucleus, which in turn floats in cell wall-enclosed cytoplasm.

“The new structural finds, along with previous genetic and morphological work, confirm that mimivirus is an odd mix of genes and parts found in viruses, bacteria and even eukaryotes, the organisms that sequester their DNA in a nucleus,” write the researchers.

What does this mean, then, for the mimivirus’ official status, which has caused some researchers to call for a redefinition of virus?

Perhaps that’s the wrong question to ask. Eugene Koonin, a National Center for Biotechnology Information researcher who reported last year that, in another viral first, mimiviruses can actually become infected by other viruses, was nonplused by arguments over mimivirus classification.

“They’re part of the biosphere, and that’s more than enough for me,” he said at the time.

See Also:

• Even Viruses Get the Blues

Citation: “Structural Studies of the Giant Mimivirus.” By Chuan Xiao, Yurii G. Kuznetsov, Siyang Sun, Susan L. Hafenstein, Victor A. Kostyuchenko, Paul R. Chipman, Marie Suzan-Monti, Didier Raoult, Alexander McPherson, Michael G. Rossmann. PLoS Biology, April 28, 2009.

Image: PLoS Biology

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