The first Extreme Viruses were discovered in the early 1980s by Wolfram Zillig of the Max Planck Institute for Biochemistry in Germany.  He isolated a virus that infected Sulfolobus,  an Archaeal microbe found in the acidic, near-boiling hot springs from around the world.  Zillig & Stedman soon learned that everywhere Sulfolobus lived, so did the viruses that infected them.   Some 6 different distinct kinds have been collected.  Some are just 24 nm wide by 2,000 nm in length. To date some 30 viruses of Sulfolobus have been described (Zellig et al., Extremophiles -2:131-140, 1998) 

    All Extreme Viruses are referred to as crenarchaeotal viruses and belong to three or four novel virus families, which were created to account for their unique features. The Fuselloviridae [SSV2] comprise lemon-shaped viruses (Fig 1a) with a core consisting of DNA, about 15kbp in size, and a basic DNA-binding protein wrapped into an envelope containing at least two hydrophobic coat proteins and probably host lipids.  The complete genome sequence of an SSV1 is known.  Two very similar viruses, SIRV1 & SIRV2 have a stiff rod-shapes, and are called the Rudiviruses (Fig. 1e). They contain linear, double-stranded DNA that forms a tubelike double helix with a DNA-binding protein.  A third group, the Lipothrixviridae (Fig. 1c) are normally flexible filaments, ranging in length from 0.4 to more than 2um & in width from 20 to 40 nm.  They contain linear double-stranded DNA. A final group of unique extreme viruses, the SNDV (Fig. 1b), has been found in a novel Sulfolobus isolate from New Zealand. These viri differ in many features from all other Sulfolobus viruses. Stedman & Zillig propose to assign it to a new genus, the Guttavirus.   It has the shape of a droplet with a dense beard of thin filaments on its pointed end (Fig. 1b). Its circular DNA is 20 kbp in size and has unique DNA sequences such that many restriction enzymes can cleave it either partially or not at all.

    One extreme virus, has already been turned into a potential tool for biotechnology. Working with researchers from Denmark (2000), Stedman added the genes for an enzyme from an extremophile microbe called Pyrococcus into an Extreme-Virus . Then he used these modified viruses to infect Sulfolobus. The new genes were integrated into the bacterial chromosome and turned the Sulfolobus into a tiny factory for making this  enzyme, which can break down cellulose, an important step in some food processing and in papermaking. The commercial appeal for food technology is that the enzyme works at very high temperatures, and the preporatory chemistry runs quicker when hot & thus more profitable.  Moreover, Extreme-Viruses have been sources of enzymes, particularly DNA & RNA polymerases, as tools for biotechnology. Finally, they have been used as a basis for the development of multiple cloning and expression vectors,

    Astrobiologist's have suggested that life may have evolved from particles much like Extreme-Viruses (Baruch Blumberg, director of NASA's Astrobiology Institute). Extreme-Viruses, living in Earth's most otherworldly locales, could carry hints about how life on other planets might emerge.

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Electron photomicrographs of a purified preparation of the fusellovirus SSV2 (a) shows "rosettes" containing large SSV2 and small SSVx particles  (arrows); the rudivirus SIRV2 (d); the unassigned viruses TTV4 (c); and SNDV (floating genus Guttavirus; b), all negatively stained. All except b and e, same magnification.

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The rod-shaped Rudivirus (SIRV2) contain linear, double-stranded DNA that forms a tubelike double helix with one DNA-binding protein. One turn of the SIRV2 superhelix measures 4.3nm and comprises 16.5 turns of B DNA. The DNA is thus compressed by a factor of 12.7. The tube is plugged on both ends and each plug carries three tail fibers (Fig.e).

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