The SARS virus may be an amalgam of two different viruses, according to a new analysis of its genome. The virus appears to be half mammalian, half avian.
Where SARS came from, or why it emerged all of a sudden in November 2002, is still unclear. Researchers have found a virus very similar to the one isolated from humans in civet cats and two other animals sold at a market in southern China (ScienceNOW, 23 May). But it's unknown which, if any, of these species is the natural reservoir.
To get a better handle on the virus's past, John Stavrinides and David Guttman of the University of Toronto in Canada compared the genes encoding four SARS proteins to the same genes in all other known coronaviruses--which includes two human pathogens as well as viruses of eight farm and lab animals. Two SARS proteins, called M and N, were most closely related to the avian coronaviruses, they concluded, while another one, called PP1ab, resembles mammalian viruses. A fourth protein, called Spike, or S, appears to be half-mammalian, half-avian, the researchers say. In the SARS genome, the S gene is sandwiched between PP1ab on one side and M and N on the other, and determines the virus's host preference. That suggests the two viruses came together at a spot in the S gene, the authors say.
The results partially agree with another study, published a few months ago by Joshua Rest and David Mindell of the University of Michigan, Ann Arbor. That paper also suggested that SARS was a combination of viruses from various hosts, but suggested the joining had occurred in another protein, called RDRP.
Neither analysis convinces Alexander Gorbalenya, a coronavirus researcher at Leiden University Medical Center in the Netherlands. Even if the SARS protein is a mosaic of two viruses, Gorbalenya says, the recombination may have occurred a long time ago and may not have anything to do with its recent emergence as a human disease.
Guttman, Rest, and Gorbalenya all agree that the key to understanding where SARS came from is to find more coronaviruses and look at their genomes. That's easier said than done, however. Beyond the virus isolated from a civet cat, no new viruses have been isolated since SARS broke out, and in China, which may hold crucial clues to SARS's origins, the search has been slow to take off (Science, 31 October, p. 766).
WHO's SARS page