Two strains of the SARS virus sequenced

Two teams of researchers have sequenced the genome of two strains of the virus believed to be responsible for severe acute respiratory syndrome (SARS). Their results confirm the suspicion that it is a previously unknown coronavirus. Knowledge of the genome, which has been made publicly available, should help in designing new diagnostic tests and eventually a vaccine.

Both teams report their findings in the current issue of the journal Science, which has made the articles freely available on the internet.

Because of the urgency of the problem, the work was conducted with a degree of openness and cooperation rarely seen in the genomic sequencing community. Both teams have posted the full sequence on the website of the National Center for Biotechnology Information, part of the US National Library of Medicine.

Two different strains of the virus, provisionally called SARS-CoV, have been sequenced. The Tor2 strain was isolated in Toronto, Canada, the city hit hardest in the West by the epidemic (www.sciencemag.org/cgi/rapidpdf/1085953v1.pdf).

One millionth of a gram of purified viral genetic material was delivered last month to the Genome Sciences Centre of British Columbia's Cancer Agency in Vancouver, which has been working around the clock ever since.

The Urbani strain was sequenced by the US Centers for Communicable Diseases in Atlanta, Georgia (www.sciencemag.org/cgi/rapidpdf/1085952v1.pdf). It is a common strain in Asia, named after Carlo Urbani, who was an infectious disease specialist at the World Health Organization's office in the Vietnamese capital, Hanoi. He helped to identify the disease as a coronavirus but later died from it.

The differences between the two strains turn out to be minor. Both comprise about 30000 nucleotides, making the genome of SARS-CoV the largest of any RNA virus. It is possible but unlikely that the differences are a result of sequencing errors.

As more strains are sequenced, the degree of difference between them will provide vital clues to the rate of mutation. Although all other known coronaviruses have been allocated to one of three serotypes, both teams of microbiologists believe the new virus belongs in a fourth category of its own.

The structural differences from other coronaviruses, and the lack of evidence of recombination, suggest that the SARS virus is not a result of other viruses swapping DNA with a previously benign coronavirus that already lived unnoticed in humans.

Rather, the researchers say, the evidence indicates that SARS is genuinely new in humans and until recently inhabited an unknown animal species, probably in Guangdong province, China.