Researchers studying the evolution of the severe acute respiratory syndrome (SARS) coronavirus claim that it may be an amalgam of two different viruses, one from birds and the other from mammals. They also suggest that the two viruses came together at a spot in a gene that determines the virulence of the virus and host preference.
John Stavrinides and David Guttman from the University of Toronto, Canada, did a detailed phylo-genetic analysis of the genes that make up the SARS coronavirus genome and compared the sequences of the proteins encoded by these genes with the same genes in all other known coronaviruses (avian, feline, canine, porcine, murine, and human). Analysis of two (matrix [M] and nucleocapsid [N]) of the four virus proteins supported a descent from an avian-like coronavirus ancestor. The phylogeny of another protein (PPlab replicative polyprotein) was markedly different from that of the M and N proteins and indicated that it originated from a mammalian-like ancestral virus. A fourth, host-determining, protein called surface-spike glycoprotein (S) was seen to be a mosaic originating from avian and mammal coronaviruses, the authors report in the Journal of Virology (2004; 78: 76–82).
In the SARS genome, the S gene is sandwiched between PP lab on one side and M and N on the other, determining the human-receptor-binding site. “We propose that a recombination event likely occurred within the S gene. [Since] the S protein is responsible for host specificity, this event may have been the critical step in the switch to a human host and the subsequent emergence of this new pathogen”, say Stavrinides and Guttman.
According to David Mindell, University of Michigan in Ann Arbor, USA, this work “provides evidence indicating recombination among different coronavirus lineages”. However, he added that even though recombination “does provide a mechanism for adaptive change among viruses, there is…no evidence linking any particular recombination event to the recent emergence of SARS in humans”. Speaking at a meeting of the Royal Society, London, UK, on January 13, Eddie Holmes (Oxford University, UK) said that “If recombination has occurred, it's so ancient it's irrelevent to SARS emergence”. Both the paper's authors and Mindell agree that an accurate identification of the zoonotic source will require extensive sampling of coronaviruses from a broad range of hosts.