Two genera, Coronavirus and Torovirus, contain viruses infecting humans. The genus Coronavirus includes two human serotypes causing respiratory disease, the prototype strains being HCV-229E and HCV-OC43, which arc clearly distiniguishable from one another by neutralization or hemagglutination inhibition. The status of the "human enteric coronaviruses" and human toroviruses has yet to be determined.Viruses that are classified in the Coronaviridae fulfill the following criteria:
• A positive-strand, nonsegmented genome of about 30,000 nucleotides. The genome contains an unusually large RNA polymerase gene (about pro tein gene, and a nucleocapsid protein gene. These genes are arranged on the genome in a specific order (as described above in the 5' ~ 3' direction) but may be interspersed with additional genes encoding further structural or nonstructural proteins.
• A virion envelope bearing pronounced surface projections. These projections are composed of the large surface glycoprotein (Mr = -200 kDa) that characteristically exhibits a coiled-coil structure in the carboxy-terminal, membrane-anchoring half.
• An integral membrane protein (Mr = -25 kDa). This pro tein characteristically has three membrane-spanning regions in the amino-terminal half.
• A 3' coterminal set of four or more intracellular subgenomic mRNAs. Only the open reading frames (ORFs) contained within the 5' unique region of each mRNA (i.e., the region not found in the next smallest mRNA) are expressed as protein.
• An RNA polymerase gene composed of two overlapping ORFs. Expression of the downstream ORF is mediated by (-1) ribosomal frameshifting. The features that distinguish coronaviruses and toroviruses are:
• The size of the nucleocapsid protein (Mr = -60 kDa and -18 kDa, respectively) and the shape of the helical nucleocapsid structure (extended or tubular, respectively).
• The absence of aleader sequence at the 5' end of torovirus mRNAs.
• With some limited exceptions, a lack of sequence similarity in the coronavirus and torovirus gene products. At the present time 13 virus es are classified as species in the Coronaviridae (Cavanagh et a1., 1994). These are listed with their acronyms and natural hosts.
The prototype of the coronavirus genus is avian infectious bronchitis virus.The name "coronavirus" is derived from the solar corona-like (1. corona = crown) appearance of virus particles in negatively stained electron micrographs. The prototype of the torovirus genus is Berne virus (BEV) and the name "torovirus" is derived from the curved tubular (1. torus = lowest convex molding in the base of a column) morphology of the nucleocapsid structure. In addition to the viruses listed in Table I, two viruses, rat coronavirus (RCV) and rabbit coronavirus (RbCV), are considered as tentative species of the coronavirus genus. Three more viruses, feline enteric coronavirus (FECV), sialoacryoadenitis virus (SADV), and porcine respiratory coronavirus (PRCV), are often discussed in the literature as coronavirus species, but the available data suggest that they could be equaHy weH considered as variants of fehne infectious peritonitis virus (FIPV), RCV, and transmissible gastroenteritis virus (TGEV), respectively.
In relation to the toro virus genus, there are reports of an enveloped virus in the stools of patients with gastroenteritis that resembles Breda viruses (Beards etal., 1984). However, this virus is not yet recognized as a member of the genus.
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