The classic caliciviruses and the NLVs are small (27-40 nm diameter), nonenveloped particles which are readily distinguishable from each other by their differing morphologies when examined by electron microscopy. They have a buoyant density of 1.33-1.41 g/cm3 (7,11). Classic caliciviruses or Sapporo-like viruses were originally named for the 32 calyces or cup-shaped depressions on the virion surface giving a Star of David-like appearance (12). Norwalk-like viruses have a less clearly defined or more ragged morphology. Although morphological differences are readily apparent, classic calicivirus and NLVs apparently have some common antigenic epitopes as well as similar geno-mic structures (1,13,14). The virion of the NLV is composed of a highly variable outer coat (capsid), a small basic protein, and a 7.7-kb positive-polarity, single-stranded RNA genome (13). X-ray crystallography performed on recombinant capsid-like particles demonstrated that the capsid has a t = 3 symmetry and is assembled from 180 monomeric units of a single 56.6 kDa protein (15-17). Several NLV genomes have been cloned and sequenced. All genomes encode three open reading frames (orf) (18-22). The first orf encodes a large polyprotein, which is proteolytically processed into a number of different proteins including a helicase, a cysteine proteinase, and an RNA-dependent RNA polymerase. The second and third orfs encode the capsid protein and small basic proteins, respectively. The function of the small basic protein has not formally been ascribed; however, its basic amino acid composition coupled with its presence in the virion suggests it may be an RNA-binding protein (3). A long poly A tail is located at the 3' end of the genome.

Human caliciviruses are presently divided into three genogroups based on antigenic and genome sequence differences, as well as slight differences in length and alignment of the open reading frames. Genogroup I includes the prototypical Norwalk virus, as well as Desert Shield, Cruise Ship, Montgomery County, and Southampton viruses. Genogroup II, of which Snow Mountain is the prototype, includes the Gwynedd, Toronto, Hawaii, Lordsdale, Mexico, Camberwell, Melksham, and White River viruses. Collectively genogroups I and II comprise the NLVs. The classic human caliciviruses are classified as genogroup III. These include the prototypical Sapporo virus, as well as the Houston, London, and Parkville virus strains (4,23). Despite genomic similarities, these three genogroups are quite divergent and a considerable degree of genetic variation is readily observed among individual strains within these three genogroups (4,23-29). For example, when 16 orf 1 PCR amplicons from genogroup I viruses were sequenced, the nucleotide identity was as low as 64% (30). The comparison of 20 genotype II strains revealed as little as 61% nucleotide sequence identity in orf 1, while analyses of orfs 2 and 3 for genogroups I and II also demonstrated a high degree of diversity (30). Although genogroup I viruses are common in the United States, genogroup II viruses may predominate as the leading cause of infections within the United States (26,29). Geno-group III caliciviruses cause gastroenteritis, principally in infants, and are less commonly associated with foodborne transmission than viruses in genogroups I and II (12).

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