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Figure 2 Genome organization of human caliciviruses. (A) Genome organization of Norwalk virus. The first ORF encodes the nonstructural proteins; ORF 2 encodes the capsid protein; and ORF 3 encodes a small basic protein. (B) Genome organization of Manchester virus (Liu et al, 1997). ORF 1 encodes the nonstructural proteins and the capsid protein, followed by ORF 2 (the homologue of the Norwalk virus ORF 3) encoding a small basic protein. ORF 3 is encoded by an out-of-frame sequence within ORF 1.

Figure 2 Genome organization of human caliciviruses. (A) Genome organization of Norwalk virus. The first ORF encodes the nonstructural proteins; ORF 2 encodes the capsid protein; and ORF 3 encodes a small basic protein. (B) Genome organization of Manchester virus (Liu et al, 1997). ORF 1 encodes the nonstructural proteins and the capsid protein, followed by ORF 2 (the homologue of the Norwalk virus ORF 3) encoding a small basic protein. ORF 3 is encoded by an out-of-frame sequence within ORF 1.

one of the two human calicivirus genera, either in the 'Norwalk-like viruses' (Norwalk virus, Lordsdale virus and Southampton virus) or in the 'Sapporo-like viruses' (Manchester virus). These viruses contain a positive-sense polyadenylated single-stranded RNA of approximately 7.6 kb (Fig. 2).

The genome of 'Norwalk-like viruses' is organized in three major open reading frames (ORFs). For

Norwalk virus, the first ORF at the 5' end encodes a large polyprotein of 1738 amino acids (aa) with a predicted molecular weight of 193.5 (193.5 K). This polyprotein contains short motifs of similarity with the 2C (helicase), 3C (cysteine protease), and 3D (RNA-dependent RNA polymerase) proteins of pi-cornaviruses (Fig. 3). Thus, the 5' end of the genome of the 'Norwalk-like viruses' codes for a precursor of

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