Giardiavirus is a small, nonenveloped virus comprising a monopartite double-stranded RNA genome, a major protein of 100 kDa, and a less abundant polypeptide of 190 kDa. It can be Isolated from the culture supernatant of Giardia lwnblia, a parasitic flagellate in human and other mammals, and effclently infects other virus-free G. lamblia. A single-stranded copy of the viral RNA can be electroporated into uninfected G. amblia cells to complete the viral replication cyde. Giardiavirus genomic cDNA of 6100 nt was constructed and its sequence revealed the presence of two large open reading frames that are separated by a -1 frameshift and share an overlap of 220 nt. The 3' open reading frame contains all consensus RNA-dependent RNA polymerase sequence motifs. A heptamer-pseudoknot structure similar to those found at ribosomal slippage sites in retroviruses and yeast killer virus was identified within this overlap. Immunostudies using antisera against synthesized peptides from four regions in the two open reading frames indicated that the 100-and 190-kDa viral proteins share a common domain in the amino-te l region.But the 190-kDa protein makes a -1 switch ofits reading frame beyond the presumed sippage heptamer and is therefore a -1

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