GIARDIAVIRUS DOUBLE-STRANDED-RNA GENOME ENCODES A CAPSID POLYPEPTIDE AND A GAG POL-LIKE FUSION PROTEIN BY A TRANSLATION FRAMESHIFT

Giardiavirus is a small, nonenveloped virus comprising a monopartite d ouble-stranded RNA genome, a major protein of 100 kDa, and a less abun dant polypeptide of 190 kDa. It can be isolated from the culture super natant of Giardia lamblia, a parasitic flagellate in human and other m ammals, and efficiently infects other virus-free G. lamblia. A single- stranded copy of the viral RNA can be electroporated into uninfected G . lamblia cells to complete the viral replication cycle. 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 f rameshift and share an overlap of 220 nt. The 3' open reading frame co ntains all consensus RNA-dependent RNA polymerase sequence motifs. A h eptamer-pseudoknot structure similar to those found at ribosomal slipp age sites in retroviruses and yeast killer virus was identified within this overlap. Immunostudies using antisera against synthesized peptid es 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 rminal region. But the 190-kDa protein makes a -1 switch of its readin g frame beyond the presumed slippage heptamer and is therefore a -1 fr ameshift fusion protein similar to the gag-pol fusion protein found in retroviruses.