A new type of virus from cultured Drosophila cells: characterization and use in studies of the heat-shock response.

An infectious virus isolated from the cultured Drosophila melanogster cell line, Schneider 2-L, appears to be a member of a new group of animal viruses. The virus, HPS-1 has a genome composed of a single segment of double-stranded RNA approximately 6 kb in length. Virions are particles approximately 36 nm in diameter. They contain only two proteins and no lipid coat. The major protein, presumably the viral coat protein, is 120,000 daltons. A 200,000 dalton protein is present in much lower quantities. Two other proteins, synthesized in virus-producing cells, are encoded in the viral genome but not included in the mature virion. Synthesis of viral proteins is not affected by the heat-shock-induced translational control that inhibits translation of most normal mRNAs but allows protein synthesis on heat-shock mRNAs. The viral mRNAs thus appear to share the structural features of heat-shock mRNAs which permit heir translation in heat-shocked cells. Viral RNA serves as a probe to study heat-shock translational control. Cells heat-shocked in the presence of actinomycin D cannot transcribe heat-shock mRNA and therefore cannot make heat-shock proteins. Although these cells are making neither heat-shock mRNA nor heat-shock protein, the translational control appears fully induced. The normal cell proteins are not made in these cells but viral proteins are synthesized. These results indicate that the heat shock-induced proteins are not components of the translational control mechanism.