Introduction

Taxonomic Structure of the Family

Virion Properties

Morphology

Virions exhibit isometric symmetry, and are 30-40  nm in diameter, with no envelope or surface projections.

Physicochemical and Physical Properties

Virion buoyant density in CsCl is 1.33-1.43  g/cm³. Additional components with different sedimentation coefficients are found in preparations of some viruses in the genus Totivirus. These consist of particles containing satellite or defective dsRNA.

Nucleic Acid

Virions contain a single molecule of linear uncapped dsRNA, 4.6-7.0  kbp in size.

Proteins

Virions contain a single major capsid protein, with an Mr of 70-100 10³. Virion-associated RNA polymerase activity is present.

Lipids

None reported.

Carbohydrates

None reported.

Genome Organization and Replication

The genome contains two large, usually overlapping, ORFs; the 5-proximal encodes the major capsid protein (Gag) and the 3-proximal encodes an RNA-dependent RNA polymerase (RDRP). The virion-associated RNA-dependent RNA polymerase catalyzes in vitro end-to-end transcription of dsRNA to produce mRNA for capsid protein and RDRP, by a conservative mechanism. When provided with viral (+) strand template, the RDRP specifically binds (+) strands and catalyzes (-) strand synthesis to form dsRNA. When provided with viral dsRNA, the RDRP carries out in vitro transcription in a template-specific reaction. The polymerase is usually expressed as a gag-pol-like fusion protein involving two ORFs.

Biological Properties

The viruses are associated with latent infections of their fungal or protozoal hosts.