Structural comparisons of empty and full cytoplasmic polyhedrosis virus. Protein-RNA interactions and implications for endogenous RNA transcription mechanism.

Viruses in the family Reoviridae are capable of transcription within the intact capsids. As the only single-shelled and thus the simplest member of the Reoviridae, cytoplasmic polyhedrosis virus (CPV) provides an attractive system for studying endogenous transcription. We report the structures of the full and empty CPV determined at 13-A resolution by electron cryomicroscopy. The structure of the empty CPV reveals a density attributed to the transcription enzyme complex, which is attached to the internal surface of the capsid shell below each of the 12 turrets. The full capsid has an identical capsid shell but contains additional internal densities contributed by the genomic double-stranded (ds) RNA. The RNA densities proximal to the capsid shell are organized into layers with a dodecahedral appearance, suggesting a genome organization of dsRNA segments each having a cone shape spooling around a transcription enzyme complex. Our structures also suggest that the capsid shell serves as a scaffold for appropriate positioning of the RNA genome, whereas nascent mRNA release takes place through the constricted central channel of the turret. Based on these observations, a detailed moving template transcription mechanism is proposed that may provide insight into the well coordinated and highly efficient endogenous RNA transcription of dsRNA viruses.