FIG. 9.

Working model for transcriptional polarity in Cystoviridae. Production of plus-sense RNA transcripts is favored in both Pol/dsRNA and particle-based transcription systems due to at least two independent factors: Pol specificity for the 3′ terminus and the availability of the terminus in a single-stranded initiation-competent form. The latter can be controlled either through more-efficient fraying of transcription initiation end of dsRNA segments (depicted for the Pol/dsRNA system) or through the presence of stable intramolecular secondary structure at the 3′ termini of plus strands and the absence of that at the 3′ termini of minus strands (shown for a partially unwound dsRNA segment within the subviral particle).