Abstact

Double-stranded (ds)RNA viruses replicate and transcribe their genome within a proteinaceous viral capsid to evade host cell defenses. While Reovirales members use conservative transcription, most dsRNA viruses, including cystoviruses, utilize semi-conservative transcription, in which a newly synthesized positive strand replaces the parental positive strand, which is released as mRNA. Here, we visualize semi-conservative transcription activation in cystovirus ɸ6 double-layered particles using cryogenic electron microscopy. We observe nucleotide-triggered disassembly of the domain-swapped outer capsid layer, subsequent expansion of the inner capsid layer, and stepwise assembly of transcription complexes at the opposing poles of the spooled dsRNA genome. These complexes consist of the viral polymerases embedded into a triskelion formed by the minor protein P7, which we show as essential for continuous transcription. The packaging hexamers proximal to the transcription sites channel the viral mRNA exit. Our results define the complex molecular pathway from the quiescent state to activated semi-conservative transcription.