Conservative transcription in three steps visualized in a double-stranded RNA virus

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Double-stranded RNA (dsRNA) viruses, like those in the Reoviridae family, utilize endogenous RNA transcription.
• The mechanisms by which RNA-dependent RNA polymerase (RdRp) initiates and regulates transcription within the viral capsid remain poorly understood due to a lack of structural data for intermediate states.

Purpose of the Study:

• To elucidate the dynamic process of endogenous RNA transcription by capturing in situ structures of the RdRp-RNA complex at various functional states.
• To understand how the RdRp coordinates transcription initiation, elongation, and termination within the cytoplasmic polyhedrosis virus (CPV) capsid.

Main Methods:

• High-resolution (2.8-3.5 Å) cryo-electron microscopy was used to capture five distinct RdRp-RNA complex structures.
• Structures were obtained under seven different experimental conditions to represent quiescent, initiation, early elongation, elongation, and abortive transcription states of CPV.

Main Results:

• The study identified a 'Y'-form RNA structure at the initiation state and a complete transcription bubble during elongation.
• Three major conformational transitions of the RdRp were observed during state transitions, detailing the dynamic process of RNA synthesis.
• The findings provide unprecedented structural insights into the step-by-step mechanism of viral RNA transcription.

Conclusions:

• The captured structures support a novel 'ouroboros' model for endogenous conservative transcription in dsRNA viruses.
• This work significantly advances our understanding of the fundamental molecular mechanisms governing viral RNA replication and provides a structural basis for future antiviral drug development.