Cryo-EM maps reveal how influenza polymerase snatches caps from host transcripts

New cryo-EM structures show influenza polymerase (FluPol) binds a transcribing host Pol II–DSIF complex to perform cap snatching. The PA endonuclease sits near the Pol II RNA exit, DSIF stabilizes the interaction, and the capped RNA is channeled from the cap-binding domain to the endonuclease before cleavage. After cleavage, the 3′ end of the primer is directed toward FluPol’s polymerase active site, yielding a primer ready for transcription initiation in a state that resembles the pre-initiation complex. Mutations at FluPol–Pol II–DSIF interfaces reduce polymerase activity in cells, defining a three-step mechanism and a window during early transcription when cap snatching can occur. These findings clarify how host transcription is hijacked for viral mRNA synthesis and suggest targets for antiviral strategies.