A structure-based model for the complete transcription cycle of influenza polymerase.

Influenza polymerase uses unique mechanisms to synthesise capped and polyadenylated viral mRNAs from the genomic RNA template, which is packaged inside ribonucleoprotein particles (RNPs). Here we visualise by cryo-electron-microscopy the conformational dynamics of the polymerase during the complete transcription cycle from pre-initiation to termination, focussing on the template trajectory. After exiting the active site cavity, the template 3' extremity rebinds into a specific site on the polymerase surface. Here it remains sequestered during all subsequent transcription steps, forcing the template to loop out as it further translocates. At termination, the strained connection between the bound template 5' end and the active site results in polyadenylation by stuttering at uridine 17. Upon product dissociation, further conformational changes release the trapped template, allowing recycling into the pre-initiation state. Influenza polymerase thus performs transcription whilst tightly binding to and protecting both template ends, allowing efficient production of multiple mRNAs from a single RNP.