File:Viru0105 0436 fig001.jpg

English: Schematic model for the Pol II transcription cycle in cells infected with influenza virus and the role of the viral RNA polymerase in the inhibition and degradation of Pol II. The model depicts the large subunit of Pol II with the C-terminal domain (CTD) which is differentially phosphorylated during the transcription cycle (indicated by red and green dots). During the transcription cycle cellular factors involved in capping, splicing and polyadenylation associate with the CTD. The polyadenylation signal AATAAA is indicated. In influenza virus infected cells, the viral RNA polymerase (composed of the PB1, PB2 and PA subunits) binds to the serine-5 phosphorylated CTD of Pol II engaged in transcription initiation. On one hand, this association allows the viral RNA polymerase to access the 5′ cap structure of nascent Pol II transcripts and on the other, it results in the premature termination of Pol II. Arrested Pol II is ubiquitylated (Ub) by the cellular machinery and eventually is degraded by the proteasome. Modified from Chan et al.27