FIGURE 1.

Cap-snatching transcription mechanism of influenza polymerase. The PA-PB1-PB2 complex is localized in the nucleus of the infected cell. During transcription, the PB2 subunit binds the 5′,7-methylguanosine cap of a host pre-mRNA molecule (red), which is subsequently cleaved 10–15 nucleotides downstream by the PA endonuclease. The resulting short capped RNA primer is used to initiate polymerization by the RNA-dependent RNA polymerase of the PB1 subunit using 5′- and 3′-bound vRNA (green) as template, resulting in capped, polyadenylated, chimeric mRNA molecules (red and blue) that are exported to the cytoplasm for translation into viral proteins.