Specific residues of PB2 and PA influenza virus polymerase subunits confer the ability for RNA polymerase II degradation and virus pathogenicity in mice.

UNLABELLED: Influenza virus transcription requires functional coupling with cellular transcription for the cap-snatching process. Despite this fact, RNA polymerase II (RNAP II) is degraded during infection in a process triggered by the viral polymerase. Reassortant viruses from the A/PR/8/34 (PR8) strain that induce (hvPR8) or do not induce (lvPR8) RNAP II degradation led to the identification of PA and PB2 subunits as responsible for the degradation process. Three changes in the PB2 sequence (I105M, N456D, and I504V) and two in PA (Q193H and I550L) differentiate PA and PB2 of lvPR8 from those of hvPR8. Using recombinant viruses, we observed that changes at position 504 of PB2, together with 550 of PA, confer the ability on lvPR8 for RNAP II degradation and, conversely, abolish hvPR8 degradation capacity. Since hvPR8 is more pathogenic than lvPR8 in mice, we tested the potential contribution of RNAP II degradation in a distant viral strain, the 2009 pandemic A/California/04/09 (CAL) virus, whose PA and PB2 subunits are of avian origin. As in the hvPR8 virus, mutations at positions 504 of PB2 and 550 of PA in CAL virus abolished its RNAP II degradation capacity. Moreover, in an in vivo model, the CAL-infected mice lost more body weight, and 75% lethality was observed in this situation compared with 100% survival in mutant-CAL- or mock-infected animals. These results confirm the involvement of specific PB2 and PA residues in RNAP II degradation, which correlates with pathogenicity in mice of viruses containing human or avian polymerase PB2 and PA subunits. IMPORTANCE: The influenza virus polymerase induces the degradation of RNAP II, which probably cooperates to avoid the antiviral response. Here, we have characterized two specific residues located in the PA and PB2 polymerase subunits that mediate this degradation in different influenza viruses. Moreover, a clear correlation between RNAP II degradation and in vivo pathogenicity in mice was observed, indicating that the degradative process constitutes a viral pathogenicity factor.