Attenuated Strains of Influenza A Viruses Do Not Induce Degradation of RNA Polymerase II

Abstract

We have previously shown that infection with laboratory-passaged strains of influenza virus causes both specific degradation of the largest subunit of the RNA polymerase II complex (RNAP II) and inhibition of host cell transcription. When infection with natural human and avian isolates belonging to different antigenic subtypes was examined, we observed that all of these viruses efficiently induce the proteolytic process. To evaluate whether this process is a general feature of nonattenuated viruses, we studied the behavior of the influenza virus strains A/PR8/8/34 (PR8) and the cold-adapted A/Ann Arbor/6/60 (AA), which are currently used as the donor strains for vaccine seeds due to their attenuated phenotype. We have observed that upon infection with these strains, degradation of the RNAP II does not occur. Moreover, by runoff experiments we observe that PR8 has a reduced ability to inhibit cellular mRNA transcription. In addition, a hypervirulent PR8 (hvPR8) variant that multiplies much faster than standard PR8 (lvPR8) in infected cells and is more virulent in mice than the parental PR8 virus, efficiently induces RNAP II degradation. Studies with reassortant viruses containing defined genome segments of both hvPR8 and lvPR8 indicate that PA and PB2 subunits individually contribute to the ability of influenza virus to degrade the RNAP II. In addition, recently it has been reported that the inclusion of PA or PB2 from hvPR8 in lvPR8 recombinant viruses, highly increases their pathogenicity. Together, the data indicate that the capacity of the influenza virus to degrade RNAP II and inhibit the host cell transcription machinery is a feature of influenza A viruses that might contribute to their virulence.The genome of the influenza A viruses consists of eight single-stranded RNA segments of negative polarity, encoding a total of 11 proteins. Upon entry into susceptible cells, infecting ribonucleoprotein complexes (RNPs) are transported to the nucleus, where transcription and replication take place. Replication of viral RNAs (vRNAs) involves the synthesis of positivestrand replicative intermediates (cRNAs) that are exact copies of the virion RNAs (for a review, see reference 15). A functional link between viral and cellular transcription has been proposed since influenza virus mRNA transcription is initiated using short capped RNA oligonucleotides as primers that are obtained by endonucleolytic cleavage of de novo-synthesized cellular premRNAs (6, 56). This cap-snatching process is performed by the viral polymerase, a heterotrimeric complex comprised of the PB1, PB2, and PA subunits (15,30,40).The carboxy-terminal domain (CTD) of the largest subunit of the RNA polymerase II (RNAP II) complex plays an essential role in cellular transcription. This domain is differentially phosphorylated during the transcription cycle, dynamically permitting or impeding its association with a large number of factors (27). Two major forms of RNAP II can be found in cells when the CTD of its largest subunit is hyperphosphorylated or hyp...

Cyclin T1/CDK9 Interacts with Influenza A Virus Polymerase and Facilitates Its Association with Cellular RNA Polymerase II

“…Previous studies found that influenza virus infection causes the specific degradation of the hypophosphorylated form of the largest subunit of Pol II. The degradation was dependent on viral polymerase and the proteolytic activity of PA (26,27). A recent study reported that the hyperphosphorylated form of Pol II was also degraded during influenza virus infection and that the proteasome pathway was involved in the degradation.…”

“…It was proposed that Pol II degradation may represent a viral strategy to subvert the host antiviral system (31). Other studies pointed out that attenuated influenza virus strains did not cause Pol II degradation, suggesting that the Pol II degradation may be linked with virulence (26). We also found that the level of the hypophosphorylated form of Pol II was decreased in A/WSN/33-infected cells compared to that in uninfected cells (data not shown), but the Finally, CDK9 activity may be modified during influenza virus infection.…”

Interferon Antagonist NSs of La Crosse Virus Triggers a DNA Damage Response-like Degradation of Transcribing RNA Polymerase II

“…Two other viruses, herpes simplex virus and influenza A virus, were previously shown to trigger degradation of RNAP II (61)(62)(63)(64). However, for both these viruses, levels of total RNAP II or of IIa are affected rather than preferentially IIo as in the case of LACV.…”

“…However, for both these viruses, levels of total RNAP II or of IIa are affected rather than preferentially IIo as in the case of LACV. Herpes simplex virus achieves RNAP II degradation through the proteasomal system (61,62), whereas for influenza A virus the proteolytic viral protein PA is responsible (63,64). It should be noted that, in contrast to LACV, these viruses are directly dependent on RNAP II activity.…”

Specific Residues of PB2 and PA Influenza Virus Polymerase Subunits Confer the Ability for RNA Polymerase II Degradation and Virus Pathogenicity in Mice

“…More recently, a degradative process affecting RNA polymerase II (RNAP II) (9)(10)(11), as well as CHD6 chromatin remodeler (12), has been reported. Degradation and disabling of host cell factors involved in genome expression may contribute to hijacking the metabolism of the infected cell and to suppressing the establishment of the host antiviral defense against viral pathogens, contributing to viral pathogenicity.…”