RESEARCH SUMMARY

Influenza virus employs an unusual RNA transcription mechanism that uses as primers short-capped oligonucleotides scavenged from newly synthesised RNA polymerase II (RNAP II) transcripts; this fact entirely decides its life cycle. It first requires functional coupling between viral and cellular transcription machineries. Second, viral and cell mRNAs contain a 5' cap structure and a 3' polyA tail and are thus structurally equivalent; influenza virus must therefore have developed sophisticated strategies to discriminate and favour translation of its own mRNA. As a consequence of its transcription mechanism and the nature of the viral mRNA, influenza virus thus requires a complex system of virus-host factor interactions to complete a successful viral replication cycle. The viral polymerase plays a key role establishing productive interactions with host-cell factors involved in both cell transcription and translation.

Of the cell factors that interact with viral polymerase, we have been involved in the characterization of two transcription related factors: hCLE, a positive modulator of the RNAP II and CHD6, a chromatin remodeller. We observed that whereas hCLE also positively modulates influenza virus replication, CHD6 is a negative modulator.

Although viral and cell transcription are functionally coupled, degradation of cell RNAP II occurs once synthesis of viral mRNAs is complete, probably avoiding competition; this degradation appears to be a virulence marker. Using recombinant viruses containing polymerase segments from strains that do or do not induce RNAP II degradation, we observed that PA and PB2 polymerase subunits contribute individually to the degradation process.

Among the mechanisms that influenza virus uses to ensure selective translation of its mRNA, the eIF4F translation initiation complex has a key role. We characterized the influenza virus mRNA translation requirements for the components in this complex. Whereas eIF4A and eIF4G components are absolutely necessary, the cap-binding factor eIF4E is dispensable. Despite the fact that influenza virus mRNAs are capped, their translation is therefore independent of the cell cap-binding factor used by cellular mRNAs, and the viral polymerase might play the role of specific viral cap-binding complex for viral mRNA translation.