[Classification and genome structure of influenza virus]

Influenza A and B viruses contain eight negative-strand RNA segments, while influenza C virus contains seven, each of which encodes 1 or 2 proteins. The RNA segments possess untranslated regions(UTRs) at 3' and 5' ends. The UTRs are composed of highly conserved terminal nucleotides and segment-specific nonconserved nucleotides located adjacent to the open reading frame of the viral RNAs. They are responsible for transcription, translation, and replication of viral RNA. We recently found that the sequences at both ends of the coding regions are important for efficient packaging of the RNA segments into influenza A virus particles. This information allowed us to generate recombinant viruses with chimeric RNA segments containing foreign gene sequences, providing a rational design for an influenza virus vector.