Evolution of the influenza A virus untranslated regions.

In case of the influenza virus, untranslated regions are considered to possess plural functions. The specific packaging model suggests that each of eight segments contains a unique "packaging signal", situated within the untranslated region and coding regions at the 5' and 3' ends of the genomic RNA. In addition to packaging signal, UTRs play key roles in polyadenylation and signals for genomic replication. In the present study, we report the evolutionary characteristics of untranslated regions of influenza virus. We analyzed 574 sequences of the human influenza A (H3N2) virus. The sequence of the untranslated region is highly conserved. Our analysis produced several observations regarding the untranslated region: (1) it has an extremely low divergence; (2) its evolutionary speed is not associated with the protein(s) it encodes; (3) its evolutionary rate is smaller than that of the open reading frame. We revealed the evolutionary characteristics of the untranslated region; this is the first study on the non-coding region of the influenza virus from the perspective of evolutionary molecular biology. In the future, clarifying the detailed mechanisms of packaging as well as the function of the RNA sequence in the untranslated region will increase the understanding of viral life cycles and evolution.