A functional sequence-specific interaction between influenza A virus genomic RNA segments.

Influenza A viruses cause annual influenza epidemics and occasional severe pandemics. Their genome is segmented into eight fragments, which offers evolutionary advantages but complicates genomic packaging. The existence of a selective packaging mechanism, in which one copy of each viral RNA is specifically packaged into each virion, is suspected, but its molecular details remain unknown. Here, we identified a direct intermolecular interaction between two viral genomic RNA segments of an avian influenza A virus using in vitro experiments. Using silent trans-complementary mutants, we then demonstrated that this interaction takes place in infected cells and is required for optimal viral replication. Disruption of this interaction did not affect the HA titer of the mutant viruses, suggesting that the same amount of viral particles was produced. However, it nonspecifically decreased the amount of viral RNA in the viral particles, resulting in an eightfold increase in empty viral particles. Competition experiments indicated that this interaction favored copackaging of the interacting viral RNA segments. The interaction we identified involves regions not previously designated as packaging signals and is not widely conserved among influenza A virus. Combined with previous studies, our experiments indicate that viral RNA segments can promote the selective packaging of the influenza A virus genome by forming a sequence-dependent supramolecular network of interactions. The lack of conservation of these interactions might limit genetic reassortment between divergent influenza A viruses.