Abstract: Our molecular analysis revealed that two amino acid changes in the polymerase complex (a
glutamate-to-lysine substitution at position 627 of
PB2 and a
threonine-to-isoleucine substitution at position 97 of
PA) were associated with the increased pathogenicity; the
PB2 E627K mutation was responsible for the initial virulence conversion (0 to 100% lethality), while the
PA T97I mutation acted as an accessory for the increased virulence.