Schematic diagram of chimeric and single amino acid PB2 mutants and virus titers in mouse lungs and brains.

<p>Differences of amino acid residues in PB2 between TS and V virus were shown as single-letter amino acid codes with their positions indicated at the top of the diagram. The red and blue bars indicate the amino acid regions originated from TS or V, respectively. Virus titers are presented by log₁₀ geometric mean EID₅₀/ml ± SEM (The detection limit is 10¹ EID₅₀/ml).